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many years of experimentation brought it to relative perfection
in 1839, in which year the famous daguerreotype first brought the
matter to popular attention. In the same year Mr. Fox Talbot read
a paper on the subject before the Royal Society, and soon
afterwards the efforts of Herschel and numerous other natural
philosophers contributed to the advancement of the new method.

In 1843 Dr. John W. Draper, the famous English-American chemist
and physiologist, showed that by photography the Fraunhofer lines
in the solar spectrum might be mapped with absolute accuracy;
also proving that the silvered film revealed many lines invisible
to the unaided eye. The value of this method of observation was
recognized at once, and, as soon as the spectroscope was
perfected, the photographic method, in conjunction with its use,
became invaluable to the chemist. By this means comparisons of
spectra may be made with a degree of accuracy not otherwise
obtainable; and, in case of the stars, whole clusters of spectra
may be placed on record at a single observation.

As the examination of the sun and stars proceeded, chemists were
amazed or delighted, according to their various preconceptions,
to witness the proof that many familiar terrestrial elements are
to be found in the celestial bodies. But what perhaps surprised
them most was to observe the enormous preponderance in the
sidereal bodies of the element hydrogen. Not only are there vast
quantities of this element in the sun's atmosphere, but some
other suns appeared to show hydrogen lines almost exclusively in
their spectra. Presently it appeared that the stars of which
this is true are those white stars, such as Sirius, which had
been conjectured to be the hottest; whereas stars that are only
red-hot, like our sun, show also the vapors of many other
elements, including iron and other metals.

In 1878 Professor J. Norman Lockyer, in a paper before the Royal
Society, called attention to the possible significance of this
series of observations. He urged that the fact of the sun showing
fewer elements than are observed here on the cool earth, while
stars much hotter than the sun show chiefly one element, and that
one hydrogen, the lightest of known elements, seemed to give
color to the possibility that our alleged elements are really
compounds, which at the temperature of the hottest stars may be
decomposed into hydrogen, the latter "element" itself being also
doubtless a compound, which might be resolved under yet more
trying conditions.

Here, then, was what might be termed direct experimental evidence
for the hypothesis of Prout. Unfortunately, however, it is
evidence of a kind which only a few experts are competent to
discuss--so very delicate a matter is the spectral analysis of
the stars. What is still more unfortunate, the experts do not
agree among themselves as to the validity of Professor Lockyer's
conclusions. Some, like Professor Crookes, have accepted them
with acclaim, hailing Lockyer as "the Darwin of the inorganic
world," while others have sought a different explanation of the
facts he brings forward. As yet it cannot be said that the
controversy has been brought to final settlement. Still, it is
hardly to be doubted that now, since the periodic law has seemed
to join hands with the spectroscope, a belief in the compound
nature of the so-called elements is rapidly gaining ground among
chemists. More and more general becomes the belief that the
Daltonian atom is really a compound radical, and that back of the
seeming diversity of the alleged elements is a single form of
primordial matter. Indeed, in very recent months, direct
experimental evidence for this view has at last come to hand,
through the study of radio-active substances. In a later chapter
we shall have occasion to inquire how this came about.



An epoch in physiology was made in the eighteenth century by the
genius and efforts of Albrecht von Haller (1708-1777), of Berne,
who is perhaps as worthy of the title "The Great" as any
philosopher who has been so christened by his contemporaries
since the time of Hippocrates. Celebrated as a physician, he was
proficient in various fields, being equally famed in his own time
as poet, botanist, and statesman, and dividing his attention
between art and science.

As a child Haller was so sickly that he was unable to amuse
himself with the sports and games common to boys of his age, and
so passed most of his time poring over books. When ten years of
age he began writing poems in Latin and German, and at fifteen
entered the University of Tubingen. At seventeen he wrote
learned articles in opposition to certain accepted doctrines, and
at nineteen he received his degree of doctor. Soon after this he
visited England, where his zeal in dissecting brought him under
suspicion of grave-robbery, which suspicion made it expedient for
him to return to the Continent. After studying botany in Basel
for some time he made an extended botanical journey through
Switzerland, finally settling in his native city, Berne, as a
practising physician. During this time he did not neglect either
poetry or botany, publishing anonymously a collection of poems.

In 1736 he was called to Gottingen as professor of anatomy,
surgery, chemistry, and botany. During his labors in the
university he never neglected his literary work, sometimes living
and sleeping for days and nights together in his library, eating
his meals while delving in his books, and sleeping only when
actually compelled to do so by fatigue. During all this time he
was in correspondence with savants from all over the world, and
it is said of him that he never left a letter of any kind

Haller's greatest contribution to medical science was his famous
doctrine of irritability, which has given him the name of "father
of modern nervous physiology," just as Harvey is called "the
father of the modern physiology of the blood." It has been said
of this famous doctrine of irritability that "it moved all the
minds of the century--and not in the departments of medicine
alone--in a way of which we of the present day have no
satisfactory conception, unless we compare it with our modern

The principle of general irritability had been laid down by
Francis Glisson (1597-1677) from deductive studies, but Haller
proved by experiments along the line of inductive methods that
this irritability was not common to all "fibre as well as to the
fluids of the body," but something entirely special, and peculiar
only to muscular substance. He distinguished between irritability
of muscles and sensibility of nerves. In 1747 he gave as the
three forces that produce muscular movements: elasticity, or
"dead nervous force"; irritability, or "innate nervous force";
and nervous force in itself. And in 1752 he described one
hundred and ninety experiments for determining what parts of the
body possess "irritability"--that is, the property of contracting
when stimulated. His conclusion that this irritability exists in
muscular substance alone and is quite independent of the nerves
proceeding to it aroused a controversy that was never definitely
settled until late in the nineteenth century, when Haller's
theory was found to be entirely correct.

It was in pursuit of experiments to establish his theory of
irritability that Haller made his chief discoveries in embryology
and development. He proved that in the process of incubation of
the egg the first trace of the heart of the chick shows itself in
the thirty-eighth hour, and that the first trace of red blood
showed in the forty-first hour. By his investigations upon the
lower animals he attempted to confirm the theory that since the
creation of genus every individual is derived from a preceding
individual--the existing theory of preformation, in which he
believed, and which taught that "every individual is fully and
completely preformed in the germ, simply growing from microscopic
to visible proportions, without developing any new parts."

In physiology, besides his studies of the nervous system, Haller
studied the mechanism of respiration, refuting the teachings of
Hamberger (1697-1755), who maintained that the lungs contract
independently. Haller, however, in common with his
contemporaries, failed utterly to understand the true function of
the lungs. The great physiologist's influence upon practical
medicine, while most profound, was largely indirect. He was a
theoretical rather than a practical physician, yet he is credited
with being the first physician to use the watch in counting the


A great contemporary of Haller was Giovanni Battista Morgagni
(1682-1771), who pursued what Sydenham had neglected, the
investigation in anatomy, thus supplying a necessary counterpart
to the great Englishman's work. Morgagni's investigations were
directed chiefly to the study of morbid anatomy--the study of the
structure of diseased tissue, both during life and post mortem,
in contrast to the normal anatomical structures. This work cannot
be said to have originated with him; for as early as 1679 Bonnet
had made similar, although less extensive, studies; and later
many investigators, such as Lancisi and Haller, had made
post-mortem studies. But Morgagni's De sedibus et causis
morborum per anatomen indagatis was the largest, most accurate,
and best-illustrated collection of cases that had ever been
brought together, and marks an epoch in medical science. From the
time of the publication of Morgagni's researches, morbid anatomy
became a recognized branch of the medical science, and the effect
of the impetus thus given it has been steadily increasing since
that time.


William Hunter (1718-1783) must always be remembered as one of
the greatest physicians and anatomists of the eighteenth century,
and particularly as the first great teacher of anatomy in
England; but his fame has been somewhat overshadowed by that of
his younger brother John.

Hunter had been intended and educated for the Church, but on the
advice of the surgeon William Cullen he turned his attention to
the study of medicine. His first attempt at teaching was in 1746,
when he delivered a series of lectures on surgery for the Society
of Naval Practitioners. These lectures proved so interesting and
instructive that he was at once invited to give others, and his
reputation as a lecturer was soon established. He was a natural
orator and story-teller, and he combined with these attractive
qualities that of thoroughness and clearness in demonstrations,
and although his lectures were two hours long he made them so
full of interest that his pupils seldom tired of listening. He
believed that he could do greater good to the world by "publicly
teaching his art than by practising it," and even during the last
few days of his life, when he was so weak that his friends
remonstrated against it, he continued his teaching, fainting from
exhaustion at the end of his last lecture, which preceded his
death by only a few days.

For many years it was Hunter's ambition to establish a museum
where the study of anatomy, surgery, and medicine might be
advanced, and in 1765 he asked for a grant of a plot of ground
for this purpose, offering to spend seven thousand pounds on its,
erection besides endowing it with a professorship of anatomy. Not
being able to obtain this grant, however, he built a house, in
which were lecture and dissecting rooms, and his museum. In this
museum were anatomical preparations, coins, minerals, and
natural-history specimens.

Hunter's weakness was his love of controversy and his resentment
of contradiction. This brought him into strained relations with
many of the leading physicians of his time, notably his own
brother John, who himself was probably not entirely free from
blame in the matter. Hunter is said to have excused his own
irritability on the grounds that being an anatomist, and
accustomed to "the passive submission of dead bodies,"
contradictions became the more unbearable. Many of the
physiological researches begun by him were carried on and
perfected by his more famous brother, particularly his
investigations of the capillaries, but he added much to the
anatomical knowledge of several structures of the body, notably
as to the structure of cartilages and joints.


In Abbot Islip's chapel in Westminster Abbey, close to the
resting-place of Ben Jonson, rest the remains of John Hunter
(1728-1793), famous in the annals of medicine as among the
greatest physiologists and surgeons that the world has ever
produced: a man whose discoveries and inventions are counted by
scores, and whose field of research was only limited by the
outermost boundaries of eighteenth-century science, although his
efforts were directed chiefly along the lines of his profession.

Until about twenty years of age young Hunter had shown little
aptitude for study, being unusually fond of out-door sports and
amusements; but about that time, realizing that some occupation
must be selected, he asked permission of his brother William to
attempt some dissections in his anatomical school in London. To
the surprise of his brother he made this dissection unusually
well; and being given a second, he acquitted himself with such
skill that his brother at once predicted that he would become a
great anatomist. Up to this time he had had no training of any
kind to prepare him for his professional career, and knew little
of Greek or Latin--languages entirely unnecessary for him, as he
proved in all of his life work. Ottley tells the story that,
when twitted with this lack of knowledge of the "dead languages"
in after life, he said of his opponent, "I could teach him that
on the dead body which he never knew in any language, dead or

By his second year in dissection he had become so skilful that he
was given charge of some of the classes in his brother's school;
in 1754 he became a surgeon's pupil in St. George's Hospital, and
two years later house-surgeon. Having by overwork brought on
symptoms that seemed to threaten consumption, he accepted the
position of staff-surgeon to an expedition to Belleisle in 1760,
and two years later was serving with the English army at
Portugal. During all this time he was constantly engaged in
scientific researches, many of which, such as his observations of
gun-shot wounds, he put to excellent use in later life. On
returning to England much improved in health in 1763, he entered
at once upon his career as a London surgeon, and from that time
forward his progress was a practically uninterrupted series of
successes in his profession.

Hunter's work on the study of the lymphatics was of great service
to the medical profession. This important net-work of minute
vessels distributed throughout the body had recently been made
the object of much study, and various students, including Haller,
had made extensive investigations since their discovery by
Asellius. But Hunter, in 1758, was the first to discover the
lymphatics in the neck of birds, although it was his brother
William who advanced the theory that the function of these
vessels was that of absorbents. One of John Hunter's pupils,
William Hewson (1739-1774), first gave an account, in 1768, of
the lymphatics in reptiles and fishes, and added to his teacher's
investigations of the lymphatics in birds. These studies of the
lymphatics have been regarded, perhaps with justice, as Hunter's
most valuable contributions to practical medicine.

In 1767 he met with an accident by which he suffered a rupture of
the tendo Achillis--the large tendon that forms the attachment of
the muscles of the calf to the heel. From observations of this
accident, and subsequent experiments upon dogs, he laid the
foundation for the now simple and effective operation for the
cure of club feet and other deformities involving the tendons.
In 1772 he moved into his residence at Earlscourt, Brompton,
where he gathered about him a great menagerie of animals, birds,
reptiles, insects, and fishes, which he used in his physiological
and surgical experiments. Here he performed a countless number of
experiments--more, probably, than "any man engaged in
professional practice has ever conducted." These experiments
varied in nature from observations of the habits of bees and
wasps to major surgical operations performed upon hedgehogs,
dogs, leopards, etc. It is said that for fifteen years he kept a
flock of geese for the sole purpose of studying the process of
development in eggs.

Hunter began his first course of lectures in 1772, being forced
to do this because he had been so repeatedly misquoted, and
because he felt that he could better gauge his own knowledge in
this way. Lecturing was a sore trial to him, as he was extremely
diffident, and without writing out his lectures in advance he was
scarcely able to speak at all. In this he presented a marked
contrast to his brother William, who was a fluent and brilliant
speaker. Hunter's lectures were at best simple readings of the
facts as he had written them, the diffident teacher seldom
raising his eyes from his manuscript and rarely stopping until
his complete lecture had been read through. His lectures were,
therefore, instructive rather than interesting, as he used
infinite care in preparing them; but appearing before his classes
was so dreaded by him that he is said to have been in the habit
of taking a half-drachm of laudanum before each lecture to nerve
him for the ordeal. One is led to wonder by what name he shall
designate that quality of mind that renders a bold and fearless
surgeon like Hunter, who is undaunted in the face of hazardous
and dangerous operations, a stumbling, halting, and "frightened"
speaker before a little band of, at most, thirty young medical
students. And yet this same thing is not unfrequently seen among
the boldest surgeons.

Hunter's Operation for the Cure of Aneurisms

It should be an object-lesson to those who, ignorantly or
otherwise, preach against the painless vivisection as practised
to-day, that by the sacrifice of a single deer in the cause of
science Hunter discovered a fact in physiology that has been the
means of saving thousands of human lives and thousands of human
bodies from needless mutilation. We refer to the discovery of the
"collateral circulation" of the blood, which led, among other
things, to Hunter's successful operation upon aneurisms.

Simply stated, every organ or muscle of the body is supplied by
one large artery, whose main trunk distributes the blood into its
lesser branches, and thence through the capillaries. Cutting off
this main artery, it would seem, should cut off entirely the
blood-supply to the particular organ which is supplied by this
vessel; and until the time of Hunter's demonstration this belief
was held by most physiologists. But nature has made a provision
for this possible stoppage of blood-supply from a single source,
and has so arranged that some of the small arterial branches
coming from the main supply-trunk are connected with other
arterial branches coming from some other supply-trunk. Under
normal conditions the main arterial trunks supply their
respective organs, the little connecting arterioles playing an
insignificant part. But let the main supply-trunk be cut off or
stopped for whatever reason, and a remarkable thing takes place.
The little connecting branches begin at once to enlarge and draw
blood from the neighboring uninjured supply-trunk, This
enlargement continues until at last a new route for the
circulation has been established, the organ no longer depending
on the now defunct original arterial trunk, but getting on as
well as before by this "collateral" circulation that has been

The thorough understanding of this collateral circulation is one
of the most important steps in surgery, for until it was
discovered amputations were thought necessary in such cases as
those involving the artery supplying a leg or arm, since it was
supposed that, the artery being stopped, death of the limb and
the subsequent necessity for amputation were sure to follow.
Hunter solved this problem by a single operation upon a deer, and
his practicality as a surgeon led him soon after to apply this
knowledge to a certain class of surgical cases in a most
revolutionary and satisfactory manner.

What led to Hunter's far-reaching discovery was his investigation
as to the cause of the growth of the antlers of the deer. Wishing
to ascertain just what part the blood-supply on the opposite
sides of the neck played in the process of development, or,
perhaps more correctly, to see what effect cutting off the main
blood-supply would have, Hunter had one of the deer of Richmond
Park caught and tied, while he placed a ligature around one of
the carotid arteries--one of the two principal arteries that
supply the head with blood. He observed that shortly after this
the antler (which was only half grown and consequently very
vascular) on the side of the obliterated artery became cold to
the touch--from the lack of warmth-giving blood. There was
nothing unexpected in this, and Hunter thought nothing of it
until a few days later, when he found, to his surprise, that the
antler had become as warm as its fellow, and was apparently
increasing in size. Puzzled as to how this could be, and
suspecting that in some way his ligature around the artery had
not been effective, he ordered the deer killed, and on
examination was astonished to find that while his ligature had
completely shut off the blood-supply from the source of that
carotid artery, the smaller arteries had become enlarged so as to
supply the antler with blood as well as ever, only by a different

Hunter soon had a chance to make a practical application of the
knowledge thus acquired. This was a case of popliteal aneurism,
operations for which had heretofore proved pretty uniformly
fatal. An aneurism, as is generally understood, is an enlargement
of a certain part of an artery, this enlargement sometimes
becoming of enormous size, full of palpitating blood, and likely
to rupture with fatal results at any time. If by any means the
blood can be allowed to remain quiet for even a few hours in this
aneurism it will form a clot, contract, and finally be absorbed
and disappear without any evil results. The problem of keeping
the blood quiet, with the heart continually driving it through
the vessel, is not a simple one, and in Hunter's time was
considered so insurmountable that some surgeons advocated
amputation of any member having an aneurism, while others cut
down upon the tumor itself and attempted to tie off the artery
above and below. The first of these operations maimed the patient
for life, while the second was likely to prove fatal.

In pondering over what he had learned about collateral
circulation and the time required for it to become fully
established, Hunter conceived the idea that if the blood-supply
was cut off from above the aneurism, thus temporarily preventing
the ceaseless pulsations from the heart, this blood would
coagulate and form a clot before the collateral circulation could
become established or could affect it. The patient upon whom he
performed his now celebrated operation was afflicted with a
popliteal aneurism--that is, the aneurism was located on the
large popliteal artery just behind the knee-joint. Hunter,
therefore, tied off the femoral, or main supplying artery in the
thigh, a little distance above the aneurism. The operation was
entirely successful, and in six weeks' time the patient was able
to leave the hospital, and with two sound limbs. Naturally the
simplicity and success of this operation aroused the attention of
Europe, and, alone, would have made the name of Hunter immortal
in the annals of surgery. The operation has ever since been
called the "Hunterian" operation for aneurism, but there is
reason to believe that Dominique Anel (born about 1679) performed
a somewhat similar operation several years earlier. It is
probable, however, that Hunter had never heard of this work of
Anel, and that his operation was the outcome of his own
independent reasoning from the facts he had learned about
collateral circulation. Furthermore, Hunter's mode of operation
was a much better one than Anel's, and, while Anel's must claim
priority, the credit of making it widely known will always be

The great services of Hunter were recognized both at home and
abroad, and honors and positions of honor and responsibility were
given him. In 1776 he was appointed surgeon-extraordinary to the
king; in 1783 he was elected a member of the Royal Society of
Medicine and of the Royal Academy of Surgery at Paris; in 1786 he
became deputy surgeon-general of the army; and in 1790 he was
appointed surgeon-general and inspector-general of hospitals. All
these positions he filled with credit, and he was actively
engaged in his tireless pursuit of knowledge and in discharging
his many duties when in October, 1793, he was stricken while
addressing some colleagues, and fell dead in the arms of a


Hunter's great rival among contemporary physiologists was the
Italian Lazzaro Spallanzani (1729-1799), one of the most
picturesque figures in the history of science. He was not
educated either as a scientist or physician, devoting, himself at
first to philosophy and the languages, afterwards studying law,
and later taking orders. But he was a keen observer of nature and
of a questioning and investigating mind, so that he is remembered
now chiefly for his discoveries and investigations in the
biological sciences. One important demonstration was his
controversion of the theory of abiogenesis, or "spontaneous
generation," as propounded by Needham and Buffon. At the time of
Needham's experiments it had long been observed that when animal
or vegetable matter had lain in water for a little time--long
enough for it to begin to undergo decomposition--the water became
filled with microscopic creatures, the "infusoria animalculis."
This would tend to show, either that the water or the animal or
vegetable substance contained the "germs" of these minute
organisms, or else that they were generated spontaneously. It was
known that boiling killed these animalcules, and Needham agreed,
therefore, that if he first heated the meat or vegetables, and
also the water containing them, and then placed them in
hermetically scaled jars--if he did this, and still the
animalcules made their appearance, it would be proof-positive
that they had been generated spontaneously. Accordingly be made
numerous experiments, always with the same results--that after a
few days the water was found to swarm with the microscopic
creatures. The thing seemed proven beyond question--providing, of
course, that there had been no slips in the experiments.

But Abbe Spallanzani thought that he detected such slips in
Needham's experiment. The possibility of such slips might come
in several ways: the contents of the jar might not have been
boiled for a sufficient length of time to kill all the germs, or
the air might not have been excluded completely by the sealing
process. To cover both these contingencies, Spallanzani first
hermetically sealed the glass vessels and then boiled them for
three-quarters of an hour. Under these circumstances no
animalcules ever made their appearance--a conclusive
demonstration that rendered Needham's grounds for his theory at
once untenable.[2]

Allied to these studies of spontaneous generation were
Spallanzani's experiments and observations on the physiological
processes of generation among higher animals. He experimented
with frogs, tortoises, and dogs; and settled beyond question the
function of the ovum and spermatozoon. Unfortunately he
misinterpreted the part played by the spermatozoa in believing
that their surrounding fluid was equally active in the
fertilizing process, and it was not until some forty years later
(1824) that Dumas corrected this error.


Among the most interesting researches of Spallanzani were his
experiments to prove that digestion, as carried on in the
stomach, is a chemical process. In this he demonstrated, as Rene
Reaumur had attempted to demonstrate, that digestion could be
carried on outside the walls of the stomach as an ordinary
chemical reaction, using the gastric juice as the reagent for
performing the experiment. The question as to whether the stomach
acted as a grinding or triturating organ, rather than as a
receptacle for chemical action, had been settled by Reaumur and
was no longer a question of general dispute. Reaumur had
demonstrated conclusively that digestion would take place in the
stomach in the same manner and the same time if the substance to
be digested was protected from the peristalic movements of the
stomach and subjected to the action of the gastric juice only. He
did this by introducing the substances to be digested into the
stomach in tubes, and thus protected so that while the juices of
the stomach could act upon them freely they would not be affected
by any movements of the organ.

Following up these experiments, he attempted to show that
digestion could take place outside the body as well as in it, as
it certainly should if it were a purely chemical process. He
collected quantities of gastric juice, and placing it in suitable
vessels containing crushed grain or flesh, kept the mixture at
about the temperature of the body for several hours. After
repeated experiments of this kind, apparently conducted with
great care, Reaumur reached the conclusion that "the gastric
juice has no more effect out of the living body in dissolving or
digesting the food than water, mucilage, milk, or any other bland
fluid."[3] Just why all of these experiments failed to
demonstrate a fact so simple does not appear; but to Spallanzani,
at least, they were by no means conclusive, and he proceeded to
elaborate upon the experiments of Reaumur. He made his
experiments in scaled tubes exposed to a certain degree of heat,
and showed conclusively that the chemical process does go on,
even when the food and gastric juice are removed from their
natural environment in the stomach. In this he was opposed by
many physiologists, among them John Hunter, but the truth of his
demonstrations could not be shaken, and in later years we find
Hunter himself completing Spallanzani's experiments by his
studies of the post-mortem action of the gastric juice upon the
stomach walls.

That Spallanzani's and Hunter's theories of the action of the
gastric juice were not at once universally accepted is shown by
an essay written by a learned physician in 1834. In speaking of
some of Spallanzani's demonstrations, he writes: "In some of the
experiments, in order to give the flesh or grains steeped in the
gastric juice the same temperature with the body, the phials were
introduced under the armpits. But this is not a fair mode of
ascertaining the effects of the gastric juice out of the body;
for the influence which life may be supposed to have on the
solution of the food would be secured in this case. The
affinities connected with life would extend to substances in
contact with any part of the system: substances placed under the
armpits are not placed at least in the same circumstances with
those unconnected with a living animal." But just how this writer
reaches the conclusion that "the experiments of Reaumur and
Spallanzani give no evidence that the gastric juice has any
peculiar influence more than water or any other bland fluid in
digesting the food"[4] is difficult to understand.

The concluding touches were given to the new theory of digestion
by John Hunter, who, as we have seen, at first opposed
Spallanzani, but who finally became an ardent champion of the
chemical theory. Hunter now carried Spallanzani's experiments
further and proved the action of the digestive fluids after
death. For many years anatomists had been puzzled by pathological
lesion of the stomach, found post mortem, when no symptoms of any
disorder of the stomach had been evinced during life. Hunter
rightly conceived that these lesions were caused by the action of
the gastric juice, which, while unable to act upon the living
tissue, continued its action chemically after death, thus
digesting the walls of the stomach in which it had been formed.
And, as usual with his observations, be turned this discovery to
practical use in accounting for certain phenomena of digestion.
The following account of the stomach being digested after death
was written by Hunter at the desire of Sir John Pringle, when he
was president of the Royal Society, and the circumstance which
led to this is as follows: "I was opening, in his presence, the
body of a patient of his own, where the stomach was in part
dissolved, which appeared to him very unaccountable, as there had
been no previous symptom that could have led him to suspect any
disease in the stomach. I took that opportunity of giving him my
ideas respecting it, and told him that I had long been making
experiments on digestion, and considered this as one of the facts
which proved a converting power in the gastric juice. . . . There
are a great many powers in nature which the living principle does
not enable the animal matter, with which it is combined, to
resist--viz., the mechanical and most of the strongest chemical
solvents. It renders it, however, capable of resisting the powers
of fermentation, digestion, and perhaps several others, which are
well known to act on the same matter when deprived of the living
principle and entirely to decompose it. "

Hunter concludes his paper with the following paragraph: "These
appearances throw considerable light on the principle of
digestion, and show that it is neither a mechanical power, nor
contractions of the stomach, nor heat, but something secreted in
the coats of the stomach, and thrown into its cavity, which there
animalizes the food or assimilates it to the nature of the blood.
The power of this juice is confined or limited to certain
substances, especially of the vegetable and animal kingdoms; and
although this menstruum is capable of acting independently of the
stomach, yet it is indebted to that viscus for its


It is a curious commentary on the crude notions of mechanics of
previous generations that it should have been necessary to prove
by experiment that the thin, almost membranous stomach of a
mammal has not the power to pulverize, by mere attrition, the
foods that are taken into it. However, the proof was now for the
first time forthcoming, and the question of the general character
of the function of digestion was forever set at rest. Almost
simultaneously with this great advance, corresponding progress
was made in an allied field: the mysteries of respiration were
at last cleared up, thanks to the new knowledge of chemistry. The
solution of the problem followed almost as a matter of course
upon the advances of that science in the latter part of the
century. Hitherto no one since Mayow, of the previous century,
whose flash of insight had been strangely overlooked and
forgotten, had even vaguely surmised the true function of the
lungs. The great Boerhaave had supposed that respiration is
chiefly important as an aid to the circulation of the blood; his
great pupil, Haller, had believed to the day of his death in 1777
that the main purpose of the function is to form the voice. No
genius could hope to fathom the mystery of the lungs so long as
air was supposed to be a simple element, serving a mere
mechanical purpose in the economy of the earth.

But the discovery of oxygen gave the clew, and very soon all the
chemists were testing the air that came from the lungs--Dr.
Priestley, as usual, being in the van. His initial experiments
were made in 1777, and from the outset the problem was as good as
solved. Other experimenters confirmed his results in all their
essentials--notably Scheele and Lavoisier and Spallanzani and
Davy. It was clearly established that there is chemical action
in the contact of the air with the tissue of the lungs; that some
of the oxygen of the air disappears, and that carbonic-acid gas
is added to the inspired air. It was shown, too, that the blood,
having come in contact with the air, is changed from black to red
in color. These essentials were not in dispute from the first.
But as to just what chemical changes caused these results was the
subject of controversy. Whether, for example, oxygen is actually
absorbed into the blood, or whether it merely unites with carbon
given off from the blood, was long in dispute.

Each of the main disputants was biased by his own particular
views as to the moot points of chemistry. Lavoisier, for
example, believed oxygen gas to be composed of a metal oxygen
combined with the alleged element heat; Dr. Priestley thought it
a compound of positive electricity and phlogiston; and Humphry
Davy, when he entered the lists a little later, supposed it to be
a compound of oxygen and light. Such mistaken notions naturally
complicated matters and delayed a complete understanding of the
chemical processes of respiration. It was some time, too, before
the idea gained acceptance that the most important chemical
changes do not occur in the lungs themselves, but in the ultimate
tissues. Indeed, the matter was not clearly settled at the close
of the century. Nevertheless, the problem of respiration had
been solved in its essentials. Moreover, the vastly important
fact had been established that a process essentially identical
with respiration is necessary to the existence not only of all
creatures supplied with lungs, but to fishes, insects, and even
vegetables--in short, to every kind of living organism.


Some interesting experiments regarding vegetable respiration were
made just at the close of the century by Erasmus Darwin, and
recorded in his Botanic Garden as a foot-note to the verse:

"While spread in air the leaves respiring play."

These notes are worth quoting at some length, as they give a
clear idea of the physiological doctrines of the time (1799),
while taking advance ground as to the specific matter in

"There have been various opinions," Darwin says, "concerning the
use of the leaves of plants in the vegetable economy. Some have
contended that they are perspiratory organs. This does not seem
probable from an experiment of Dr. Hales, Vegetable Statics, p.
30. He, found, by cutting off branches of trees with apples on
them and taking off the leaves, that an apple exhaled about as
much as two leaves the surfaces of which were nearly equal to the
apple; whence it would appear that apples have as good a claim to
be termed perspiratory organs as leaves. Others have believed
them excretory organs of excrementitious juices, but as the vapor
exhaled from vegetables has no taste, this idea is no more
probable than the other; add to this that in most weathers they
do not appear to perspire or exhale at all.

"The internal surface of the lungs or air-vessels in men is said
to be equal to the external surface of the whole body, or almost
fifteen square feet; on this surface the blood is exposed to the
influence of the respired air through the medium, however, of a
thin pellicle; by this exposure to the air it has its color
changed from deep red to bright scarlet, and acquires something
so necessary to the existence of life that we can live scarcely a
minute without this wonderful process.

"The analogy between the leaves of plants and the lungs or gills
of animals seems to embrace so many circumstances that we can
scarcely withhold our consent to their performing similar

"1. The great surface of leaves compared to that of the trunk
and branches of trees is such that it would seem to be an organ
well adapted for the purpose of exposing the vegetable juices to
the influence of the air; this, however, we shall see afterwards
is probably performed only by their upper surfaces, yet even in
this case the surface of the leaves in general bear a greater
proportion to the surface of the tree than the lungs of animals
to their external surfaces.

"2. In the lung of animals the blood, after having been exposed
to the air in the extremities of the pulmonary artery, is changed
in color from deep red to bright scarlet, and certainly in some
of its essential properties it is then collected by the pulmonary
vein and returned to the heart. To show a similarity of
circumstances in the leaves of plants, the following experiment
was made, June 24, 1781. A stalk with leaves and seed-vessels of
large spurge (Euphorbia helioscopia) had been several days placed
in a decoction of madder (Rubia tinctorum) so that the lower part
of the stem and two of the undermost leaves were immersed in it.
After having washed the immersed leaves in clear water I could
readily discover the color of the madder passing along the middle
rib of each leaf. The red artery was beautifully visible on the
under and on the upper surface of the leaf; but on the upper side
many red branches were seen going from it to the extremities of
the leaf, which on the other side were not visible except by
looking through it against the light. On this under side a system
of branching vessels carrying a pale milky fluid were seen coming
from the extremities of the leaf, and covering the whole under
side of it, and joining two large veins, one on each side of the
red artery in the middle rib of the leaf, and along with it
descending to the foot-stalk or petiole. On slitting one of these
leaves with scissors, and having a magnifying-glass ready, the
milky blood was seen oozing out of the returning veins on each
side of the red artery in the middle rib, but none of the red
fluid from the artery.

"All these appearances were more easily seen in a leaf of Picris
treated in the same manner; for in this milky plant the stems and
middle rib of the leaves are sometimes naturally colored reddish,
and hence the color of the madder seemed to pass farther into the
ramifications of their leaf-arteries, and was there beautifully
visible with the returning branches of milky veins on each side."

Darwin now goes on to draw an incorrect inference from his

"3. From these experiments," he says, "the upper surface of the
leaf appeared to be the immediate organ of respiration, because
the colored fluid was carried to the extremities of the leaf by
vessels most conspicuous on the upper surface, and there changed
into a milky fluid, which is the blood of the plant, and then
returned by concomitant veins on the under surface, which were
seen to ooze when divided with scissors, and which, in Picris,
particularly, render the under surface of the leaves greatly
whiter than the upper one."

But in point of fact, as studies of a later generation were to
show, it is the under surface of the leaf that is most abundantly
provided with stomata, or "breathing-pores." From the stand-point
of this later knowledge, it is of interest to follow our author a
little farther, to illustrate yet more fully the possibility of
combining correct observations with a faulty inference.

"4. As the upper surface of leaves constitutes the organ of
respiration, on which the sap is exposed in the termination of
arteries beneath a thin pellicle to the action of the atmosphere,
these surfaces in many plants strongly repel moisture, as cabbage
leaves, whence the particles of rain lying over their surfaces
without touching them, as observed by Mr. Melville (Essays
Literary and Philosophical: Edinburgh), have the appearance of
globules of quicksilver. And hence leaves with the upper
surfaces on water wither as soon as in the dry air, but continue
green for many days if placed with the under surface on water, as
appears in the experiments of Monsieur Bonnet (Usage des
Feuilles). Hence some aquatic plants, as the water-lily
(Nymphoea), have the lower sides floating on the water, while the
upper surfaces remain dry in the air.

"5. As those insects which have many spiracula, or breathing
apertures, as wasps and flies, are immediately suffocated by
pouring oil upon them, I carefully covered with oil the surfaces
of several leaves of phlomis, of Portugal laurel, and balsams,
and though it would not regularly adhere, I found them all die in
a day or two.

"It must be added that many leaves are furnished with muscles
about their foot-stalks, to turn their surfaces to the air or
light, as mimosa or Hedysarum gyrans. From all these analogies I
think there can be no doubt but that leaves of trees are their
lungs, giving out a phlogistic material to the atmosphere, and
absorbing oxygen, or vital air.

"6. The great use of light to vegetation would appear from this
theory to be by disengaging vital air from the water which they
perspire, and thence to facilitate its union with their blood
exposed beneath the thin surface of their leaves; since when pure
air is thus applied it is probable that it can be more readily
absorbed. Hence, in the curious experiments of Dr. Priestley and
Mr. Ingenhouz, some plants purified less air than others--that
is, they perspired less in the sunshine; and Mr. Scheele found
that by putting peas into water which about half covered them
they converted the vital air into fixed air, or carbonic-acid
gas, in the same manner as in animal respiration.

"7. The circulation in the lungs or leaves of plants is very
similar to that of fish. In fish the blood, after having passed
through their gills, does not return to the heart as from the
lungs of air-breathing animals, but the pulmonary vein taking the
structure of an artery after having received the blood from the
gills, which there gains a more florid color, distributes it to
the other parts of their bodies. The same structure occurs in the
livers of fish, whence we see in those animals two circulations
independent of the power of the heart--viz., that beginning at
the termination of the veins of the gills and branching through
the muscles, and that which passes through the liver; both which
are carried on by the action of those respective arteries and

Darwin is here a trifle fanciful in forcing the analogy between
plants and animals. The circulatory system of plants is really
not quite so elaborately comparable to that of fishes as he
supposed. But the all-important idea of the uniformity underlying
the seeming diversity of Nature is here exemplified, as elsewhere
in the writings of Erasmus Darwin; and, more specifically, a
clear grasp of the essentials of the function of respiration is
fully demonstrated.


Several causes conspired to make exploration all the fashion
during the closing epoch of the eighteenth century. New aid to
the navigator had been furnished by the perfected compass and
quadrant, and by the invention of the chronometer; medical
science had banished scurvy, which hitherto had been a perpetual
menace to the voyager; and, above all, the restless spirit of the
age impelled the venturesome to seek novelty in fields altogether
new. Some started for the pole, others tried for a northeast or
northwest passage to India, yet others sought the great
fictitious antarctic continent told of by tradition. All these of
course failed of their immediate purpose, but they added much to
the world's store of knowledge and its fund of travellers' tales.

Among all these tales none was more remarkable than those which
told of strange living creatures found in antipodal lands. And
here, as did not happen in every field, the narratives were often
substantiated by the exhibition of specimens that admitted no
question. Many a company of explorers returned more or less laden
with such trophies from the animal and vegetable kingdoms, to the
mingled astonishment, delight, and bewilderment of the closet
naturalists. The followers of Linnaeus in the "golden age of
natural history," a few decades before, had increased the number
of known species of fishes to about four hundred, of birds to one
thousand, of insects to three thousand, and of plants to ten
thousand. But now these sudden accessions from new territories
doubled the figure for plants, tripled it for fish and birds, and
brought the number of described insects above twenty thousand.
Naturally enough, this wealth of new material was sorely puzzling
to the classifiers. The more discerning began to see that the
artificial system of Linnaeus, wonderful and useful as it had
been, must be advanced upon before the new material could be
satisfactorily disposed of. The way to a more natural system,
based on less arbitrary signs, had been pointed out by Jussieu in
botany, but the zoologists were not prepared to make headway
towards such a system until they should gain a wider
understanding of the organisms with which they had to deal
through comprehensive studies of anatomy. Such studies of
individual forms in their relations to the entire scale of
organic beings were pursued in these last decades of the century,
but though two or three most important generalizations were
achieved (notably Kaspar Wolff's conception of the cell as the
basis of organic life, and Goethe's all-important doctrine of
metamorphosis of parts), yet, as a whole, the work of the
anatomists of the period was germinative rather than
fruit-bearing. Bichat's volumes, telling of the recognition of
the fundamental tissues of the body, did not begin to appear till
the last year of the century. The announcement by Cuvier of the
doctrine of correlation of parts bears the same date, but in
general the studies of this great naturalist, which in due time
were to stamp him as the successor of Linnaeus, were as yet only
fairly begun.



We have seen that the focal points of the physiological world
towards the close of the eighteenth century were Italy and
England, but when Spallanzani and Hunter passed away the scene
shifted to France. The time was peculiarly propitious, as the
recent advances in many lines of science had brought fresh data
for the student of animal life which were in need of
classification, and, as several minds capable of such a task were
in the field, it was natural that great generalizations should
have come to be quite the fashion. Thus it was that Cuvier came
forward with a brand-new classification of the animal kingdom,
establishing four great types of being, which he called
vertebrates, mollusks, articulates, and radiates. Lamarck had
shortly before established the broad distinction between animals
with and those without a backbone; Cuvier's Classification
divided the latter--the invertebrates--into three minor groups.
And this division, familiar ever since to all students of
zoology, has only in very recent years been supplanted, and then
not by revolution, but by a further division, which the elaborate
recent studies of lower forms of life seemed to make desirable.

In the course of those studies of comparative anatomy which led
to his new classification, Cuvier's attention was called
constantly to the peculiar co-ordination of parts in each
individual organism. Thus an animal with sharp talons for
catching living prey--as a member of the cat tribe--has also
sharp teeth, adapted for tearing up the flesh of its victim, and
a particular type of stomach, quite different from that of
herbivorous creatures. This adaptation of all the parts of the
animal to one another extends to the most diverse parts of the
organism, and enables the skilled anatomist, from the observation
of a single typical part, to draw inferences as to the structure
of the entire animal--a fact which was of vast aid to Cuvier in
his studies of paleontology. It did not enable Cuvier, nor does
it enable any one else, to reconstruct fully the extinct animal
from observation of a single bone, as has sometimes been
asserted, but what it really does establish, in the hands of an
expert, is sufficiently astonishing.

"While the study of the fossil remains of the greater quadrupeds
is more satisfactory," he writes, "by the clear results which it
affords, than that of the remains of other animals found in a
fossil state, it is also complicated with greater and more
numerous difficulties. Fossil shells are usually found quite
entire, and retaining all the characters requisite for comparing
them with the specimens contained in collections of natural
history, or represented in the works of naturalists. Even the
skeletons of fishes are found more or less entire, so that the
general forms of their bodies can, for the most part, be
ascertained, and usually, at least, their generic and specific
characters are determinable, as these are mostly drawn from their
solid parts. In quadrupeds, on the contrary, even when their
entire skeletons are found, there is great difficulty in
discovering their distinguishing characters, as these are chiefly
founded upon their hairs and colors and other marks which have
disappeared previous to their incrustation. It is also very rare
to find any fossil skeletons of quadrupeds in any degree
approaching to a complete state, as the strata for the most part
only contain separate bones, scattered confusedly and almost
always broken and reduced to fragments, which are the only means
left to naturalists for ascertaining the species or genera to
which they have belonged.

"Fortunately comparative anatomy, when thoroughly understood,
enables us to surmount all these difficulties, as a careful
application of its principles instructs us in the correspondences
and dissimilarities of the forms of organized bodies of different
kinds, by which each may be rigorously ascertained from almost
every fragment of its various parts and organs.

"Every organized individual forms an entire system of its own,
all the parts of which naturally correspond, and concur to
produce a certain definite purpose, by reciprocal reaction, or by
combining towards the same end. Hence none of these separate
parts can change their forms without a corresponding change in
the other parts of the same animal, and consequently each of
these parts, taken separately, indicates all the other parts to
which it has belonged. Thus, as I have elsewhere shown, if the
viscera of an animal are so organized as only to be fitted for
the digestion of recent flesh, it is also requisite that the jaws
should be so constructed as to fit them for devouring prey; the
claws must be constructed for seizing and tearing it to pieces;
the teeth for cutting and dividing its flesh; the entire system
of the limbs, or organs of motion, for pursuing and overtaking
it; and the organs of sense for discovering it at a distance.
Nature must also have endowed the brain of the animal with
instincts sufficient for concealing itself and for laying plans
to catch its necessary victims. . . . . . . . . .

"To enable the animal to carry off its prey when seized, a
corresponding force is requisite in the muscles which elevate the
head, and this necessarily gives rise to a determinate form of
the vertebrae to which these muscles are attached and of the
occiput into which they are inserted. In order that the teeth of
a carnivorous animal may be able to cut the flesh, they require
to be sharp, more or less so in proportion to the greater or less
quantity of flesh that they have to cut. It is requisite that
their roots should be solid and strong, in proportion to the
quantity and size of the bones which they have to break to
pieces. The whole of these circumstances must necessarily
influence the development and form of all the parts which
contribute to move the jaws. . . . . . . . . .

After these observations, it will be easily seen that similar
conclusions may be drawn with respect to the limbs of carnivorous
animals, which require particular conformations to fit them for
rapidity of motion in general; and that similar considerations
must influence the forms and connections of the vertebrae and
other bones constituting the trunk of the body, to fit them for
flexibility and readiness of motion in all directions. The bones
also of the nose, of the orbit, and of the ears require certain
forms and structures to fit them for giving perfection to the
senses of smell, sight, and hearing, so necessary to animals of
prey. In short, the shape and structure of the teeth regulate the
forms of the condyle, of the shoulder-blade, and of the claws, in
the same manner as the equation of a curve regulates all its
other properties; and as in regard to any particular curve all
its properties may be ascertained by assuming each separate
property as the foundation of a particular equation, in the same
manner a claw, a shoulder-blade, a condyle, a leg or arm bone, or
any other bone separately considered, enables us to discover the
description of teeth to which they have belonged; and so also
reciprocally we may determine the forms of the other bones from
the teeth. Thus commencing our investigations by a careful
survey of any one bone by itself, a person who is sufficiently
master of the laws of organic structure may, as it were,
reconstruct the whole animal to which that bone belonged."[1]

We have already pointed out that no one is quite able to perform
the necromantic feat suggested in the last sentence; but the
exaggeration is pardonable in the enthusiast to whom the
principle meant so much and in whose hands it extended so far.

Of course this entire principle, in its broad outlines, is
something with which every student of anatomy had been familiar
from the time when anatomy was first studied, but the full
expression of the "law of co-ordination," as Cuvier called it,
had never been explicitly made before; and, notwithstanding its
seeming obviousness, the exposition which Cuvier made of it in
the introduction to his classical work on comparative anatomy,
which was published during the first decade of the nineteenth
century, ranks as a great discovery. It is one of those
generalizations which serve as guideposts to other discoveries.


Much the same thing may be said of another generalization
regarding the animal body, which the brilliant young French
physician Marie Francois Bichat made in calling attention to the
fact that each vertebrate organism, including man, has really two
quite different sets of organs--one set under volitional control,
and serving the end of locomotion, the other removed from
volitional control, and serving the ends of the "vital processes"
of digestion, assimilation, and the like. He called these sets of
organs the animal system and the organic system, respectively.
The division thus pointed out was not quite new, for Grimaud,
professor of physiology in the University of Montpellier, had
earlier made what was substantially the same classification of
the functions into "internal or digestive and external or
locomotive"; but it was Bichat's exposition that gave currency to
the idea.

Far more important, however, was another classification which
Bichat put forward in his work on anatomy, published just at the
beginning of the last century. This was the division of all
animal structures into what Bichat called tissues, and the
pointing out that there are really only a few kinds of these in
the body, making up all the diverse organs. Thus muscular organs
form one system; membranous organs another; glandular organs a
third; the vascular mechanism a fourth, and so on. The
distinction is so obvious that it seems rather difficult to
conceive that it could have been overlooked by the earliest
anatomists; but, in point of fact, it is only obvious because now
it has been familiarly taught for almost a century. It had never
been given explicit expression before the time of Bichat, though
it is said that Bichat himself was somewhat indebted for it to
his master, Desault, and to the famous alienist Pinel.

However that may be, it is certain that all subsequent anatomists
have found Bichat's classification of the tissues of the utmost
value in their studies of the animal functions. Subsequent
advances were to show that the distinction between the various
tissues is not really so fundamental as Bichat supposed, but that
takes nothing from the practical value of the famous

It was but a step from this scientific classification of tissues
to a similar classification of the diseases affecting them, and
this was one of the greatest steps towards placing medicine on
the plane of an exact science. This subject of these branches
completely fascinated Bichat, and he exclaimed, enthusiastically:
"Take away some fevers and nervous trouble, and all else belongs
to the kingdom of pathological anatomy." But out of this
enthusiasm came great results. Bichat practised as he preached,
and, believing that it was only possible to understand disease by
observing the symptoms carefully at the bedside, and, if the
disease terminated fatally, by post-mortem examination, he was so
arduous in his pursuit of knowledge that within a period of less
than six months he had made over six hundred autopsies--a record
that has seldom, if ever, been equalled. Nor were his efforts
fruitless, as a single example will suffice to show. By his
examinations he was able to prove that diseases of the chest,
which had formerly been classed under the indefinite name
"peripneumonia," might involve three different structures, the
pleural sac covering the lungs, the lung itself, and the
bronchial tubes, the diseases affecting these organs being known
respectively as pleuritis, pneumonia, and bronchitis, each one
differing from the others as to prognosis and treatment. The
advantage of such an exact classification needs no demonstration.


At the same time when these broad macroscopical distinctions were
being drawn there were other workers who were striving to go even
deeper into the intricacies of the animal mechanism with the aid
of the microscope. This undertaking, however, was beset with
very great optical difficulties, and for a long time little
advance was made upon the work of preceding generations. Two
great optical barriers, known technically as spherical and
chromatic aberration--the one due to a failure of the rays of
light to fall all in one plane when focalized through a lens, the
other due to the dispersive action of the lens in breaking the
white light into prismatic colors--confronted the makers of
microscopic lenses, and seemed all but insuperable. The making of
achromatic lenses for telescopes had been accomplished, it is
true, by Dolland in the previous century, by the union of lenses
of crown glass with those of flint glass, these two materials
having different indices of refraction and dispersion. But, aside
from the mechanical difficulties which arise when the lens is of
the minute dimensions required for use with the microscope, other
perplexities are introduced by the fact that the use of a wide
pencil of light is a desideratum, in order to gain sufficient
illumination when large magnification is to be secured.

In the attempt to overcome those difficulties, the foremost
physical philosophers of the time came to the aid of the best
opticians. Very early in the century, Dr. (afterwards Sir David)
Brewster, the renowned Scotch physicist, suggested that certain
advantages might accrue from the use of such gems as have high
refractive and low dispersive indices, in place of lenses made of
glass. Accordingly lenses were made of diamond, of sapphire, and
so on, and with some measure of success. But in 1812 a much more
important innovation was introduced by Dr. William Hyde
Wollaston, one of the greatest and most versatile, and, since the
death of Cavendish, by far the most eccentric of English natural
philosophers. This was the suggestion to use two plano-convex
lenses, placed at a prescribed distance apart, in lieu of the
single double-convex lens generally used. This combination
largely overcame the spherical aberration, and it gained
immediate fame as the "Wollaston doublet."

To obviate loss of light in such a doublet from increase of
reflecting surfaces, Dr. Brewster suggested filling the
interspace between the two lenses with a cement having the same
index of refraction as the lenses themselves--an improvement of
manifest advantage. An improvement yet more important was made by
Dr. Wollaston himself in the introduction of the diaphragm to
limit the field of vision between the lenses, instead of in front
of the anterior lens. A pair of lenses thus equipped Dr.
Wollaston called the periscopic microscope. Dr. Brewster
suggested that in such a lens the same object might be attained
with greater ease by grinding an equatorial groove about a thick
or globular lens and filling the groove with an opaque cement.
This arrangement found much favor, and came subsequently to be
known as a Coddington lens, though Mr. Coddington laid no claim
to being its inventor.

Sir John Herschel, another of the very great physicists of the
time, also gave attention to the problem of improving the
microscope, and in 1821 he introduced what was called an
aplanatic combination of lenses, in which, as the name implies,
the spherical aberration was largely done away with. It was
thought that the use of this Herschel aplanatic combination as an
eyepiece, combined with the Wollaston doublet for the objective,
came as near perfection as the compound microscope was likely
soon to come. But in reality the instrument thus constructed,
though doubtless superior to any predecessor, was so defective
that for practical purposes the simple microscope, such as the
doublet or the Coddington, was preferable to the more complicated

Many opticians, indeed, quite despaired of ever being able to
make a satisfactory refracting compound microscope, and some of
them had taken up anew Sir Isaac Newton's suggestion in reference
to a reflecting microscope. In particular, Professor Giovanni
Battista Amici, a very famous mathematician and practical
optician of Modena, succeeded in constructing a reflecting
microscope which was said to be superior to any compound
microscope of the time, though the events of the ensuing years
were destined to rob it of all but historical value. For there
were others, fortunately, who did not despair of the
possibilities of the refracting microscope, and their efforts
were destined before long to be crowned with a degree of success
not even dreamed of by any preceding generation.

The man to whom chief credit is due for directing those final
steps that made the compound microscope a practical implement
instead of a scientific toy was the English amateur optician
Joseph Jackson Lister. Combining mathematical knowledge with
mechanical ingenuity, and having the practical aid of the
celebrated optician Tulley, he devised formulae for the
combination of lenses of crown glass with others of flint glass,
so adjusted that the refractive errors of one were corrected or
compensated by the other, with the result of producing lenses of
hitherto unequalled powers of definition; lenses capable of
showing an image highly magnified, yet relatively free from those
distortions and fringes of color that had heretofore been so
disastrous to true interpretation of magnified structures.

Lister had begun his studies of the lens in 1824, but it was not
until 1830 that he contributed to the Royal Society the famous
paper detailing his theories and experiments. Soon after this
various continental opticians who had long been working along
similar lines took the matter up, and their expositions, in
particular that of Amici, introduced the improved compound
microscope to the attention of microscopists everywhere. And it
required but the most casual trial to convince the experienced
observers that a new implement of scientific research had been
placed in their hands which carried them a long step nearer the
observation of the intimate physical processes which lie at the
foundation of vital phenomena. For the physiologist this
perfection of the compound microscope had the same significance
that the, discovery of America had for the fifteenth-century
geographers--it promised a veritable world of utterly novel
revelations. Nor was the fulfilment of that promise long delayed.

Indeed, so numerous and so important were the discoveries now
made in the realm of minute anatomy that the rise of histology to
the rank of an independent science may be said to date from this
period. Hitherto, ever since the discovery of magnifying-glasses,
there had been here and there a man, such as Leuwenhoek or
Malpighi, gifted with exceptional vision, and perhaps unusually
happy in his conjectures, who made important contributions to the
knowledge of the minute structure of organic tissues; but now of
a sudden it became possible for the veriest tyro to confirm or
refute the laborious observations of these pioneers, while the
skilled observer could step easily beyond the barriers of vision
that hitherto were quite impassable. And so, naturally enough,
the physiologists of the fourth decade of the nineteenth century
rushed as eagerly into the new realm of the microscope as, for
example, their successors of to-day are exploring the realm of
the X-ray.

Lister himself, who had become an eager interrogator of the
instrument he had perfected, made many important discoveries, the
most notable being his final settlement of the long-mooted
question as to the true form of the red corpuscles of the human
blood. In reality, as everybody knows nowadays, these are
biconcave disks, but owing to their peculiar figure it is easily
possible to misinterpret the appearances they present when seen
through a poor lens, and though Dr. Thomas Young and various
other observers had come very near the truth regarding them,
unanimity of opinion was possible only after the verdict of the
perfected microscope was given.

These blood corpuscles are so infinitesimal in size that
something like five millions of them are found in each cubic
millimetre of the blood, yet they are isolated particles, each
having, so to speak, its own personality. This, of course, had
been known to microscopists since the days of the earliest
lenses. It had been noticed, too, by here and there an observer,
that certain of the solid tissues seemed to present something of
a granular texture, as if they, too, in their ultimate
constitution, were made up of particles. And now, as better and
better lenses were constructed, this idea gained ground
constantly, though for a time no one saw its full significance.
In the case of vegetable tissues, indeed, the fact that little
particles encased a membranous covering, and called cells, are
the ultimate visible units of structure had long been known. But
it was supposed that animal tissues differed radically from this
construction. The elementary particles of vegetables "were
regarded to a certain extent as individuals which composed the
entire plant, while, on the other hand, no such view was taken of
the elementary parts of animals."


In the year 1833 a further insight into the nature of the
ultimate particles of plants was gained through the observation
of the English microscopist Robert Brown, who, in the course of
his microscopic studies of the epidermis of orchids, discovered
in the cells "an opaque spot," which he named the nucleus.
Doubtless the same "spot" had been seen often enough before by
other observers, but Brown was the first to recognize it as a
component part of the vegetable cell and to give it a name.

"I shall conclude my observations on Orchideae," said Brown,
"with a notice of some points of their general structure, which
chiefly relate to the cellular tissue. In each cell of the
epidermis of a great part of this family, especially of those
with membranous leaves, a single circular areola, generally
somewhat more opaque than, the membrane of the cell, is
observable. This areola, which is more or less distinctly
granular, is slightly convex, and although it seems to be on the
surface is in reality covered by the outer lamina of the cell.
There is no regularity as to its place in the cell; it is not
unfrequently, however, central or nearly so.

"As only one areola belongs to each cell, and as in many cases
where it exists in the common cells of the epidermis, it is also
visible in the cutaneous glands or stomata, and in these is
always double--one being on each side of the limb--it is highly
probable that the cutaneous gland is in all cases composed of two
cells of peculiar form, the line of union being the longitudinal
axis of the disk or pore.

"This areola, or nucleus of the cell as perhaps it might be
termed, is not confined to the epidermis, being also found, not
only in the pubescence of the surface, particularly when jointed,
as in cypripedium, but in many cases in the parenchyma or
internal cells of the tissue, especially when these are free from
the deposition of granular matter.

"In the compressed cells of the epidermis the nucleus is in a
corresponding degree flattened; but in the internal tissue it is
often nearly spherical, more or less firmly adhering to one of
the walls, and projecting into the cavity of the cell. In this
state it may not unfrequently be found. in the substance of the
column and in that of the perianthium.

"The nucleus is manifest also in the tissue of the stigma, where
in accordance with the compression of the utriculi, it has an
intermediate form, being neither so much flattened as in the
epidermis nor so convex as it is in the internal tissue of the

"I may here remark that I am acquainted with one case of apparent
exception to the nucleus being solitary in each utriculus or
cell--namely, in Bletia Tankervilliae. In the utriculi of the
stigma of this plant, I have generally, though not always, found
a second areola apparently on the surface, and composed of much
larger granules than the ordinary nucleus, which is formed of
very minute granular matter, and seems to be deep seated.

"Mr. Bauer has represented the tissue of the stigma, in the
species of Bletia, both before and, as he believes, after
impregnation; and in the latter state the utriculi are marked
with from one to three areolae of similar appearance.

"The nucleus may even be supposed to exist in the pollen of this
family. In the early stages of its formation, at least a minute
areola is of ten visible in the simple grain, and in each of the
constituent parts of cells of the compound grain. But these
areolae may perhaps rather be considered as merely the points of
production of the tubes.

"This nucleus of the cell is not confined to orchideae, but is
equally manifest in many other monocotyledonous families; and I
have even found it, hitherto however in very few cases, in the
epidermis of dicotyledonous plants; though in this primary
division it may perhaps be said to exist in the early stages of
development of the pollen. Among monocotyledons, the orders in
which it is most remarkable are Liliaceae, Hemerocallideae,
Asphodeleae, Irideae, and Commelineae.

"In some plants belonging to this last-mentioned family,
especially in Tradascantia virginica, and several nearly related
species, it is uncommonly distinct, not in the epidermis and in
the jointed hairs of the filaments, but in the tissue of the
stigma, in the cells of the ovulum even before impregnation, and
in all the stages of formation of the grains of pollen, the
evolution of which is so remarkable in tradascantia.

"The few indications of the presence of this nucleus, or areola,
that I have hitherto met with in the publications of botanists
are chiefly in some figures of epidermis, in the recent works of
Meyen and Purkinje, and in one case, in M. Adolphe Broigniart's
memoir on the structure of leaves. But so little importance
seems to be attached to it that the appearance is not always
referred to in the explanations of the figures in which it is
represented. Mr. Bauer, however, who has also figured it in the
utriculi of the stigma of Bletia Tankervilliae has more
particularly noticed it, and seems to consider it as only visible
after impregnation."[2]


That this newly recognized structure must be important in the
economy of the cell was recognized by Brown himself, and by the
celebrated German Meyen, who dealt with it in his work on
vegetable physiology, published not long afterwards; but it
remained for another German, the professor of botany in the
University of Jena, Dr. M. J. Schleiden, to bring the nucleus to
popular attention, and to assert its all-importance in the
economy of the cell.

Schleiden freely acknowledged his indebtedness to Brown for first
knowledge of the nucleus, but he soon carried his studies of that
structure far beyond those of its discoverer. He came to believe
that the nucleus is really the most important portion of the
cell, in that it is the original structure from which the
remainder of the cell is developed. Hence he named it the
cytoblast. He outlined his views in an epochal paper published
in Muller's Archives in 1838, under title of "Beitrage zur
Phytogenesis." This paper is in itself of value, yet the most
important outgrowth of Schleiden's observations of the nucleus
did not spring from his own labors, but from those of a friend to
whom he mentioned his discoveries the year previous to their
publication. This friend was Dr. Theodor Schwann, professor of
physiology in the University of Louvain.

At the moment when these observations were communicated to him
Schwann was puzzling over certain details of animal histology
which he could not clearly explain. His great teacher, Johannes
Muller, had called attention to the strange resemblance to
vegetable cells shown by certain cells of the chorda dorsalis
(the embryonic cord from which the spinal column is developed),
and Schwann himself had discovered a corresponding similarity in
the branchial cartilage of a tadpole. Then, too, the researches
of Friedrich Henle had shown that the particles that make up the
epidermis of animals are very cell-like in appearance. Indeed,
the cell-like character of certain animal tissues had come to be
matter of common note among students of minute anatomy. Schwann
felt that this similarity could not be mere coincidence, but he
had gained no clew to further insight until Schleiden called his
attention to the nucleus. Then at once he reasoned that if there
really is the correspondence between vegetable and animal tissues
that he suspected, and if the nucleus is so important in the
vegetable cell as Schleiden believed, the nucleus should also be
found in the ultimate particles of animal tissues.

Schwann's researches soon showed the entire correctness of this
assumption. A closer study of animal tissues under the microscope
showed, particularly in the case of embryonic tissues, that
"opaque spots" such as Schleiden described are really to be found
there in abundance--forming, indeed, a most characteristic phase
of the structure. The location of these nuclei at comparatively
regular intervals suggested that they are found in definite
compartments of the tissue, as Schleiden had shown to be the case
with vegetables; indeed, the walls that separated such cell-like
compartments one from another were in some cases visible.
Particularly was this found to be the case with embryonic
tissues, and the study of these soon convinced Schwann that his
original surmise had been correct, and that all animal tissues
are in their incipiency composed of particles not unlike the
ultimate particles of vegetables in short, of what the botanists
termed cells. Adopting this name, Schwann propounded what soon
became famous as his cell theory, under title of Mikroskopische
Untersuchungen uber die Ubereinstimmung in der Structur und dent
Wachsthum der Thiere und Pflanzen. So expeditious had been his
work that this book was published early in 1839, only a few
months after the appearance of Schleiden's paper.

As the title suggests, the main idea that actuated Schwann was to
unify vegetable and animal tissues. Accepting cell-structure as
the basis of all vegetable tissues, he sought to show that the
same is true of animal tissues, all the seeming diversities of
fibre being but the alteration and development of what were
originally simple cells. And by cell Schwann meant, as did
Schleiden also, what the word ordinarily implies--a cavity walled
in on all sides. He conceived that the ultimate constituents of
all tissues were really such minute cavities, the most important
part of which was the cell wall, with its associated nucleus. He
knew, indeed, that the cell might be filled with fluid contents,
but he regarded these as relatively subordinate in importance to
the wall itself. This, however, did not apply to the nucleus,
which was supposed to lie against the cell wall and in the
beginning to generate it. Subsequently the wall might grow so
rapidly as to dissociate itself from its contents, thus becoming
a hollow bubble or true cell; but the nucleus, as long as it
lasted, was supposed to continue in contact with the cell wall.
Schleiden had even supposed the nucleus to be a constituent part
of the wall, sometimes lying enclosed between two layers of its
substance, and Schwann quoted this view with seeming approval.
Schwann believed, however, that in the mature cell the nucleus
ceased to be functional and disappeared.

The main thesis as to the similarity of development of vegetable
and animal tissues and the cellular nature of the ultimate
constitution of both was supported by a mass of carefully
gathered evidence which a multitude of microscopists at once
confirmed, so Schwann's work became a classic almost from the
moment of its publication. Of course various other workers at
once disputed Schwann's claim to priority of discovery, in
particular the English microscopist Valentin, who asserted, not
without some show of justice, that he was working closely along
the same lines. Put so, for that matter, were numerous others,
as Henle, Turpin, Du-mortier, Purkinje, and Muller, all of whom
Schwann himself had quoted. Moreover, there were various
physiologists who earlier than any of these had foreshadowed the
cell theory--notably Kaspar Friedrich Wolff, towards the close of
the previous century, and Treviranus about 1807, But, as we have
seen in so many other departments of science, it is one thing to
foreshadow a discovery, it is quite another to give it full
expression and make it germinal of other discoveries. And when
Schwann put forward the explicit claim that "there is one
universal principle of development for the elementary parts, of
organisms, however different, and this principle is the formation
of cells," he enunciated a doctrine which was for all practical
purposes absolutely new and opened up a novel field for the
microscopist to enter. A most important era in physiology dates
from the publication of his book in 1839.


That Schwann should have gone to embryonic tissues for the
establishment of his ideas was no doubt due very largely to the
influence of the great Russian Karl Ernst von Baer, who about ten
years earlier had published the first part of his celebrated work
on embryology, and whose ideas were rapidly gaining ground,
thanks largely to the advocacy of a few men, notably Johannes
Muller, in Germany, and William B. Carpenter, in England, and to
the fact that the improved microscope had made minute anatomy
popular. Schwann's researches made it plain that the best field
for the study of the animal cell is here, and a host of explorers
entered the field. The result of their observations was, in the
main, to confirm the claims of Schwann as to the universal
prevalence of the cell. The long-current idea that animal tissues
grow only as a sort of deposit from the blood-vessels was now
discarded, and the fact of so-called plantlike growth of animal
cells, for which Schwann contended, was universally accepted. Yet
the full measure of the affinity between the two classes of cells
was not for some time generally apprehended.

Indeed, since the substance that composes the cell walls of
plants is manifestly very different from the limiting membrane of
the animal cell, it was natural, so long as the, wall was
considered the most essential part of the structure, that the
divergence between the two classes of cells should seem very
pronounced. And for a time this was the conception of the matter
that was uniformly accepted. But as time went on many observers
had their attention called to the peculiar characteristics of the
contents of the cell, and were led to ask themselves whether
these might not be more important than had been supposed. In
particular, Dr. Hugo von Mohl, professor of botany in the
University of Tubingen, in the course of his exhaustive studies
of the vegetable cell, was impressed with the peculiar and
characteristic appearance of the cell contents. He observed
universally within the cell "an opaque, viscid fluid, having
granules intermingled in it," which made up the main substance of
the cell, and which particularly impressed him because under
certain conditions it could be seen to be actively in motion, its
parts separated into filamentous streams.

Von Mohl called attention to the fact that this motion of the
cell contents had been observed as long ago as 1774 by
Bonaventura Corti, and rediscovered in 1807 by Treviranus, and
that these observers had described the phenomenon under the "most
unsuitable name of 'rotation of the cell sap.' Von Mohl
recognized that the streaming substance was something quite
different from sap. He asserted that the nucleus of the cell lies
within this substance and not attached to the cell wall as
Schleiden had contended. He saw, too, that the chlorophyl
granules, and all other of the cell contents, are incorporated
with the "opaque, viscid fluid," and in 1846 he had become so
impressed with the importance of this universal cell substance
that be gave it the name of protoplasm. Yet in so doing he had no
intention of subordinating the cell wall. The fact that Payen, in
1844, had demonstrated that the cell walls of all vegetables,
high or low, are composed largely of one substance, cellulose,
tended to strengthen the position of the cell wall as the really
essential structure, of which the protoplasmic contents were only
subsidiary products.

Meantime, however, the students of animal histology were more and
more impressed with the seeming preponderance of cell contents
over cell walls in the tissues they studied. They, too, found
the cell to be filled with a viscid, slimy fluid capable of
motion. To this Dujardin gave the name of sarcode. Presently it
came to be known, through the labors of Kolliker, Nageli,
Bischoff, and various others, that there are numerous lower forms
of animal life which seem to be composed of this sarcode, without
any cell wall whatever. The same thing seemed to be true of
certain cells of higher organisms, as the blood corpuscles.
Particularly in the case of cells that change their shape
markedly, moving about in consequence of the streaming of their
sarcode, did it seem certain that no cell wall is present, or
that, if present, its role must be insignificant.

And so histologists came to question whether, after all, the cell
contents rather than the enclosing wall must not be the really
essential structure, and the weight of increasing observations
finally left no escape from the conclusion that such is really
the case. But attention being thus focalized on the cell
contents, it was at once apparent that there is a far closer
similarity between the ultimate particles of vegetables and those
of animals than had been supposed. Cellulose and animal membrane
being now regarded as more by-products, the way was clear for the
recognition of the fact that vegetable protoplasm and animal
sarcode are marvellously similar in appearance and general
properties. The closer the observation the more striking seemed
this similarity; and finally, about 1860, it was demonstrated by
Heinrich de Bary and by Max Schultze that the two are to all
intents and purposes identical. Even earlier Remak had reached a
similar conclusion, and applied Von Mohl's word protoplasm to
animal cell contents, and now this application soon became
universal. Thenceforth this protoplasm was to assume the utmost
importance in the physiological world, being recognized as the
universal "physical basis of life," vegetable and animal alike.
This amounted to the logical extension and culmination of
Schwann's doctrine as to the similarity of development of the two
animate kingdoms. Yet at the, same time it was in effect the
banishment of the cell that Schwann had defined. The word cell
was retained, it is true, but it no longer signified a minute
cavity. It now implied, as Schultze defined it, "a small mass of
protoplasm endowed with the attributes of life." This definition
was destined presently to meet with yet another modification, as
we shall see; but the conception of the protoplasmic mass as the
essential ultimate structure, which might or might not surround
itself with a protective covering, was a permanent addition to
physiological knowledge. The earlier idea had, in effect,
declared the shell the most important part of the egg; this
developed view assigned to the yolk its true position.

In one other important regard the theory of Schleiden and Schwann
now became modified. This referred to the origin of the cell.
Schwann had regarded cell growth as a kind of crystallization,
beginning with the deposit of a nucleus about a granule in the
intercellular substance--the cytoblastema, as Schleiden called
it. But Von Mohl, as early as 1835, had called attention to the
formation of new vegetable cells through the division of a
pre-existing cell. Ehrenberg, another high authority of the time,
contended that no such division occurs, and the matter was still
in dispute when Schleiden came forward with his discovery of
so-called free cell-formation within the parent cell, and this
for a long time diverted attention from the process of division
which Von Mohl had described. All manner of schemes of
cell-formation were put forward during the ensuing years by a
multitude of observers, and gained currency notwithstanding Von
Mohl's reiterated contention that there are really but two ways
in which the formation of new cells takes place--namely, "first,
through division of older cells; secondly, through the formation
of secondary cells lying free in the cavity of a cell."

But gradually the researches of such accurate observers as Unger,
Nageli, Kolliker, Reichart, and Remak tended to confirm the
opinion of Von Mohl that cells spring only from cells, and
finally Rudolf Virchow brought the matter to demonstration about
1860. His Omnis cellula e cellula became from that time one of
the accepted data of physiology. This was supplemented a little
later by Fleming's Omnis nucleus e nucleo, when still more
refined methods of observation had shown that the part of the
cell which always first undergoes change preparatory to new
cell-formation is the all-essential nucleus. Thus the nucleus was
restored to the important position which Schwann and Schleiden
had given it, but with greatly altered significance. Instead of
being a structure generated de novo from non-cellular substance,
and disappearing as soon as its function of cell-formation was
accomplished, the nucleus was now known as the central and
permanent feature of every cell, indestructible while the cell
lives, itself the division-product of a pre-existing nucleus, and
the parent, by division of its substance, of other generations of
nuclei. The word cell received a final definition as "a small
mass of protoplasm supplied with a nucleus."

In this widened and culminating general view of the cell theory
it became clear that every animate organism, animal or vegetable,
is but a cluster of nucleated cells, all of which, in each
individual case, are the direct descendants of a single
primordial cell of the ovum. In the developed individuals of
higher organisms the successive generations of cells become
marvellously diversified in form and in specific functions; there
is a wonderful division of labor, special functions being chiefly
relegated to definite groups of cells; but from first to last
there is no function developed that is not present, in a
primitive way, in every cell, however isolated; nor does the
developed cell, however specialized, ever forget altogether any
one of its primordial functions or capacities. All physiology,
then, properly interpreted, becomes merely a study of cellular
activities; and the development of the cell theory takes its
place as the great central generalization in physiology of the
nineteenth century. Something of the later developments of this
theory we shall see in another connection.


Just at the time when the microscope was opening up the paths
that were to lead to the wonderful cell theory, another novel
line of interrogation of the living organism was being put
forward by a different set of observers. Two great schools of
physiological chemistry had arisen--one under guidance of Liebig
and Wohler, in Germany, the other dominated by the great French
master Jean Baptiste Dumas. Liebig had at one time contemplated
the study of medicine, and Dumas had achieved distinction in
connection with Prevost, at Geneva, in the field of pure
physiology before he turned his attention especially to
chemistry. Both these masters, therefore, and Wohler as well,
found absorbing interest in those phases of chemistry that have
to do with the functions of living tissues; and it was largely
through their efforts and the labors of their followers that the
prevalent idea that vital processes are dominated by unique laws
was discarded and physiology was brought within the recognized
province of the chemist. So at about the time when the microscope
had taught that the cell is the really essential structure of the
living organism, the chemists had come to understand that every
function of the organism is really the expression of a chemical
change--that each cell is, in short, a miniature chemical
laboratory. And it was this combined point of view of anatomist
and chemist, this union of hitherto dissociated forces, that made
possible the inroads into the unexplored fields of physiology
that were effected towards the middle of the nineteenth century.

One of the first subjects reinvestigated and brought to proximal
solution was the long-mooted question of the digestion of foods.
Spallanzani and Hunter had shown in the previous century that
digestion is in some sort a solution of foods; but little advance
was made upon their work until 1824, when Prout detected the
presence of hydrochloric acid in the gastric juice. A decade
later Sprott and Boyd detected the existence of peculiar glands
in the gastric mucous membrane; and Cagniard la Tour and Schwann
independently discovered that the really active principle of the
gastric juice is a substance which was named pepsin, and which
was shown by Schwann to be active in the presence of hydrochloric

Almost coincidently, in 1836, it was discovered by Purkinje and
Pappenheim that another organ than the stomach--namely, the
pancreas--has a share in digestion, and in the course of the
ensuing decade it came to be known, through the efforts of
Eberle, Valentin, and Claude Bernard, that this organ is
all-important in the digestion of starchy and fatty foods. It was
found, too, that the liver and the intestinal glands have each an
important share in the work of preparing foods for absorption, as
also has the saliva--that, in short, a coalition of forces is
necessary for the digestion of all ordinary foods taken into the

And the chemists soon discovered that in each one of the
essential digestive juices there is at least one substance having
certain resemblances to pepsin, though acting on different kinds
of food. The point of resemblance between all these essential
digestive agents is that each has the remarkable property of
acting on relatively enormous quantities of the substance which
it can digest without itself being destroyed or apparently even
altered. In virtue of this strange property, pepsin and the
allied substances were spoken of as ferments, but more recently
it is customary to distinguish them from such organized ferments
as yeast by designating them enzymes. The isolation of these
enzymes, and an appreciation of their mode of action, mark a long
step towards the solution of the riddle of digestion, but it must
be added that we are still quite in the dark as to the real
ultimate nature of their strange activity.

In a comprehensive view, the digestive organs, taken as a whole,
are a gateway between the outside world and the more intimate
cells of the organism. Another equally important gateway is
furnished by the lungs, and here also there was much obscurity
about the exact method of functioning at the time of the revival
of physiological chemistry. That oxygen is consumed and carbonic
acid given off during respiration the chemists of the age of
Priestley and Lavoisier had indeed made clear, but the mistaken
notion prevailed that it was in the lungs themselves that the
important burning of fuel occurs, of which carbonic acid is a
chief product. But now that attention had been called to the
importance of the ultimate cell, this misconception could not
long hold its ground, and as early as 1842 Liebig, in the course
of his studies of animal heat, became convinced that it is not in
the lungs, but in the ultimate tissues to which they are
tributary, that the true consumption of fuel takes place.
Reviving Lavoisier's idea, with modifications and additions,
Liebig contended, and in the face of opposition finally
demonstrated, that the source of animal heat is really the
consumption of the fuel taken in through the stomach and the
lungs. He showed that all the activities of life are really the
product of energy liberated solely through destructive processes,
amounting, broadly speaking, to combustion occurring in the
ultimate cells of the organism. Here is his argument:


"The oxygen taken into the system is taken out again in the same
forms, whether in summer or in winter; hence we expire more
carbon in cold weather, and when the barometer is high, than we
do in warm weather; and we must consume more or less carbon in
our food in the same proportion; in Sweden more than in Sicily;
and in our more temperate climate a full eighth more in winter
than in summer.

"Even when we consume equal weights of food in cold and warm
countries, infinite wisdom has so arranged that the articles of
food in different climates are most unequal in the proportion of
carbon they contain. The fruits on which the natives of the South
prefer to feed do not in the fresh state contain more than twelve
per cent. of carbon, while the blubber and train-oil used by the
inhabitants of the arctic regions contain from sixty-six to
eighty per cent. of carbon.

"It is no difficult matter, in warm climates, to study moderation
in eating, and men can bear hunger for a long time under the
equator; but cold and hunger united very soon exhaust the body.

"The mutual action between the elements of the food and the
oxygen conveyed by the circulation of the blood to every part of
the body is the source of animal heat.

"All living creatures whose existence depends on the absorption
of oxygen possess within themselves a source of heat independent
of surrounding objects.

"This truth applies to all animals, and extends besides to the
germination of seeds, to the flowering of plants, and to the
maturation of fruits. It is only in those parts of the body to
which arterial blood, and with it the oxygen absorbed in
respiration, is conveyed that heat is produced. Hair, wool, or
feathers do not possess an elevated temperature. This high
temperature of the animal body, or, as it may be called,
disengagement of heat, is uniformly and under all circumstances
the result of the combination of combustible substance with

"In whatever way carbon may combine with oxygen, the act of
combination cannot take place without the disengagement of heat.
It is a matter of indifference whether the combination takes
place rapidly or slowly, at a high or at a low temperature; the
amount of heat liberated is a constant quantity. The carbon of
the food, which is converted into carbonic acid within the body,
must give out exactly as much heat as if it had been directly
burned in the air or in oxygen gas; the only difference is that
the amount of heat produced is diffused over unequal times. In
oxygen the combustion is more rapid and the heat more intense; in
air it is slower, the temperature is not so high, but it
continues longer.

"It is obvious that the amount of heat liberated must increase or
diminish with the amount of oxygen introduced in equal times by
respiration. Those animals which respire frequently, and
consequently consume much oxygen, possess a higher temperature
than others which, with a body of equal size to be heated, take
into the system less oxygen. The temperature of a child (102
degrees) is higher than that of an adult (99.5 degrees). That of
birds (104 to 105.4 degrees) is higher than that of quadrupeds
(98.5 to 100.4 degrees), or than that of fishes or amphibia,
whose proper temperature is from 3.7 to 2.6 degrees higher than
that of the medium in which they live. All animals, strictly
speaking, are warm-blooded; but in those only which possess lungs
is the temperature of the body independent of the surrounding

"The most trustworthy observations prove that in all climates, in
the temperate zones as well as at the equator or the poles, the
temperature of the body in man, and of what are commonly called
warm-blooded animals, is invariably the same; yet how different
are the circumstances in which they live.

"The animal body is a heated mass, which bears the same relation
to surrounding objects as any other heated mass. It receives heat
when the surrounding objects are hotter, it loses heat when they
are colder than itself. We know that the rapidity of cooling
increases with the difference between the heated body and that of
the surrounding medium--that is, the colder the surrounding
medium the shorter the time required for the cooling of the
heated body. How unequal, then, must be the loss of heat of a man
at Palermo, where the actual temperature is nearly equal to that
of the body, and in the polar regions, where the external
temperature is from 70 to 90 degrees lower.

"Yet notwithstanding this extremely unequal loss of heat,
experience has shown that the blood of an inhabitant of the
arctic circle has a temperature as high as that of the native of
the South, who lives in so different a medium. This fact, when
its true significance is perceived, proves that the heat given
off to the surrounding medium is restored within the body with
great rapidity. This compensation takes place more rapidly in
winter than in summer, at the pole than at the equator.

"Now in different climates the quantity of oxygen introduced into
the system of respiration, as has been already shown, varies
according to the temperature of the external air; the quantity of
inspired oxygen increases with the loss of heat by external
cooling, and the quantity of carbon or hydrogen necessary to
combine with this oxygen must be increased in like ratio. It is
evident that the supply of heat lost by cooling is effected by
the mutual action of the elements of the food and the inspired
oxygen, which combine together. To make use of a familiar, but
not on that account a less just illustration, the animal body
acts, in this respect, as a furnace, which we supply with fuel.
It signifies nothing what intermediate forms food may assume,
what changes it may undergo in the body, the last change is
uniformly the conversion of carbon into carbonic acid and of its
hydrogen into water; the unassimilated nitrogen of the food,
along with the unburned or unoxidized carbon, is expelled in the
excretions. In order to keep up in a furnace a constant
temperature, we must vary the supply of fuel according to the
external temperature--that is, according to the supply of oxygen.

"In the animal body the food is the fuel; with a proper supply of
oxygen we obtain the heat given out during its oxidation or


Further researches showed that the carriers of oxygen, from the
time of its absorption in the lungs till its liberation in the
ultimate tissues, are the red corpuscles, whose function had been
supposed to be the mechanical one of mixing of the blood. It
transpired that the red corpuscles are composed chiefly of a
substance which Kuhne first isolated in crystalline form in 1865,
and which was named haemoglobin--a substance which has a
marvellous affinity for oxygen, seizing on it eagerly at the
lungs vet giving it up with equal readiness when coursing among
the remote cells of the body. When freighted with oxygen it
becomes oxyhaemoglobin and is red in color; when freed from its
oxygen it takes a purple hue; hence the widely different
appearance of arterial and venous blood, which so puzzled the
early physiologists.

This proof of the vitally important role played by the red-blood
corpuscles led, naturally, to renewed studies of these
infinitesimal bodies. It was found that they may vary greatly in
number at different periods in the life of the same individual,
proving that they may be both developed and destroyed in the
adult organism. Indeed, extended observations left no reason to
doubt that the process of corpuscle formation and destruction may
be a perfectly normal one--that, in short, every red-blood
corpuscle runs its course and dies like any more elaborate
organism. They are formed constantly in the red marrow of bones,
and are destroyed in the liver, where they contribute to the
formation of the coloring matter of the bile. Whether there are
other seats of such manufacture and destruction of the corpuscles
is not yet fully determined. Nor are histologists agreed as to
whether the red-blood corpuscles themselves are to be regarded as
true cells, or merely as fragments of cells budded out from a
true cell for a special purpose; but in either case there is not
the slightest doubt that the chief function of the red corpuscle
is to carry oxygen.

If the oxygen is taken to the ultimate cells before combining
with the combustibles it is to consume, it goes without saying
that these combustibles themselves must be carried there also.
Nor could it be in doubt that the chiefest of these ultimate
tissues, as regards, quantity of fuel required, are the muscles.
A general and comprehensive view of the organism includes, then,
digestive apparatus and lungs as the channels of fuel-supply;
blood and lymph channels as the transportation system; and muscle
cells, united into muscle fibres, as the consumption furnaces,
where fuel is burned and energy transformed and rendered
available for the purposes of the organism, supplemented by a set
of excretory organs, through which the waste products--the
ashes--are eliminated from the system.

But there remain, broadly speaking, two other sets of organs
whose size demonstrates their importance in the economy of the
organism, yet whose functions are not accounted for in this
synopsis. These are those glandlike organs, such as the spleen,
which have no ducts and produce no visible secretions, and the
nervous mechanism, whose central organs are the brain and spinal
cord. What offices do these sets of organs perform in the great
labor-specializing aggregation of cells which we call a living

As regards the ductless glands, the first clew to their function
was given when the great Frenchman Claude Bernard (the man of
whom his admirers loved to say, "He is not a physiologist merely;
he is physiology itself") discovered what is spoken of as the
glycogenic function of the liver. The liver itself, indeed, is
not a ductless organ, but the quantity of its biliary output
seems utterly disproportionate to its enormous size, particularly
when it is considered that in the case of the human species the
liver contains normally about one-fifth of all the blood in the
entire body. Bernard discovered that the blood undergoes a change
of composition in passing through the liver. The liver cells
(the peculiar forms of which had been described by Purkinje,
Henle, and Dutrochet about 1838) have the power to convert
certain of the substances that come to them into a starchlike
compound called glycogen, and to store this substance away till
it is needed by the organism. This capacity of the liver cells
is quite independent of the bile-making power of the same cells;
hence the discovery of this glycogenic function showed that an
organ may have more than one pronounced and important specific
function. But its chief importance was in giving a clew to those
intermediate processes between digestion and final assimilation
that are now known to be of such vital significance in the
economy of the organism.

In the forty odd years that have elapsed since this pioneer
observation of Bernard, numerous facts have come to light showing
the extreme importance of such intermediate alterations of
food-supplies in the blood as that performed by the liver. It has

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