This page contains affiliate links. As Amazon Associates we earn from qualifying purchases.
  • 1897
Buy it on Amazon FREE Audible 30 days

passes down in front of the spine to the pelvis, where it divides into two main branches, which supply the pelvis and the lower limbs.

The descending aorta, while passing downwards, gives off arteries to the different tissues and organs. Of these branches the chief are the coeliac artery, which subdivides into three great branches,–one each to supply the stomach, the liver, and the spleen; then the renal arteries, one to each kidney; and next two others, the mesenteric arteries, to the intestines. The aorta at last divides into two main branches, the common iliac arteries, which, by their subdivisions, furnish the arterial vessels for the pelvis and the lower limbs.

[Illustration: Fig. 72.–Left Cavities of the Heart.

A, B, right pulmonary veins;
with S, openings of the veins;
E, D, C, aortic valves;
R, aorta;
P, pulmonary artery;
O, pulmonic valves;
H, mitral valve;
K, columnæ carnoeæ;
M, right ventricular cavity;
N, interventricular septum.

The flow of blood in the arteries is caused by the muscular force of the heart, aided by the elastic tissues and muscular fibers of the arterial walls, and to a certain extent by the muscles themselves. Most of the great arterial trunks lie deep in the fleshy parts of the body; but their branches are so numerous and become so minute that, with a few exceptions, they penetrate all the tissues of the body,–so much so, that the point of the finest needle cannot be thrust into the flesh anywhere without wounding one or more little arteries and thus drawing blood.

188. The Veins. The veins are the blood-vessels which carry the impure blood from the various tissues of the body to the heart. They begin in the minute capillaries at the extremities of the four limbs, and everywhere throughout the body, and passing onwards toward the heart, receive constantly fresh accessions on the way from myriad other veins bringing blood from other wayside capillaries, till the central veins gradually unite into larger and larger vessels until at length they form the two great vessels which open into the right auricle of the heart.

These two great venous trunks are the inferior vena cava, bringing the blood from the trunk and the lower limbs, and the superior vena cava, bringing the blood from the head and the upper limbs. These two large trunks meet as they enter the right auricle. The four pulmonary veins, as we have learned, carry the arterial blood from the lungs to the left auricle.

[Illustration: Fig. 73.

A, part of a vein laid open, with two pairs of valves; B, longitudinal section of a vein, showing the valves closed. ]

A large vein generally accompanies its corresponding artery, but most veins lie near the surface of the body, just beneath the skin. They may be easily seen under the skin of the hand and forearm, especially in aged persons. If the arm of a young person is allowed to hang down a few moments, and then tightly bandaged above the elbow to retard the return of the blood, the veins become large and prominent.

The walls of the larger veins, unlike arteries, contain but little of either elastic or muscular tissue; hence they are thin, and when empty collapse. The inner surfaces of many of the veins are supplied with pouch-like folds, or pockets, which act as valves to impede the backward flow of the blood, while they do not obstruct blood flowing forward toward the heart. These valves can be shown by letting the forearm hang down, and sliding the finger upwards over the veins (Fig. 73).

The veins have no force-pump, like the arteries, to propel their contents towards their destination. The onward flow of the blood in them is due to various causes, the chief being the pressure behind of the blood pumped into the capillaries. Then as the pocket-like valves prevent the backward flow of the blood, the pressure of the various muscles of the body urges along the blood, and thus promotes the onward flow.

The forces which drive the blood through the arteries are sufficient to carry the blood on through the capillaries. It is calculated that the onward flow in the capillaries is about 1/50 to 1/33 of an inch in a second, while in the arteries the blood current flows about 16 inches in a second, and in the great veins about 4 inches every second.

[Illustration: Fig. 74.–The Structure of Capillaries.

Capillaries of various sizes, showing cells with nuclei]

189. The Capillaries. The capillaries are the minute, hair-like tubes, with very thin walls, which form the connection between the ending of the finest arteries and the beginning of the smallest veins. They are distributed through every tissue of the body, except the epidermis and its products, the epithelium, the cartilages, and the substance of the teeth. In fact, the capillaries form a network of the tiniest blood-vessels, so minute as to be quite invisible, at least one-fourth smaller than the finest line visible to the naked eye.

The capillaries serve as a medium to transmit the blood from the arteries to the veins; and it is through them that the blood brings nourishment to the surrounding tissues. In brief, we may regard the whole body as consisting of countless groups of little islands surrounded by ever-flowing streams of blood. The walls of the capillaries are of the most delicate structure, consisting of a single layer of cells loosely connected. Thus there is allowed the most free interchange between the blood and the tissues, through the medium of the lymph.

The number of the capillaries is inconceivable. Those in the lungs alone, placed in a continuous line, would reach thousands of miles. The thin walls of the capillaries are admirably adapted for the important interchanges that take place between the blood and the tissues.

190. The Circulation of the Blood. It is now well to study the circulation as a whole, tracing the course of the blood from a certain point until it returns to the same point. We may conveniently begin with the portion of blood contained at any moment in the right auricle. The superior and inferior venæ cavæ are busily filling the auricle with dark, impure blood. When it is full, it contracts. The passage leading to the right ventricle lies open, and through it the blood pours till the ventricle is full. Instantly this begins, in its turn, to contract. The tricuspid valve at once closes, and blocks the way backward. The blood is now forced through the open semilunar valves into the pulmonary artery.

The pulmonary artery, bringing venous blood, by its alternate expansion and recoil, draws the blood along until it reaches the pulmonary capillaries. These tiny tubes surround the air cells of the lungs, and here an exchange takes place. The impure, venous blood here gives up its _débris_ in the shape of carbon dioxid and water, and in return takes up a large amount of oxygen. Thus the blood brought to the lungs by the pulmonary arteries leaves the lungs entirely different in character and appearance. This part of the circulation is often called the lesser or pulmonic circulation.

The four pulmonary veins bring back bright, scarlet blood, and pour it into the left auricle of the heart, whence it passes through the mitral valve into the left ventricle. As soon as the left ventricle is full, it contracts. The mitral valve instantly closes and blocks the passage backward into the auricle; the blood, having no other way open, is forced through the semilunar valves into the aorta. Now red in color from its fresh oxygen, and laden with nutritive materials, it is distributed by the arteries to the various tissues of the body. Here it gives up its oxygen, and certain nutritive materials to build up the tissues, and receives certain products of waste, and, changed to a purple color, passes from the capillaries into the veins.

[Illustration: Fig. 75.–Diagram illustrating the Circulation.

1, right auricle;
2, left auricle;
3, right ventricle;
4, left ventricle;
5, vena cava superior;
6, vena cava inferior;
7, pulmonary arteries;
8, lungs;
9, pulmonary veins;
10, aorta;
11, alimentary canal;
12, liver;
13, hepatic artery;
14, portal vein;
15, hepatic vein.

All the veins of the body, except those from the lungs and the heart itself, unite into two large veins, as already described, which pour their contents into the right auricle of the heart, and thus the grand round of circulation is continually maintained. This is called the systemic circulation. The whole circuit of the blood is thus divided into two portions, very distinct from each other.

191. The Portal Circulation. A certain part of the systemic or greater circulation is often called the portal circulation, which consists of the flow of the blood from the abdominal viscera through the portal vein and liver to the hepatic vein. The blood brought to the capillaries of the stomach, intestines, spleen, and pancreas is gathered into veins which unite into a single trunk called the portal vein. The blood, thus laden with certain products of digestion, is carried to the liver by the portal vein, mingling with that supplied to the capillaries of the same organ by the hepatic artery. From these capillaries the blood is carried by small veins which unite into a large trunk, the hepatic vein, which opens into the inferior vena cava. The portal circulation is thus not an independent system, but forms a kind of loop on the systemic circulation.

The lymph-current is in a sense a slow and stagnant side stream of the blood circulation; for substances are constantly passing from the blood-vessels into the lymph spaces, and returning, although after a comparatively long interval, into the blood by the great lymphatic trunks.

Experiment 90. _To illustrate the action of the heart, and how it pumps the blood in only one direction_. Take a Davidson or Household rubber syringe. Sink the suction end into water, and press the bulb. As you let the bulb expand, it fills with water; as you press it again, a valve prevents the water from flowing back, and it is driven out in a jet along the other pipe. The suction pipe represents the veins; the bulb, the heart; and the tube end, out of which the water flows, the arteries.

[NOTE. The heart is not nourished by the blood which passes through it. The muscular substance of the heart itself is supplied with nourishment by two little arteries called the _coronary arteries_, which start from the aorta just above two of the semilunar valves. The blood is returned to the right auricle (not to either of the venæ cavæ) by the _coronary vein_.]

The longest route a portion of blood may take from the moment it leaves the left ventricle to the moment it returns to it, is through the portal circulation. The shortest possible route is through the substance of the heart itself. The mean time which the blood requires to make a complete circuit is about 23 seconds.

192. The Rhythmic Action of the Heart. To maintain a steady flow of blood throughout the body the action of the heart must be regular and methodical. The heart does not contract as a whole. The two auricles contract at the same time, and this is followed at once by the contraction of the two ventricles. While the ventricles are contracting, the auricles begin to relax, and after the ventricles contract they also relax. Now comes a pause, or rest, after which the auricles and ventricles contract again in the same order as before, and their contractions are followed by the same pause as before. These contractions and relaxations of the various parts of the heart follow one another so regularly that the result is called the rhythmic action of the heart.

The average number of beats of the heart, under normal conditions, is from 65 to 75 per minute. Now the time occupied from the instant the auricles begin to contract until after the contraction of the ventricles and the pause, is less than a second. Of this time one-fifth is occupied by the contraction of the auricles, two-fifths by the contraction of the ventricles, and the time during which the whole heart is at rest is two-fifths of the period.

193. Impulse and Sounds of the Heart. The rhythmic action of the heart is attended with various occurrences worthy of note. If the hand be laid flat over the chest wall on the left, between the fifth and sixth ribs, the heart will be felt beating. This movement is known as the beat or impulse of the heart, and can be both seen and felt on the left side. The heart-beat is unusually strong during active bodily exertion, and under mental excitement.

The impulse of the heart is due to the striking of the lower, tense part of the ventricles–the apex of the heart–against the chest wall at the moment of their vigorous contraction. It is important for the physician to know the exact place where the heart-beat should be felt, for the heart may be displaced by disease, and its impulse would indicate its new position.

Sounds also accompany the heart’s action. If the ear be applied over the region of the heart, two distinct sounds will be heard following one another with perfect regularity. Their character may be tolerably imitated by pronouncing the syllables _lubb_, _dÅ­p_. One sound is heard immediately after the other, then there is a pause, then come the two sounds again. The first is a dull, muffled sound, known as the “first sound,” followed at once by a short and sharper sound, known as the “second sound” of the heart.

The precise cause of the first sound is still doubtful, but it is made at the moment the ventricles contract. The second sound is, without doubt, caused by the sudden closure of the semilunar valves of the pulmonary artery and the aorta, at the moment when the contraction of the ventricles is completed.

[Illustration: Fig. 76.–Muscular Fibers of the Ventricles.

A, superficial fibers common to both ventricles; B, fibers of the left ventricle;
C, deep fibers passing upwards toward the base of the heart; D, fibers penetrating the left ventricle ]

The sounds of the heart are modified or masked by blowing “murmurs” when the cardiac orifices or valves are roughened, dilated, or otherwise affected as the result of disease. Hence these new sounds may often afford indications of the greatest importance to physicians in the diagnosis of heart-disease.

194. The Nervous Control of the Heart. The regular, rhythmic movement of the heart is maintained by the action of certain nerves. In various places in the substance of the heart are masses of nerve matter, called ganglia. From these ganglia there proceed, at regular intervals, discharges of nerve energy, some of which excite movement, while others seem to restrain it. The heart would quickly become exhausted if the exciting ganglia had it all their own way, while it would stand still if the restraining ganglia had full sway. The influence of one, however, modifies the other, and the result is a moderate and regular activity of the heart.

The heart is also subject to other nerve influences, but from outside of itself. Two nerves are connected with the heart, the pneumogastric and the sympathetic (secs. 271 and 265). The former appears to be connected with the restraining ganglia; the latter with the exciting ganglia. Thus, if a person were the subject of some emotion which caused fainting, the explanation would be that the impression had been conveyed to the brain, and from the brain to the heart by the pneumogastric nerves. The result would be that the heart for an instant ceases to beat. Death would be the result if the nerve influence were so great as to restrain the movements of the heart for any appreciable time.

Again, if the person were the subject of some emotion by which the heart were beating faster than usual, it would mean that there was sent from the brain to the heart by the sympathetic nerves the impression which stimulated it to increased activity.

195. The Nervous Control of the Blood-vessels. The tone and caliber of the blood-vessels are controlled by certain vaso-motor nerves, which are distributed among the muscular fibers of the walls. These nerves are governed from a center in the medulla oblongata, a part of the brain (sec. 270). If the nerves are stimulated more than usual, the muscular walls contract, and the quantity of the blood flowing through them and the supply to the part are diminished. Again, if the stimulus is less than usual, the vessels dilate, and the supply to the part is increased.

Now the vaso-motor center may be excited to increased activity by influences reaching it from various parts of the body, or even from the brain itself. As a result, the nerves are stimulated, and the vessels contract. Again, the normal influence of the vaso-motor center may be suspended for a time by what is known as the inhibitory or restraining effect. The result is that the tone of the blood-vessels becomes diminished, and their channels widen.

The effect of this power of the nervous system is to give it a certain control over the circulation in particular parts. Thus, though the force of the heart and the general average blood-pressure remain the same, the state of the circulation may be very different in different parts of the body. The importance of this local control over the circulation is of the utmost significance. Thus an organ at work needs to be more richly supplied with blood than when at rest. For example, when the salivary glands need to secrete saliva, and the stomach to pour out gastric juice, the arteries that supply these organs are dilated, and so the parts are flushed with an extra supply of blood, and thus are aroused to greater activity.

Again, the ordinary supply of blood to a part may be lessened, so that the organ is reduced to a state of inactivity, as occurs in the case of the brain during sleep. We have in the act of blushing a visible example of sudden enlargement of the smaller arteries of the face and neck, called forth by some mental emotion which acts on the vaso-motor center and diminishes its activity. The reverse condition occurs in the act of turning pale. Then the result of the mental emotion is to cause the vaso-motor nerves to exercise a more powerful control over the capillaries, thereby closing them, and thus shutting off the flow of blood.

Experiment 91. Hold up the ear of a white rabbit against the light while the animal is kept quiet and not alarmed. The red central artery can be seen coursing along the translucent organ, giving off branches which by subdivision become too small to be separately visible, and the whole ear has a pink color and is warm from the abundant blood flowing through it. Attentive observation will show also that the caliber of the main artery is not constant; at somewhat irregular periods of a minute or more it dilates and contracts a little.

[Illustration: Fig. 77.–Some of the Principal Organs of the Chest and Abdomen. (Blood vessels on the left, muscles on the right.)]

In brief, all over the body, the nervous system, by its vaso-motor centers, is always supervising and regulating the distribution of blood in the body, sending now more and now less to this or that part.

[Illustration: Fig. 78.–Capillary Blood-Vessels in the Web of a Frog’s Foot, as seen with the Microscope.]

196. The Pulse. When the finger is placed on any part of the body where an artery is located near the surface, as, for example, on the radial artery near the wrist, there is felt an intermittent pressure, throbbing with every beat of the heart. This movement, frequently visible to the eye, is the result of the alternate expansion of the artery by the wave of blood, and the recoil of the arterial walls by their elasticity. In other words, it is the wave produced by throwing a mass of blood into the arteries already full. The blood-wave strikes upon the elastic walls of the arteries, causing an increased distention, followed at once by contraction. This regular dilatation and rigidity of the elastic artery answering to the beats of the heart, is known as the pulse.

The pulse may be easily found at the wrist, the temple, and the inner side of the ankle. The throb of the two carotid arteries may be plainly felt by pressing the thumb and finger backwards on each side of the larynx. The progress of the pulse-wave must not be confused with the actual current of the blood itself. For instance, the pulse-wave travels at the rate of about 30 feet a second, and takes about 1/10 of a second to reach the wrist, while the blood itself is from 3 to 5 seconds in reaching the same place.

The pulse-wave may be compared to the wave produced by a stiff breeze on the surface of a slowly moving stream, or the jerking throb sent along a rope when shaken. The rate of the pulse is modified by age, fatigue, posture, exercise, stimulants, disease, and many other circumstances. At birth the rate is about 140 times a minute, in early infancy, 120 or upwards, in the healthy adult between 65 and 75, the most common number being 72. In the same individual, the pulse is quicker when standing than when lying down, is quickened by excitement, is faster in the morning, and is slowest at midnight. In old age the pulse is faster than in middle life; in children it is quicker than in adults.

[Illustration: Fig. 79.–Circulation in the Capillaries, as seen with the Microscope.]

As the pulse varies much in its rate and character in disease, it is to the skilled touch of the physician an invaluable help in the diagnosis of the physical condition of his patient.

Experiment 92. _To find the pulse_. Grasp the wrist of a friend, pressing with three fingers over the radius. Press three fingers over the radius in your own wrist, to feel the pulse.

Count by a watch the rate of your pulse per minute, and do the same with a friend’s pulse. Compare its characters with your own pulse.

Observe how the character and frequency of the pulse are altered by posture, muscular exercise, a prolonged, sustained, deep inspiration, prolonged expiration, and other conditions.

197. Effect of Alcoholic Liquors upon the Organs of Circulation. Alcoholic drinks exercise a destructive influence upon the heart, the circulation, and the blood itself. These vicious liquids can reach the heart only indirectly, either from the stomach by the portal vein to the liver, and thence to the heart, or else by way of the lacteals, and so to the blood through the thoracic duct. But by either course the route is direct enough, and speedy enough to accomplish a vast amount of ruinous work.

The influence of alcohol upon the heart and circulation is produced mainly through the nervous system. The inhibitory nerves, as we have seen, hold the heart in check, exercise a restraining control over it, very much as the reins control an active horse. In health this inhibitory influence is protective and sustaining. But now comes the narcotic invasion of alcoholic drinks, which paralyze the inhibitory nerves, with the others, and at once the uncontrolled heart, like the unchecked steed, plunges on to violent and often destructive results.

[Illustration: Fig. 80.–Two Principal Arteries of the Front of the Leg (Anterior Tibial and Dorsalis Pedis).]

This action, because it is quicker, has been considered also a stronger action, and the alcohol has therefore been supposed to produce a stimulating effect. But later researches lead to the conclusion that the effect of alcoholic liquors is not properly that of a stimulant, but of a narcotic paralyzant, and that while it indeed quickens, it also really weakens the heart’s action. This view would seem sustained by the fact that the more the intoxicants are pushed, the deeper are the narcotic and paralyzing effects. After having obstructed the nutritive and reparative functions of the vital fluid for many years, their effects at last may become fatal.

This relaxing effect involves not only the heart, but also the capillary system, as is shown in the complexion of the face and the color of the hands. In moderate drinkers the face is only flushed, but in drunkards it is purplish. The flush attending the early stages of drinking is, of course, not the flush of health, but an indication of disease.[34]

198. Effect upon the Heart. This forced overworking of the heart which drives it at a reckless rate, cuts short its periods of rest and inevitably produces serious heart-exhaustion. If repeated and continued, it involves grave changes of the structure of the heart. The heart muscle, endeavoring to compensate for the over-exertion, may become much thickened, making the ventricles smaller, and so fail to do its duty in properly pumping forward the blood which rushes in from the auricle. Or the heart wall may by exhaustion become thinner, making the ventricles much too large, and unable to send on the current. In still other cases, the heart degenerates with minute particles of fat deposited in its structures, and thus loses its power to propel the nutritive fluid. All three of these conditions involve organic disease of the valves, and all three often produce fatal results.

199. Effect of Alcohol on the Blood-vessels. Alcoholic liquors injure not only the heart, but often destroy the blood-vessels, chiefly the larger arteries, as the arch of the aorta or the basilar artery of the brain. In the walls of these vessels may be gradually deposited a morbid product, the result of disordered nutrition, sometimes chalky, sometimes bony, with usually a dangerous dilatation of the tube.

In other cases the vessels are weakened by an unnatural fatty deposit. Though these disordered conditions differ somewhat, the morbid results in all are the same. The weakened and stiffened arterial walls lose the elastic spring of the pulsing current. The blood fails to sweep on with its accustomed vigor. At last, owing perhaps to the pressure, against the obstruction of a clot of blood, or perhaps to some unusual strain of work or passion, the enfeebled vessel bursts, and death speedily ensues from a form of apoplexy.

[Illustration: Fig. 81.–Showing the Carotid Artery and Jugular Vein on the Right Side, with Some of their Main Branches. (Some branches of the cervical plexus, and the hypoglossal nerve are also shown.)]

[NOTE. “An alcoholic heart loses its contractile and resisting power, both through morbid changes in its nerve ganglia and in its muscle fibers. In typhoid fever, muscle changes are evidently the cause of the heart-enfeeblement; while in diphtheria, disturbances in innervation cause the heart insufficiency. ‘If the habitual use of alcohol causes the loss of contractile and resisting power by impairment of both the nerve ganglia and muscle fibers of the heart, how can it act as a heart tonic?'”–Dr. Alfred L. Loomis, Professor of Medicine in the Medical Department of the University of the City of New York.]

200. Other Results from the Use of Intoxicants. Other disastrous consequences follow the use of intoxicants, and these upon the blood. When any alcohol is present in the circulation, its greed for water induces the absorption of moisture from the red globules of the blood, the oxygen-carriers. In consequence they contract and harden, thus becoming unable to absorb, as theretofore, the oxygen in the lungs. Then, in turn, the oxidation of the waste matter in the tissues is prevented; thus the corpuscles cannot convey carbon dioxid from the capillaries, and this fact means that some portion of refuse material, not being thus changed and eliminated, must remain in the blood, rendering it impure and unfit for its proper use in nutrition. Thus, step by step, the use of alcoholics impairs the functions of the blood corpuscles, perverts nutrition, and slowly poisons the blood.

[Illustration: Fig. 82.–The Right Axillary and Brachial Arteries, with Some of their Main Branches.]

[NOTE. “Destroy or paralyze the inhibitory nerve center, and instantly its controlling effect on the heart mechanism is lost, and the accelerating agent, being no longer under its normal restraint, runs riot. The heart’s action is increased, the pulse is quickened, an excess of blood is forced into the vessels, and from their becoming engorged and dilated the face gets flushed, all the usual concomitants of a general engorgement of the circulation being the result.”–Dr. George Harley, F.R.S., an eminent English medical author.

“The habitual use of alcohol produces a deleterious influence upon the whole economy. The digestive powers are weakened, the appetite is impaired, and the muscular system is enfeebled. The blood is impoverished, and nutrition is imperfect and disordered, as shown by the flabbiness of the skin and muscles, emaciation, or an abnormal accumulation of fat.”–Dr. Austin Flint, Senior, formerly Professor of the Practice of Medicine in Bellevue Medical College, and author of many standard medical works.

“The immoderate use of the strong kind of tobacco, which soldiers affect, is often very injurious to them, especially to very young soldiers. It renders them nervous and shaky, gives rise to palpitation, and is a factor in the production of the irritable or so-called “trotting-heart” and tends to impair the appetite and digestion.”–London _Lancet_.

“I never smoke because I have seen the most efficient proofs of the injurious effects of tobacco on the nervous system.”–Dr. Brown-Sequard, the eminent French physiologist.

“Tobacco, and especially cigarettes, being a depressant upon the heart, should be positively forbidden.”–Dr. J. M. Keating, on “Physical Development,” in _Cyclopædia of the Diseases of Children_.]

201. Effect of Tobacco upon the Heart. While tobacco poisons more or less almost every organ of the body, it is upon the heart that it works its most serious wrong. Upon this most important organ its destructive effect is to depress and paralyze. Especially does this apply to the young, whose bodies are not yet knit into the vigor that can brave invasion.

The _nicotine_ of tobacco acts through the nerves that control the heart’s action. Under its baneful influence the motions of the heart are irregular, now feeble and fluttering, now thumping with apparently much force: but both these forms of disturbed action indicate an abnormal condition. Frequently there is severe pain in the heart, often dizziness with gasping breath, extreme pallor, and fainting.

The condition of the pulse is a guide to this state of the heart. In this the physician reads plainly the existence of the “tobacco heart,” an affection as clearly known among medical men as croup or measles. There are few conditions more distressing than the constant and impending suffering attending a tumultuous and fluttering heart. It is stated that one in every four of tobacco-users is subject, in some degree, to this disturbance. Test examinations of a large number of lads who had used cigarettes showed that only a very small percentage escaped cardiac trouble. Of older tobacco-users there are very few but have some warning of the hazard they invoke. Generally they suffer more or less from the tobacco heart, and if the nervous system or the heart be naturally feeble, they suffer all the more speedily and intensely.

Additional Experiments.

Experiment 93. Touch a few drops of blood fresh from the finger, with a strip of dry, smooth, neutral litmus paper, highly glazed to prevent the red corpuscles from penetrating into the test paper. Allow the blood to remain a short time; then wash it off with a stream of distilled water, when a blue spot upon a red or violet ground will be seen, indicating its _alkaline_ reaction, due chiefly to the sodium phosphate and sodium carbonate.

Experiment 94. Place on a glass slide a thin layer of defibrinated blood; try to read printed matter through it. This cannot be done.

Experiment 95. _To make blood transparent or laky_. Place in each of three test tubes two or three teaspoonfuls of defibrinated blood, obtained from Experiment 89, labeled _A, B_, and _C. A_ is for comparison. To _B_ add five volumes of water, and warm slightly, noting the change of color by reflected and transmitted light. By reflected light it is much darker,–it looks almost black; but by transmitted light it is transparent. Test this by looking at printed matter as in Experiment 94.

Experiment 96. To fifteen or twenty drops of defibrinated blood in a test tube (labeled _D_) add five volumes of a 10-per-cent solution of common salt. It changes to a very bright, florid, brick-red color. Compare its color with _A, B_, and _C_. It is opaque.

Experiment 97. Wash away the coloring matter from the twigs (see Experiment 89) with a stream of water until the fibrin becomes quite white. It is white, fibrous, and elastic. Stretch some of the fibers to show their extensibility; on freeing them, they regain their elasticity.

Experiment 98. Take some of the serum saved from Experiment 88 and note that it does not coagulate spontaneously. Boil a little in a test tube over a spirit lamp, and the albumen will coagulate.

Experiment 99. _To illustrate in a general way that blood is really a mass of red bodies which give the red color to the fluid in which they float._ Fill a clean white glass bottle two-thirds full of little red beads, and then fill the bottle full of water. At a short distance the bottle appears to be rilled with a uniformly red liquid.

Experiment 100. _To show how blood holds a mineral substance in solution_. Put an egg-shell crushed fine, into a glass of water made acid by a teaspoonful of muriatic acid. After an hour or so the egg-shell will disappear, having been dissolved in the acid water. In like manner the blood holds various minerals in solution.

Experiment 101. _To hear the sounds of the heart_. Locate the heart exactly. Note its beat. Borrow a stethoscope from some physician. Listen to the heart-beat of some friend. Note the sounds of your own heart in the same way.

Experiment 102. _To show how the pulse may be studied_. “The movements of the artery in the human body as the pulse-wave passes through it may be shown to consist in a sudden dilatation, followed by a slow contraction, interrupted by one or more secondary dilatations. This demonstration may be made by pressing a small piece of looking-glass about one centimeter square (⅔ of an inch) upon the wrist over the radial artery, in such a way that with each pulse beat the mirror may be slightly tilted. If the wrist be now held in such a position that sunlight will fall upon the mirror, a spot of light will be reflected on the opposite side of the room, and its motion upon the wall will show that the expansion of the artery is a sudden movement, while the subsequent contraction is slow and interrupted.”–Bowditch’s _Hints for Teachers of Physiology_.

[Illustration: Fig. 83.–How the Pulse may be studied by Pressing a Mirror over the Radial Artery.]

Experiment 103. _To illustrate the effect of muscular exercise in quickening the pulse_. Run up and down stairs several times. Count the pulse both before and after. Note the effect upon the rate.

Experiment 104. _To show the action of the elastic walls of the arteries._ Take a long glass or metal tube of small caliber. Fasten one end to the faucet of a water-pipe (one in a set bowl preferred) by a very short piece of rubber tube. Turn the water on and off alternately and rapidly, to imitate the intermittent discharge of the ventricles. The water will flow from the other end of the rubber pipe in jets, each jet ceasing the moment the water is shut off.

The experiment will be more successful if the rubber bulb attached to an ordinary medicine-dropper be removed, and the tapering glass tube be slipped on to the outer end of the rubber tube attached to the faucet.

Experiment 105. Substitute a piece of rubber tube for the glass tube, and repeat the preceding experiment. Now it will be found that a continuous stream flows from the tube. The pressure of water stretches the elastic tube, and when the stream is turned off, the rubber recoils on the water, and the intermittent flow is changed into a continuous stream.

Experiment 106. _To illustrate some of the phenomena of circulation._ Take a common rubber bulb syringe, of the Davidson, Household, or any other standard make. Attach a piece of rubber tube about six or eight feet long to the delivery end of the syringe.

To represent the resistance made by the capillaries to the flow of blood, slip the large end of a common glass medicine-dropper into the outer end of the rubber tube. This dropper has one end tapered to a fine point.

Place the syringe flat, without kinks or bends, on a desk or table. Press the bulb slowly and regularly. The water is thus pumped into the tube in an intermittent manner, and yet it is forced out of the tapering end of the glass tube in a steady flow.

Experiment 107. Take off the tapering glass tube, or, in the place of one long piece of rubber tube, substitute several pieces of glass tubing connected together by short pieces of rubber tubes. The obstacle to the flow has thus been greatly lessened, and the water flows out in intermittent jets to correspond to the compression of the bulb.

Chapter VIII.


202. Nature and Object of Respiration. The blood, as we have learned, not only provides material for the growth and activity of all the tissues of the body, but also serves as a means of removing from them the products of their activity. These are waste products, which if allowed to remain, would impair the health of the tissues. Thus the blood becomes impoverished both by the addition of waste material, and from the loss of its nutritive matter.

We have shown, in the preceding chapter, how the blood carries to the tissues the nourishment it has absorbed from the food. We have now to consider a new source of nourishment to the blood, _viz._, that which it receives from the oxygen of the air. We are also to learn one of the methods by which the blood gets rid of poisonous waste matters. In brief, we are to study the set of processes known as respiration, by which oxygen is supplied to the various tissues, and by which the principal waste matters, or chief products of oxidation, are removed.

Now, the tissues are continually feeding on the life-giving oxygen, and at the same time are continually producing carbon dioxid and other waste products. In fact, the life of the tissues is dependent upon a continual succession of oxidations and deoxidations. When the blood leaves the tissues, it is poorer in oxygen, is burdened with carbon dioxid, and has had its color changed from bright scarlet to purple red. This is the change from the arterial to venous conditions which has been described in the preceding chapter.

Now, as we have seen, the change from venous to arterial blood occurs in the capillaries of the lungs, the only means of communication between the pulmonary arteries and the pulmonary veins. The blood in the pulmonary capillaries is separated from the air only by a delicate tissue formed of its own wall and the pulmonary membrane. Hence a gaseous interchange, the essential step in respiration, very readily takes place between the blood and the air, by which the latter gains moisture and carbon dioxid, and loses its oxygen. These changes in the lungs also restore to the dark blood its rosy tint.

The only condition absolutely necessary to the purification of the blood is an organ having a delicate membrane, on one side of which is a thin sheet of blood, while the other side is in such contact with the air that an interchange of gases can readily take place. The demand for oxygen is, however, so incessant, and the accumulation of carbon dioxid is so rapid in every tissue of the human body, that an All-Wise Creator has provided a most perfect but complicated set of machinery to effect this wonderful purification of the blood.

We are now ready to begin the study of the arrangement and working of the respiratory apparatus. With its consideration, we complete our view of the sources of supply to the blood, and begin our study of its purification.

[Illustration: Fig. 84.–The Epiglottis.]

203. The Trachea, or Windpipe. If we look into the mouth of a friend, or into our own with a mirror, we see at the back part an arch which is the boundary line of the mouth proper. There is just behind this a similar limit for the back part of the nostrils. The funnel-shaped cavity beyond, into which both the mouth and the posterior nasal passages open, is called the pharynx. In its lower part are two openings; the trachea, or windpipe, in front, and the œsophagus behind.

The trachea is surmounted by a box-like structure of cartilage, about four and one-half inches long, called the larynx. The upper end of the larynx opens into the pharynx or throat, and is provided with a lid,– the epiglottis,–which closes under certain circumstances (secs. 137 and 349). The larynx contains the organ of voice, and is more fully described in Chapter XII.

The continuation of the larynx is the trachea, a tube about three-fourths of an inch in diameter, and about four inches long. It extends downwards along the middle line of the neck, where it may readily be felt in front, below the Adam’s apple.

[Illustration: Fig. 85.–Larynx, Trachea, and the Bronchi. (Front view.)

A, epiglottis;
B, thyroid cartilage;
C, cricoid-thyroid membrane, connecting with the cricoid cartilage below, all forming the larynx;
D, one of the rings of the trachea. ]

The walls of the windpipe are strengthened by a series of cartilaginous rings, each somewhat the shape of a horseshoe or like the letter C, being incomplete behind, where they come in contact with the œsophagus. Thus the trachea, while always open for the passage of air, admits of the distention of the food-passage.

204. The Bronchial Tubes. The lower end of the windpipe is just behind the upper part of the sternum, and there it divides into two branches, called bronchi. Each branch enters the lung of its own side, and breaks up into a great number of smaller branches, called bronchial tubes. These divide into smaller tubes, which continue subdividing till the whole lung is penetrated by the branches, the extremities of which are extremely minute. To all these branches the general name of bronchial tubes is given. The smallest are only about one-fiftieth of an inch in diameter.

[Illustration: Fig. 86.–Relative Position of the Lungs, Heart, and its Great Vessels.

A, left ventricle;
B, right ventricle;
C, left auricle;
D, right auricle;
E, superior vena cava;
F, pulmonary artery;
G, aorta;
H, arch of the aorta;
K, innominate artery;
L, right common carotid artery;
M, right subclavian artery;
N, thyroid cartilage forming upper portion of the larynx; O, trachea.

Now the walls of the windpipe, and of the larger bronchial tubes would readily collapse, and close the passage for air, but for a wise precaution. The horseshoe-shaped rings of cartilage in the trachea and the plates of cartilage in the bronchial tubes keep these passages open. Again, these air passages have elastic fibers running the length of the tubes, which allow them to stretch and bend readily with the movements of the neck.

205. The Cilia of the Air Passages. The inner surfaces of the windpipe and bronchial tubes are lined with mucous membrane, continuous with that of the throat, the mouth, and the nostrils, the secretion from which serves to keep the parts moist.

Delicate, hair-like filaments, not unlike the pile on velvet, called cilia, spring from the epithelial lining of the air tubes. Their constant wavy movement is always upwards and outwards, towards the mouth. Thus any excessive secretion, as of bronchitis or catarrh, is carried upwards, and finally expelled by coughing. In this way, the lungs are kept quite free from particles of foreign matter derived from the air. Otherwise we should suffer, and often be in danger from the accumulation of mucus and dust in the air passages. Thus these tiny cilia act as dusters which Nature uses to keep the air tubes free and clean (Fig. 5).

[Illustration: Fig. 87.–Bronchial tube, with its Divisions and Subdivisions. (Showing groups of air cells at the termination of minute bronchial tubes.)]

206. The Lungs. The lungs, the organs of respiration, are two pinkish gray structures of a light, spongy appearance, that fill the chest cavity, except the space taken up by the heart and large vessels. Between the lungs are situated the large bronchi, the œsophagus, the heart in its pericardium, and the great blood-vessels. The base of the lungs rests on the dome-like diaphragm, which separates the chest from the abdomen. This partly muscular and partly tendinous partition is a most important factor in breathing.

Each lung is covered, except at one point, with an elastic serous membrane in a double layer, called the pleura. One layer closely envelops the lung, at the apex of which it is reflected to the wall of the chest cavity of its own side, which it lines. The two layers thus form between them a Closed Sac a serous cavity (see Fig. 69, also note, p. 176).

[Illustration: Fig. 88.–The Lungs with the Trachea, Bronchi, and Larger Bronchial Tubes exposed. (Posterior view.)

A, division of left bronchus to upper lobe; B, left branch of the Pulmonary artery; C, left bronchus;
D, left superior pulmonary vein;
E, left inferior pulmonary vein;
F, left auricle;
K, inferior vena cava;
L, division of right bronchus to lower lobe; M, right inferior pulmonary vein;
N, right superior pulmonary vein;
O, right branch of the pulmonary artery; P, division of right bronchus to upper lobe; R, left ventricle;
S, right ventricle.

In health the two pleural surfaces of the lungs are always in contact, and they secrete just enough serous fluid to allow the surfaces to glide smoothly upon each other. Inflammation of this membrane is called _pleurisy_. In this disease the breathing becomes very painful, as the secretion of glairy serum is suspended, and the dry and inflamed surfaces rub harshly upon each other.

The root of the lung, as it is called, is formed by the bronchi, two pulmonary arteries, and two pulmonary veins. The nerves and lymphatic vessels of the lung also enter at the root. If we only remember that all the bronchial tubes, great and small, are hollow, we may compare the whole system to a short bush or tree growing upside down in the chest, of which the trachea is the trunk, and the bronchial tubes the branches of various sizes.

207. Minute Structure of the Lungs. If one of the smallest bronchial tubes be traced in its tree-like ramifications, it will be found to end in an irregular funnel-shaped passage wider than itself. Around this passage are grouped a number of honeycomb-like sacs, the air cells[35] or alveoli of the lungs. These communicate freely with the passage, and through it with the bronchial branches, but have no other openings. The whole arrangement of passages and air cells springing from the end of a bronchial tube, is called an ultimate lobule. Now each lobule is a very small miniature of a whole lung, for by the grouping together of these lobules another set of larger lobules is formed.

[Illustration: Fig. 89.

A, diagrammatic representation of the ending of a bronchial tube in air sacs or alveoli;
B, termination of two bronchial tubes in enlargement beset with air sacs (_Huxley_);
C, diagrammatic view of an air sac.

a lies within sac and points to epithelium lining wall; b, partition between two adjacent sacs, in which run capillaries; c, elastic connective tissue (_Huxley_). ]

In like manner countless numbers of these lobules, bound together by connective tissue, are grouped after the same fashion to form by their aggregation the lobes of the lung. The right lung has three such lobes; and the left, two. Each lobule has a branch of the pulmonary artery entering it, and a similar rootlet of the pulmonary vein leaving it. It also receives lymphatic vessels, and minute twigs of the pulmonary plexus of nerves.

[Illustration: Fig. 90.–Diagram to illustrate the Amounts of Air contained by the Lungs in Various Phases of Ordinary and of Forced Respiration.]

The walls of the air cells are of extreme thinness, consisting of delicate elastic and connective tissue, and lined inside by a single layer of thin epithelial cells. In the connective tissue run capillary vessels belonging to the pulmonary artery and veins. Now these delicate vessels running in the connective tissue are surrounded on all sides by air cells. It is evident, then, that the blood flowing through these capillaries is separated from the air within the cells only by the thin walls of the vessels, and the delicate tissues of the air cells.

This arrangement is perfectly adapted for an interchange between the blood in the capillaries and the air in the air cells. This will be more fully explained in sec. 214.

208. Capacity of the Lungs. In breathing we alternately take into and expel from the lungs a certain quantity of air. With each quiet inspiration about 30 cubic inches of air enter the lungs, and 30 cubic inches pass out with each expiration. The air thus passing into and out of the lungs is called tidal air. After an ordinary inspiration, the lungs contain about 230 cubic inches of air. By taking a deep inspiration, about 100 cubic inches more can be taken in. This extra amount is called complemental air.

After an ordinary expiration, about 200 cubic inches are left in the lungs, but by forced expiration about one-half of this may be driven out. This is known as supplemental air. The lungs can never be entirely emptied of air, about 75 to 100 cubic inches always remaining. This is known as the residual air.

The air that the lungs of an adult man are capable of containing is thus composed:

Complemental air 100 cubic inches. Tidal ” 30 ” “
Supplemental ” 100 ” “
Residual ” 100 ” “
Total capacity of lungs 330 ” “

If, then, a person proceeds, after taking the deepest possible breath, to breath out as much as he can, he expels:

Complemental air 100 cubic inches. Tidal ” 30 ” “
Supplemental ” 100 ” “

This total of 230 cubic inches forms what is called the vital capacity of the chest (Fig. 90).

209. The Movements of Breathing. The act of breathing consists of a series of rhythmical movements, succeeding one another in regular order. In the first movement, inspiration, the chest rises, and there is an inrush of fresh air; this is at once followed by expiration, the falling of the chest walls, and the output of air. A pause now occurs, and the same breathing movements are repeated.

The entrance and the exit of air into the respiratory passages are accompanied with peculiar sounds which are readily heard on placing the ear at the chest wall. These sounds are greatly modified in various pulmonary diseases, and hence are of great value to the physician in making a correct diagnosis.

In a healthy adult, the number of respirations should be from 16 to 18 per minute, but they vary with age, that of a newly born child being 44 for the same time. Exercise increases the number, while rest diminishes it. In standing, the rate is more than when lying at rest. Mental emotion and excitement quicken the rate. The number is smallest during sleep. Disease has a notable effect upon the frequency of respirations. In diseases involving the lungs, bronchial tubes, and the pleura, the rate may be alarmingly increased, and the pulse is quickened in proportion.

210. The Mechanism of Breathing. The chest is a chamber with bony walls, the ribs connecting in front with the breastbone, and behind with the spine. The spaces between the ribs are occupied by the intercostal muscles, while large muscles clothe the entire chest. The diaphragm serves as a movable floor to the chest, which is an air-tight chamber with movable walls and floor. In this chamber are suspended the lungs, the air cells of which communicate with the outside through the bronchial passages, but have no connection with the chest cavity. The thin space between the lungs and the rib walls, called the pleural cavity, is in health a vacuum.

Now, when the diaphragm contracts, it descends and thus increases the depth of the chest cavity. A quantity of air is now drawn into the lungs and causes them to expand, thus filling up the increased space. As soon as the diaphragm relaxes, returning to its arched position and reducing the size of the chest cavity, the air is driven from the lungs, which then diminish in size. After a short pause, the diaphragm again contracts, and the same round of operation is constantly repeated.

The walls of the chest being movable, by the contractions of the intercostals and other muscles, the ribs are raised and the breastbone pushed forward. The chest cavity is thus enlarged from side to side and from behind forwards. Thus, by the simultaneous descent of the diaphragm and the elevation of the ribs, the cavity of the chest is increased in three directions,–downwards, side-ways, and from behind forwards.

It is thus evident that inspiration is due to a series of muscular contractions. As soon as the contractions cease, the elastic lung tissue resumes its original position, just as an extended rubber band recovers itself. As a result, the original size of the chest cavity is restored, and the inhaled air is driven from the lungs. Expiration may then be regarded as the result of an elastic recoil, and not of active muscular contractions.

[Illustration: Fig. 91.–Diagrammatic Section of the Trunk. (Showing the expansion of the chest and the movement of the ribs by action of the lungs.) [The dotted lines indicate the position during inspiration.]]

211. Varieties of Breathing. This is the mechanism of quiet, normal respiration. When the respiration is difficult, additional forces are brought into play. Thus when the windpipe and bronchial tubes are obstructed, as in croup, asthma, or consumption, many additional muscles are made use of to help the lungs to expand. The position which asthmatics often assume, with arms raised to grasp something for support, is from the need of the sufferer to get a fixed point from which the muscles of the arm and chest may act forcibly in raising the ribs, and thus securing more comfortable breathing.

The visible movements of breathing vary according to circumstances. In infants the action of the diaphragm is marked, and the movements of the abdomen are especially obvious. This is called abdominal breathing. In women the action of the ribs as they rise and fall, is emphasized more than in men, and this we call costal breathing. In young persons and in men, the respiration not usually being impeded by tight clothing, the breathing is normal, being deep and abdominal.

Disease has a marked effect upon the mode of breathing. Thus, when children suffer from some serious chest disease, the increased movements of the abdominal walls seem distressing. So in fracture of the ribs, the surgeon envelops the overlying part of the chest with long strips of firm adhesive plaster to restrain the motions of chest respiration, that they may not disturb the jagged ends of the broken bones. Again, in painful diseases of the abdomen, the sufferer instinctively suspends the abdominal action and relies upon the chest breathing. These deviations from the natural movements of respiration are useful to the physician in ascertaining the seat of disease.

212. The Nervous Control of Respiration. It is a matter of common experience that one’s breath may be held for a short time, but the need of fresh air speedily gets the mastery, and a long, deep breath is drawn. Hence the efforts of criminals to commit suicide by persistent restraint of their breathing, are always a failure. At the very worst, unconsciousness ensues, and then respiration is automatically resumed. Thus a wise Providence defeats the purpose of crime. The movements of breathing go on without our attention. In sleep the regularity of respiration is even greater than when awake. There is a particular part of the nervous system that presides over the breathing function. It is situated in that part of the brain called the medulla oblongata, and is fancifully called the “vital knot” (sec. 270). It is injury to this respiratory center which proves fatal in cases of broken neck.

From this nerve center there is sent out to the nerves that supply the diaphragm and other muscles of breathing, a force which stimulates them to regular contraction. This breathing center is affected by the condition of the blood. It is stimulated by an excess of carbon dioxid in the blood, and is quieted by the presence of oxygen.

Experiment 108. _To locate the lungs_. Mark out the boundaries of the lungs by “sounding” them; that is, by _percussion_, as it is called. This means to put the forefinger of the left hand across the chest or back, and to give it a quick, sharp rap with two or three fingers. Note where it sounds hollow, resonant. This experiment can be done by the student with only imperfect success, until practice brings some skill.

Experiment 109. Borrow a stethoscope, and listen to the respiration over the chest on the right side. This is known as _auscultation_. Note the difference of the sounds in inspiration and in expiration. Do not confuse the heart sounds with those of respiration. The respiratory murmurs may be heard fairly well by applying the ear flat to the chest, with only one garment interposed.

Experiment 110. Get a sheep’s lungs, with the windpipe attached. Ask for the heart and lungs all in one mass. Take pains to examine the specimen first, and accept only a good one. Parts are apt to be hastily snipped or mangled. Examine the windpipe. Note the horseshoe-shaped rings of cartilage in front, which serve to keep it open.

Experiment 111. Examine one bronchus, carefully dissecting away the lung tissue with curved scissors. Follow along until small branches of the bronchial tubes are reached. Take time for the dissection, and save the specimen in dilute alcohol. Put pieces of the lung tissue in a basin of water, and note that they float.

The labored breathing of suffocation and of lung diseases is due to the excessive stimulation of this center, caused by the excess of carbon dioxid in the blood. Various mental influences from the brain itself, as the emotions of alarm or joy or distress, modify the action of the respiratory center.

Again, nerves of sensation on the surface of the body convey influences to this nerve center and lead to its stimulation, resulting in a vigorous breathing movement. Thus a dash of cold water on the face or neck of a fainting person instantly produces a deep, long-drawn breath. Certain drugs, as opium, act to reduce the activity of this nerve center. Hence, in opium poisoning, special attention should be paid to keeping up the respiration. The condition of the lungs themselves is made known to the breathing center, by messages sent along the branches of the great pneumogastric nerve (page 276), leading from the lungs to the medulla oblongata.

213. Effects of Respiration upon the Blood. The blood contains three gases, partly dissolved in it and partly in chemical union with certain of its constituents. These are oxygen, carbon dioxid, and nitrogen. The latter need not be taken into account. The oxygen is the nourishing material which the tissues require to carry on their work. The carbon dioxid is a waste substance which the tissues produce by their activity, and which the blood carries away from them.

As before shown, the blood as it flows through the tissues loses most of its oxygen, and carbon dioxid takes its place. Now if the blood is to maintain its efficiency in this respect, it must always be receiving new supplies of oxygen, and also have some mode of throwing off its excess of carbon dioxid. This, then, is the double function of the process of respiration. Again, the blood sent out from the left side of the heart is of a bright scarlet color. After its work is done, and the blood returns to the right side of the heart, it is of a dark purple color. This change in color takes place in the capillaries, and is due to the fact that there the blood gives up most of its oxygen to the tissues and receives from them a great deal of carbon dioxid.

In brief, while passing through the capillaries of the lungs the blood has been changed from the venous to the arterial blood. That is to say, the blood in its progress through the lungs has rid itself of its excess of carbon dioxid and obtained a fresh supply of oxygen.[36]

214. Effects of Respiration upon the Air in the Lungs. It is well known that if two different liquids be placed in a vessel in contact with each other and left undisturbed, they do not remain separate, but gradually mix, and in time will be perfectly combined. This is called diffusion of liquids. The same thing occurs with gases, though the process is not visible. This is known as the diffusion of gases. It is also true that two liquids will mingle when separated from each other by a membrane (sec. 129). In a similar manner two gases, especially if of different densities, may mingle even when separated from each other by a membrane.

In a general way this explains the respiratory changes that occur in the blood in the lungs. Blood containing oxygen and carbon dioxid is flowing in countless tiny streams through the walls of the air cells of the lungs. The air cells themselves contain a mixture of the same two gases. A thin, moist membrane, well adapted to allow gaseous diffusion, separates the blood from the air. This membrane is the delicate wall of the capillaries and the epithelium of the air cells. By experiment it has been found that the pressure of oxygen in the blood is less than that in the air cells, and that the pressure of carbon dioxid gas in the blood is greater than that in the air cells. As a result, a diffusion of gases ensues. The blood gains oxygen and loses carbon dioxid, while the air cells lose oxygen and gain the latter gas.

[Illustration: Fig. 92.–Capillary Network of the Air Cells and Origin of the Pulmonary Veins.

A, small branch of pulmonary artery; B, twigs of the pulmonary artery anastomosing to form peripheral network of the primitive air cells;
C, capillary network around the walls of the air sacs; D, branches of network converging for form the veinlets of the pulmonary veins.

The blood thus becomes purified and reinvigorated, and at the same time is changed in color from purple to scarlet, from venous to arterial. It is now evident that if this interchange is to continue, the air in the cells must be constantly renewed, its oxygen restored, and its excess of carbon dioxid removed. Otherwise the process just described would be reversed, making the blood still more unfit to nourish the tissues, and more poisonous to them than before.

215. Change in the Air in Breathing. The air which we exhale during respiration differs in several important particulars from the air we inhale. Both contain chiefly the three gases, though in different quantities, as the following table shows.

Oxygen. Nitrogen. Carbon Dioxid. Inspired air contains 20.81 79.15 .04 Expired air contains 16.03 79.58 4.38

That is, expired air contains about five per cent less oxygen and five per cent more carbon dioxid than inspired air.

The temperature of expired air is variable, but generally is higher than that of inspired air, it having been in contact with the warm air passages. It is also loaded with aqueous vapor, imparted to it like the heat, not in the depth of the lungs, but in the upper air passages.

Expired air contains, besides carbon dioxid, various impurities, many of an unknown nature, and all in small amounts. When the expired air is condensed in a cold receiver, the aqueous product is found to contain organic matter, which, from the presence of _micro-organisms_, introduced in the inspired air, is apt to putrefy rapidly. Some of these organic substances are probably poisonous, either so in themselves, as produced in some manner in the breathing apparatus, or poisonous as being the products of decomposition. For it is known that various animal substances give rise, by decomposition, to distinct poisonous products known as _ptomaines_. It is possible that some of the constituents of the expired air are of an allied nature. See under “Bacteria” (Chapter XIV).

At all events, these substances have an injurious action, for an atmosphere containing simply one per cent of pure carbon dioxid has very little hurtful effect on the animal economy, but an atmosphere in which the carbon dioxid has been raised one per cent by breathing is highly injurious.

The quantity of oxygen removed from the air by the breathing of an adult person at rest amounts daily to about 18 cubic feet. About the same amount of carbon dioxid is expelled, and this could be represented by a piece of pure charcoal weighing 9 ounces. The quantity of carbon dioxid, however, varies with the age, and is increased also by external cold and by exercise, and is affected by the kind of food. The amount of water, exhaled as vapor, varies from 6 to 20 ounces daily. The average daily quantity is about one-half a pint.

216. Modified Respiratory Movements. The respiratory column of air is often used in a mechanical way to expel bodies from the upper air passages. There are also, in order to secure special ends, a number of modified movements not distinctly respiratory. The following peculiar respiratory acts call for a few words of explanation.

A sigh is a rapid and generally audible expiration, due to the elastic recoil of the lungs and chest walls. It is often caused by depressing emotions. Yawning is a deep inspiration with a stretching of the muscles of the face and mouth, and is usually excited by fatigue or drowsiness, but often occurs from a sort of contagion.

Hiccough is a sudden jerking inspiration due to the spasmodic contraction of the diaphragm and of the glottis, causing the air to rush suddenly through the larynx, and produce this peculiar sound. Snoring is caused by vibration of the soft palate during sleep, and is habitual with some, although it occurs with many when the system is unusually exhausted and relaxed.

Laughing consists of a series of short, rapid, spasmodic expirations which cause the peculiar sounds, with characteristic movements of the facial muscles. Crying, caused by emotional states, consists of sudden jerky expirations with long inspirations, with facial movements indicative of distress. In sobbing, which often follows long-continued crying, there is a rapid series of convulsive inspirations, with sudden involuntary contractions of the diaphragm. Laughter, and sometimes sobbing, like yawning, may be the result of involuntary imitation.

Experiment 112. _Simple Apparatus to Illustrate the Movements of the Lungs in the Chest_.–T is a bottle from which the bottom has been removed; D, a flexible and elastic membrane tied on the bottle, and capable of being pulled out by the string S, so as to increase the capacity of the bottle. L is a thin elastic bag representing the lungs. It communicates with the external air by a glass tube fitted air-tight through a cork in the neck of the bottle. When D is drawn down, the pressure of the external air causes L to expand. When the string is let go, L contracts again, by virtue of its elasticity.

[Illustration: Fig. 93.]

Coughing is produced by irritation in the upper part of the windpipe and larynx. A deep breath is drawn, the opening of the windpipe is closed, and immediately is burst open with a violent effort which sends a blast of air through the upper air passages. The object is to dislodge and expel any mucus or foreign matter that is irritating the air passages.

Sneezing is like coughing; the tongue is raised against the soft palate, so the air is forced through the nasal passages. It is caused by an irritation of the nostrils or eyes. In the beginning of a cold in the head, for instance, the cold air irritates the inflamed mucous membrane of the nose, and causes repeated attacks of sneezing.

217. How the Atmosphere is Made Impure. The air around us is constantly being made impure in a great variety of ways. The combustion of fuel, the respiration of men and animals, the exhalations from their bodies, the noxious gases and effluvia of the various industries, together with the changes of fermentation and decomposition to which all organized matter is liable,–all tend to pollute the atmosphere.

The necessity of external ventilation has been foreseen for us. The forces of nature,–the winds, sunlight, rain, and growing vegetation,–all of great power and universal distribution and application, restore the balance, and purify the air. As to the principal gases, the air of the city does not differ materially from that of rural sections. There is, however, a vastly greater quantity of dust and smoke in the air of towns. The breathing of this dust, to a greater or less extent laden with bacteria, fungi, and the germs of disease, is an ever-present and most potent menace to public and personal health. It is one of the main causes of the excess of mortality in towns and cities over that of country districts.

This is best shown in the overcrowded streets and houses of great cities, which are deprived of the purifying influence of sun and air. The fatal effect of living in vitiated air is especially marked in the mortality among infants and children living in the squalid and overcrowded sections of our great cities. The salutary effect of sunshine is shown by the fact that mortality is usually greater on the shady side of the street.

218. How the Air is Made Impure by Breathing. It is not the carbon dioxid alone that causes injurious results to health, it is more especially the organic matter thrown off in the expired air. The carbon dioxid which accompanies the organic matter is only the index. In testing the purity of air it is not difficult to ascertain the amount of carbon dioxid present, but it is no easy problem to measure the amount of organic matter. Hence it is the former that is looked for in factories, churches, schoolrooms, and when it is found to exceed .07 per cent it is known that there is a hurtful amount of organic matter present.

The air as expelled from the lungs contains, not only a certain amount of organic matter in the form of vapor, but minute solid particles of _débris_ and bacterial micro-organisms (Chap. XIV). The air thus already vitiated, after it leaves the mouth, putrefies very rapidly. It is at once absorbed by clothing, curtains, carpets, porous walls, and by many other objects. It is difficult to dislodge these enemies of health even by free ventilation. The close and disagreeable odor of a filthy or overcrowded room is due to these organic exhalations from the lungs, the skin, and the unclean clothing of the occupants.

The necessity of having a proper supply of fresh air in enclosed places, and the need of removal of impure air are thus evident. If a man were shut up in a tightly sealed room containing 425 cubic feet of air, he would be found dead or nearly so at the end of twenty-four hours. Long before this time he would have suffered from nausea, headache, dizziness, and other proofs of blood-poisoning. These symptoms are often felt by those who are confined for an hour or more in a room where the atmosphere has been polluted by a crowd of people. The unpleasant effects rapidly disappear on breathing fresh air.

219. The Effect on the Health of Breathing Foul Air. People are often compelled to remain indoors for many hours, day after day, in shops, factories, or offices, breathing air perhaps only slightly vitiated, but still recognized as “stuffy.” Such persons often suffer from ill health. The exact form of the disturbance of health depends much upon the hereditary proclivity and physical make-up of the individual. Loss of appetite, dull headache, fretfulness, persistent weariness, despondency, followed by a general weakness and an impoverished state of blood, often result.

Persons in this lowered state of health are much more prone to surfer from colds, catarrhs, bronchitis, and pneumonia than if they were living in the open air, or breathing only pure air. Thus, in the Crimean War, the soldiers who lived in tents in the coldest weather were far more free from colds and lung troubles than those who lived in tight and ill-ventilated huts. In the early fall when typhoid fever is prevalent, the grounds of large hospitals are dotted with canvas tents, in which patients suffering from this fever do much better than in the wards.

This tendency to inflammatory diseases of the air passages is aggravated by the overheated and overdried condition of the air in the room occupied. This may result from burning gas, from overheated furnaces and stoves, hot-water pipes, and other causes. Serious lung diseases, such as consumption, are more common among those who live in damp, overcrowded, or poorly ventilated homes.

220. The Danger from Pulmonary Infection. The germ of pulmonary consumption, known as the bacillus tuberculosis, is contained in the breath and the sputa from the lungs of its victims. It is not difficult to understand how these bacilli may be conveyed through the air from the lungs of the sick to those of apparently healthy people. Such persons may, however, be predisposed, either constitutionally or by defective hygienic surroundings, to fall victims to this dreaded disease. Overcrowding, poor ventilation, and dampness all tend to increase the risk of pulmonary infection.

It must not be supposed that the tubercle bacillus is necessarily transmitted directly through the air from the lungs of the sick to be implanted in the lungs of the healthy. The germs may remain for a time in the dust turn and _débris_ of damp, filthy, and overcrowded houses. In this congenial soil they retain their vitality for a long time, and possibly may take on more virulent infective properties than they possessed when expelled from the diseased lungs.[37]

[Illustration: Fig. 94. Example of a Micro-Organism–Bacillus Tuberculosis in Spotum. (Magnified about 500 diameters.)]

221. Ventilation. The question of a practicable and economical system of ventilation for our homes, schoolrooms, workshops, and public places presents many difficult and perplexing problems. It is perhaps due to the complex nature of the subject, that ventilation, as an ordinary condition of daily health, has been so much neglected. The matter is practically ignored in building ordinary houses. The continuous renewal of air receives little if any consideration, compared with the provision made to furnish our homes with heat, light, and water. When the windows are closed we usually depend for ventilation upon mere chance,–on the chimney, the fireplace, and the crevices of doors and windows. The proper ventilation of a house and its surroundings should form as prominent a consideration in the plans of builders and architects as do the grading of the land, the size of the rooms, and the cost of heating.

The object of ventilation is twofold: First, to provide for the removal of the impure air; second, for a supply of pure air. This must include a plan to provide fresh air in such a manner that there shall be no draughts or exposure of the occupants of the rooms to undue temperature. Hence, what at first might seem an easy thing to do, is, in fact, one of the most difficult of sanitary problems.

222. Conditions of Efficient Ventilation. To secure proper ventilation certain conditions must be observed. The pure air introduced should not be far below the temperature of the room, or if so, the entering current should be introduced towards the ceiling, that it may mix with the warm air.

Draughts must be avoided. If the circuit from entrance to exit is short, draughts are likely to be produced, and impure air has less chance of mixing by diffusion with the pure air. The current of air introduced should be constant, otherwise the balance may occasionally be in favor of vitiated air. If a mode of ventilation prove successful, it should not be interfered with by other means of entrance. Thus, an open door may prevent the incoming air from passing through its proper channels. It is desirable that the inlet be so arranged that it can be diminished in size or closed altogether. For instance, when the outer air is very cold, or the wind blows directly into the inlet, the amount of cold air entering it may lower the temperature of the room to an undesirable degree.

In brief, it is necessary to have a thorough mixing of pure and impure air, so that the combination at different parts of the room may be fairly uniform. To secure these results, the inlets and outlets should be arranged upon principles of ventilation generally accepted by authorities on public health. It seems hardly necessary to say that due attention must be paid to the source from which the introduced air is drawn. If it be taken from foul cellars, or from dirty streets, it may be as impure as that which it is designed to replace.

Animal Heat.

223. Animal or Vital Heat. If a thermometer, made for the purpose, be placed for five minutes in the armpit, or under the tongue, it will indicate a temperature of about 98½° F., whether the surrounding atmosphere be warm or cold. This is the natural heat of a healthy person, and in health it rarely varies more than a degree or two. But as the body is constantly losing heat by radiation and conduction, it is evident that if the standard temperature be maintained, a certain amount of heat must be generated within the body to make up for the loss externally. The heat thus produced is known as animal or vital heat.

This generation of heat is common to all living organisms. When the mass of the body is large, its heat is readily perceptible to the touch and by its effect upon the thermometer. In mammals and birds the heat-production is more active than in fishes and reptiles, and their temperatures differ in degree even in different species of the same class, according to the special organization of the animal and the general activity of its functions. The temperature of the frog may be 85° F. in June and 41° F. in January. The structure of its tissues is unaltered and their vitality unimpaired by such violent fluctuations. But in man it is necessary not only for health, but even for life, that the temperature should vary only within narrow limits around the mean of 98½° F.

We are ignorant of the precise significance of this constancy of temperature in warm-blooded animals, which is as important and peculiar as their average height, Man, undoubtedly, must possess a superior delicacy of organization, hardly revealed by structure, which makes it necessary that he should be shielded from the shocks and jars of varying temperature, that less highly endowed organisms endure with impunity.

224. Sources of Bodily Heat. The heat of the body is generated by the chemical changes, generally spoken of as those of oxidation, which are constantly going on in the tissues. Indeed, whenever protoplasmic materials are being oxidized (the process referred to in sec. 15 as katabolism) heat is being set free. These chemical changes are of various kinds, but the great source of heat is the katabolic process, known as oxidation.

The vital part of the tissues, built up from the complex classes of food, is oxidized by means of the oxygen carried by the arterial blood, and broken down into simpler bodies which at last result in urea, carbon dioxid, and water. Wherever there is life, this process of oxidation is going on, but more energetically in some tissues and organs than in others. In other words, the minutest tissue in the body is a source of heat in proportion to the activity of its chemical changes. The more active the changes, the greater is the heat produced, and the greater the amount of urea, carbon dioxid, and water eliminated. The waste caused by this oxidation must be made good by a due supply of food to be built up into protoplasmic material. For the production of heat, therefore, food is necessary. But the oxidation process is not as simple and direct as the statement of it might seem to indicate. Though complicated in its various stages, the ultimate result is as simple as in ordinary combustion outside of the body, and the products are the same.

The continual chemical changes, then, chiefly by oxidation of combustible materials in the tissues, produce an amount of heat which is efficient to maintain the temperature of the living body at about 98½° F. This process of oxidation provides not only for the heat of the body, but also for the energy required to carry on the muscular work of the animal organism.

225. Regulation of the Bodily Temperature. While bodily heat is being continually produced, it is also as continually being lost by the lungs, by the skin, and to some extent, by certain excretions. The blood, in its swiftly flowing current, carries warmth from the tissues where heat is being rapidly generated, to the tissues or organs in which it is being lost by radiation, conduction, or evaporation. Were there no arrangement by which heat could be distributed and regulated, the temperature of the body would be very unequal in different parts, and would vary at different times.

The normal temperature is maintained with slight variations throughout life. Indeed a change of more than a degree above or below the average, indicates some failure in the organism, or some unusual influence. It is evident, then, that the mechanisms which regulate the temperature of the body must be exceedingly sensitive.

The two chief means of regulating the temperature of the body are the lungs and the skin. As a means of lowering the temperature, the lungs and air passages are very inferior to the skin; although, by giving heat to the air we breathe, they stand next to the skin in importance. As a regulating power they are altogether subordinate to the skin.

Experiment 113. _To show the natural temperature of the body_. Borrow a physician’s clinical thermometer, and take your own temperature, and that of several friends, by placing the instrument under the tongue, closing the mouth, and holding it there for five minutes. It should be thoroughly cleansed after each use.

226. The Skin as a Heat-regulator. The great regulator of the bodily temperature is, undoubtedly, the skin, which performs this function by means of a self-regulating apparatus with a more or less double action. First, the skin regulates the loss of heat by means of the vaso-motor mechanism. The more blood passes through the skin, the greater will be the loss of heat by conduction, radiation, and evaporation. Hence, any action of the vaso-motor mechanism which causes dilatation of the cutaneous capillaries, leads to a larger flow of blood through the skin, and will tend to cool the body. On the other hand, when by the same mechanism the cutaneous vessels are constricted, there will be a smaller flow of blood through the skin, which will serve to check the loss of heat from the body (secs. 195 and 270).

Again, the special nerves of perspiration act directly as regulators of temperature. They increase the loss of heat when they promote the secretion of the skin, and diminish the loss when they cease to promote it.

The practical working of this heat-regulating mechanism is well shown by exercise. The bodily temperature rarely rises so much as a degree during vigorous exercise. The respiration is increased, the cutaneous capillaries become dilated from the quickened circulation, and a larger amount of blood is circulating through the skin. Besides this, the skin perspires freely. A large amount of heat is thus lost to the body, sufficient to offset the addition caused by the muscular contractions.

It is owing to the wonderful elasticity of the sweat-secreting mechanism, and to the increase in respiratory activity, and the consequent increase in the amount of watery vapor given off by the lungs, that men are able to endure for days an atmosphere warmer than the blood, and even for a short time at a temperature above that of boiling water. The temperature of a Turkish bath may be as high as 150° to 175° F. But an atmospheric temperature may be considerably below this, and yet if long continued becomes dangerous to life. In August, 1896, for instance, hundreds of persons died in this country, within a few days, from the effects of the excessive heat.

A much higher temperature may be borne in dry air than in humid air, or that which is saturated with watery vapor. Thus, a shade temperature of 100° F. in the dry air of a high plain may be quite tolerable, while a temperature of 80° F. in the moisture-laden atmosphere of less elevated regions, is oppressive. The reason is that in dry air the sweat evaporates freely, and cools the skin. In saturated air at the bodily temperature there is little loss of heat by perspiration, or by evaporation from the bodily surface.

This topic is again discussed in the description of the skin as a regulator of the bodily temperature (sec. 241).

227. Voluntary Means of Regulating the Temperature. The voluntary factor, as a means of regulating the heat loss in man, is one of great importance. Clothing retards the loss of heat by keeping in contact with it a layer of still air, which is an exceedingly bad conductor. When a man feels too warm and throws off his coat, he removes one of the badly conducting layers of air, and increases the heat loss by radiation and conduction. The vapor next the skin is thus allowed a freer access to the surface, and the loss of heat by evaporation of the sweat becomes greater. This voluntary factor by which the equilibrium is maintained must be regarded as of great importance. This power also exists in the lower animals, but to a much smaller extent. Thus a dog, on a hot day, runs out his tongue and stretches his limbs so as to increase the surface from which heat is radiated and conducted.

The production, like the loss, of heat is to a certain extent under the control of the will. Work increases the production of heat, and rest, especially sleep, lessens it. Thus the inhabitants of very hot countries seek relief during the hottest part of the day by a siesta. The quantity and quality of food also influence the production of heat. A larger quantity of food is taken in winter than in summer. Among the inhabitants of the northern and Arctic regions, the daily consumption of food is far greater than in temperate and tropical climates.

228. Effect of Alcohol upon the Lungs. It is a well recognized fact that alcohol when taken into the stomach is carried from that organ to the liver, where, by the baneful directness of its presence, it produces a speedy and often disastrous effect. But the trail of its malign power does not disappear there. From the liver it passes to the right side of the heart, and thence to the lungs, where its influence is still for harm.

In the lungs, alcohol tends to check and diminish the breathing capacity of these organs. This effect follows from the partial paralyzing influence of the stupefying agent upon the sympathetic nervous system, diminishing its sensibility to the impulse of healthful respiration. This diminished capacity for respiration is clearly shown by the use of the _spirometer_, a simple instrument which accurately records the cubic measure of the lungs, and proves beyond denial the decrease of the lung space.

“Most familiar and most dangerous is the drinking man’s inability to resist lung diseases.”–Dr. Adoph Frick, the eminent German physiologist of Zurich.

“Alcohol, instead of preventing consumption, as was once believed, reduces the vitality so much as to render the system unusually susceptible to that fatal disease.”–R. S. Tracy, M.D., Sanitary Inspector of the N. Y. City Health Dept.

“In thirty cases in which alcoholic phthisis was present a dense, fibroid, pigmented change was almost invariably present in some portion of the lung far more frequently than in other cases of phthisis.”–_Annual of Medical Sciences_.

“There is no form of consumption so fatal as that from alcohol. Medicines affect the disease but little, the most judicious diet fails, and change of air accomplishes but slight real good…. In plain terms, there is no remedy whatever for alcoholic phthisis. It may be delayed in its course, but it is never stopped; and not infrequently, instead of being delayed, it runs on to a fatal termination more rapidly than is common in any other type of the disorder.”–Dr. B. W. Richardson in _Diseases of Modern Life_.

229. Other Results of Intoxicants upon the Lungs. But a more potent injury to the lungs comes from another cause. The lungs are the arena where is carried on the ceaseless interchange of elements that is necessary to the processes of life. Here the dark venous blood, loaded with effete material, lays down its carbon burden and, with the brightening company of oxygen, begins again its circuit. But the enemy intrudes, and the use of alcohol tends to prevent this benign interchange.

The continued congestion of the lung tissue results in its becoming thickened and hardened, thus obstructing the absorption of oxygen, and the escape of carbon dioxid. Besides this, alcohol destroys the integrity of the red globules, causing them to shrink and harden, and impairing their power to receive oxygen. Thus the blood that leaves the lungs conveys an excess of the poisonous carbon dioxid, and a deficiency of the needful oxygen. This is plainly shown in the purplish countenance of the inebriate, crowded with enlarged veins. This discoloration of the face is in a measure reproduced upon the congested mucous membrane of the lungs. It is also proved beyond question by the decreased amount of carbon dioxid thrown off in the expired breath of any person who has used alcoholics.

The enfeebled respiration explains (though it is only one of the reasons) why inebriates cannot endure vigorous and prolonged exertion as can a healthy person. The hurried circulation produced by intoxicants involves in turn quickened respiration, which means more rapid exhaustion of the life forces. The use of intoxicants involves a repeated dilatation of the capillaries, which steadily diminishes their defensive power, rendering the person more liable to yield to the invasion of pulmonary diseases.[38]

230. Effect of Alcoholics upon Disease. A theory has prevailed, to a limited extent, that the use of intoxicants may act as a preventive of consumption. The records of medical science fail to show any proof whatever to support this impression. No error could be more serious or more misleading, for the truth is in precisely the opposite direction. Instead of preventing, alcohol tends to develop consumption. Many physicians of large experience record the existence of a distinctly recognized alcoholic consumption, attacking those constitutions broken down by dissipation. This form of consumption is steadily progressive, and always fatal.

The constitutional debility produced by the habit of using alcoholic beverages tends to render one a prompt victim to the more severe diseases, as pneumonia, and especially epidemical diseases, which sweep away vast numbers of victims every year.

231. Effect of Tobacco upon the Respiratory Passages. The effects of tobacco upon the throat and lungs are frequently very marked and persistent. The hot smoke must very naturally be an irritant, as the mouth and nostrils were not made as a chimney for heated and narcotic vapors. The smoke is an irritant, both by its temperature and from its destructive ingredients, the carbon soot and the ammonia which it conveys. It irritates and dries the mucous membrane of the mouth and throat, producing an unnatural thirst which becomes an enticement to the use of intoxicating liquors. The inflammation of the mouth and throat is apt to extend up the Eustachian tube, thus impairing the sense of hearing.

But even these are not all the bad effects of tobacco. The inhalation of the poisonous smoke produces unhealthful effects upon the delicate mucous membrane of the bronchial tubes and of the lungs. Upon the former the effect is to produce an irritating cough, with short breath and chronic bronchial catarrh. The pulmonary membrane is congested, taking cold becomes easy, and recovery from it tedious. Frequently the respiration is seriously disturbed, thus the blood is imperfectly aërated, and so in turn the nutrition of the entire system is impaired. The cigarette is the defiling medium through which these direful results frequently invade the system, and the easily moulded condition of youth yields readily to the destructive snare.

“The first effect of a cigar upon any one demonstrates that tobacco can poison by its smoke and through the lungs.”–London _Lancet_.

“The action of the heart and lungs is impaired by the influence of the narcotic on the nervous system, but a morbid state of the larynx, trachea, and lungs results from the direct action of the smoke.”–Dr. Laycock, Professor of Medicine in the University of Edinburgh.

Additional Experiments.

Experiment 114. _To illustrate the arrangement of the lungs and the two pleuræ._ Place a large sponge which will represent the lungs in a thin paper bag which just fits it; this will represent the pulmonary layer of the pleura. Place the sponge and paper bag inside a second paper bag, which will represent the parietal layer of the pleura. Join the mouths of the two bags. The two surfaces of the bags which are now in contact will represent the two moistened surfaces of the pleuræ, which rub together in breathing.

Experiment 115. _To show how the lungs may be filled with air._ Take one of the lungs saved from Experiment 110. Tie a glass tube six inches long into the larynx. Attach a piece of rubber to one end of the glass tube. Now inflate the lung several times, and let it collapse. When distended, examine every part of it.

Experiment 116. _To take your own bodily temperature or that of a friend._ If you cannot obtain the use of a physician’s clinical thermometer, unfasten one of the little thermometers found on so many calendars and advertising sheets. Hold it for five minutes under the tongue with the lips closed. Read it while in position or the instant it is removed. The natural temperature of the mouth is about 98½° F.

Experiment 117. _To show the vocal cords._ Get a pig’s windpipe in perfect order, from the butcher, to show the vocal cords. Once secured, it can be kept for an indefinite time in glycerine and water or dilute alcohol.

Experiment 118. _To show that the air we expire is warm._ Breathe on a thermometer for a few minutes. The mercury will rise rapidly.

Experiment 119. _To show that expired cur is moist_. Breathe on a mirror, or a knife blade, or any polished metallic surface, and note the deposit of moisture.

Experiment 120. _To show that the expired air contains carbon dioxid_. Put a glass tube into a bottle of lime water and breathe through the tube. The A liquid will soon become cloudy, because the carbon dioxid of the expired air throws down the lime held in solution.

Experiment 121. “A substitute for a clinical thermometer may be readily contrived by taking an ordinary house thermometer from its tin case, and cutting off the lower part of the scale so that the bulb may project freely. With this instrument the pupils may take their own and each other’s temperatures, and it will be found that whatever the season of the year or the temperature of the room, the thermometer in the mouth will record about 99° F. Care must, of course, be taken to keep the thermometer in the mouth till it ceases to rise, and to read while it is still in position.”–Professor H. P. Bowditch.

Experiment 122. _To illustrate the manner in which the movements of inspiration cause the air to enter the lungs._ Fit up an apparatus, as represented in Fig. 95, in which a stout glass tube is provided with a sound cork, B, and also an air-tight piston, D, resembling that of an ordinary syringe. A short tube, A, passing through the cork, has a small India-rubber bag, C, tied to it. Fit the cork in the tube while the piston is near the top. Now, by lowering the piston we increase the capacity of the cavity containing the bag. The pressure outside the bag is thus lowered, and air rushes into it through the tube, A, till a balance is restored. The bag is thus stretched. As soon as we let go the piston, the elasticity of the bag, being free to act, Movements of drives out the air just taken in, and the piston returns to its former place.

[Illustration: Fig. 95. Apparatus for Illustrating the Movements of Respiration.]

It will be noticed that in this experiment the elastic bag and its tube represent the lungs and trachea; and the glass vessel enclosing it, the thorax.

For additional experiments on the mechanics of respiration, see Chapter XV.

Chapter IX.

The Skin and the Kidneys.

232. The Elimination of Waste Products. We have traced the food from the alimentary canal into the blood. We have learned that various food materials, prepared by the digestive processes, are taken up by the branches of the portal vein, or by the lymphatics, and carried into the blood current. The nutritive material thus absorbed is conveyed by the blood plasma and the lymph to the various tissues to provide them with nourishment.

We have learned also that oxygen, taken up in the air cells of the lungs, is being continually carried to the tissues, and that the blood is purified by being deprived in the lungs of its excess of carbon dioxid. From this tissue activity, which is mainly oxidation, are formed certain waste products which, as we have seen, are absorbed by the capillaries and lymphatics and carried into the venous circulation.

In their passage through the blood and tissues, the albumens, sugars, starches, and fats are converted into carbon dioxid, water, and urea, or some closely allied body. Certain articles of food also contain small amounts of sulphur and phosphorus, which undergo oxidation into sulphates and phosphates. We speak, then, of carbon dioxid, salts, and water as waste products of the animal economy. These leave the body by one of the three main channels,–the lungs, the skin, or the kidneys.

The elimination of these products is brought about by a special apparatus called organs of excretion. The worn-out substances themselves are called excretions, as opposed to secretions, which are elaborated for use in the body. (See note, p. 121.) As already shown, the lungs are the main channels for the elimination of carbon dioxid, and of a portion of water as vapor. By the skin the body gets rid of a small portion of salts, a little carbon dioxid, and a large amount of water in the form of perspiration. From the kidneys are eliminated nearly all the urea and allied bodies, the main portion of the salts, and a large amount of water. In fact, practically all the nitrogenous waste leaves the body by the kidneys.

[Illustration: Fig. 96.–Diagrammatic Scheme to illustrate in a very General Way Absorption and Excretion.

A, represents the alimentary canal;
L, the pulmonary surface;
K, the surface of the renal epithelium; S, the skin;
o, oxygen;
h, hydrogen,;
n, nitrogen.

233. The Skin. The skin is an important and unique organ of the body. It is a blood-purifying organ as truly as are the lungs and the kidneys, while it also performs other and complex duties. It is not merely a protective covering for the surface of the body. This is indeed the most apparent, but in some respectes, the lest important, of its functions. This protective duty is necessary and efficient, as is proved by the familiar experience of the pain when a portion of the outer skin has been removed.

The skin, being richly supplied with nerves, is an important organ of sensibility and touch. In some parts it is closely attached to the structures beneath, while in others it is less firmly adherent and rests upon a variable amount of fatty tissue. It thus assists in relieving the abrupt projections and depressions of the general surface, and in giving roundness and symmetry to the entire body. The thickness of the skin varies in different parts of the body. Where exposed to pressure and friction, as on the soles of the feet and in the palms of the hands, it is much thickened.

The true skin is 1/12 to ⅛ of an inch in thickness, but in certain parts, as in the lips and ear passages, it is often not more than 1/100 of an inch thick. At the orifices of the body, as at the mouth, ears, and nose, the skin gradually passes into mucous membrane, the structure of the two being practically identical. As the skin is an outside covering, so is the mucous membrane a more delicate inside lining for all cavities into which the apertures open, as the alimentary canal and the lungs.

[Illustration: Fig. 97.–A Layer of the Cuticle from the Palm of the Hand. (Detached by maceration.)]

The skin ranks as an important organ of excretion, its product being sweat, excreted by the sweat glands. The amount of this excretion evaporated from the general surface is very considerable, and is modified as becomes necessary from the varied conditions of the temperature. The skin also plays an important part in regulating the bodily temperature(sec. 241).

234. The Cutis Vera, or True Skin. The skin is remarkably complex in its structure, and is divided into two distinct layers, which may be readily separated: the deeper layer,–the true skin, dermis, or corium; and the superficial layer, or outer skin,–the epidermis, cuticle, or scarf skin.

The true skin consists of elastic and white fibrous tissue, the bundles of which interlace in every direction. Throughout this feltwork structure which gradually passes into areolar tissue are numerous muscular fibers, as about the hair-follicles and the oil glands. When these tiny muscles contract from cold or by mental emotion, the follicles project upon the surface, producing what is called “goose flesh.”

The true skin is richly supplied with blood-vessels and nerves, as when cut it bleeds freely, and is very sensitive. The surface of the true skin is thrown into a series of minute elevations called the papillæ, upon which the outer skin is moulded. These abound in blood-vessels, lymphatics, and peculiar nerve-endings, which will be described in connection with the organ of touch (sec. 314). The papillæ are large and numerous in sensitive places, as the palms of the hands, the soles of the feet, and the fingers. They are arranged in parallel curved lines, and form the elevated ridges seen on the surface of the outer skin (Fig. 103).

235. The Epidermis, or Cuticle. Above the true skin is the epidermis. It is semi-transparent, and under the microscope resembles the scales of a fish. It is this layer that is raised by a blister.

As the epidermis has neither blood-vessels, nerves, nor lymphatics, it may be cut without bleeding or pain. Its outer surface is marked with shallow grooves which correspond to the deep furrows between the papillæ of the true skin. The inner surface is applied directly to the papillary layer of the true skin, and follows closely its inequalities. The outer skin is made up of several layers of cells, which next to the true skin are soft and active, but gradually become harder towards the surface, where they are flattened and scale-like. The upper scales are continually being rubbed off, and are replaced by deeper cells from beneath. There are new cells continually being produced in the deeper layer, which push upward the cells already existing, then gradually become dry, and are cast off as fine, white dust. Rubbing with a coarse towel after a hot bath removes countless numbers of these dead cells of the outer skin. During and after an attack of scarlet fever the patient “peels,” that is, sheds an unusual amount of the seal; cells of the cuticle.

The deeper and more active layer of the epidermis, the _mucosum_, is made up of cells some of which contain minute granules of pigment, or coloring matter, that give color to the skin. The differences in the tint, as brunette, fair, and blond, are due mainly to the amount of coloring matter in these pigment cells. In the European this amount is generally small, while in other peoples the color cells may be brown, yellow, or even black. The pinkish tint of healthy skin, and the rosy-red after a bath are due, not to the pigment cells, but to the pressure of capillaries in the true skin, the color of the blood being seen through the semi-transparent outer skin.

[Illustration: Fig. 98.–Surface of the Palm of the Hand, showing the Openings of the Sweat Glands and the Grooves between the Papillæ of the Skin. (Magnified 4 diameters.) [In the smaller figure the same epidermal surface is shown, as seen with the naked eye.]]

Experiment 123. Of course the living skin can be examined only in a general way. Stretch and pull it, and notice that it is elastic. Note any liver spots, white scars, moles, warts, etc. Examine the outer skin carefully with a strong magnifying glass. Study the papillæ on the palms. Scrape off with a sharp knife a few bits of the scarf skin, and examine them with the microscope.

236. The Hair. Hairs varying in size cover nearly the entire body, except a few portions, as the upper eyelids, the palms of the hands, and the soles of the feet.

The length and diameter of the hairs vary in different persons, especially in the long, soft hairs of the head and beard. The average number of hairs upon a square inch of the scalp is about 1000, and the number upon the entire head is estimated as about 120,000.

Healthy hair is quite elastic, and may be stretched from one-fifth to one-third more than its original length. An ordinary hair from the head will support a weight of six to seven ounces. The hair may become strongly electrified by friction, especially when brushed vigorously in cold, dry weather. Another peculiarity of the hair is that it readily absorbs moisture.

237. Structure of the Hair. The hair and the nails are structures connected with the skin, being modified forms of the epidermis. A hair is formed by a depression, or furrow, the inner walls of which consist of the infolded outer skin. This depression takes the form of a sac and is called the hair-follicle, in which the roots of the hair are embedded. At the bottom of the follicle there is an upward projection of the true skin, a papilla, which contains blood-vessels and nerves. It is covered with epidermic cells which multiply rapidly, thus accounting for the rapid growth of the hair. Around each papilla is a bulbous expansion, the hair bulb, from which the hair begins to grow.

[Illustration: Fig. 99.–Epidermis of the Foot.

It will be noticed that there are only a few orifices of the sweat glands in this region. (Magnified 8 diameters.)]

The cells on the papillæ are the means by which the hairs grow. As these are pushed upwards by new ones formed beneath, they are compressed, and the shape of the follicle determines their cylindrical growth, the shaft of the hair. So closely are these cells welded to form the cylinder, that even under a microscope the hair presents only a fibrous appearance, except in the center, where the cells are larger, forming the medulla, or pith (Fig. 106).

The medulla of the hair contains the pigment granules or coloring matter,