[Fig. 20]

Fig. 20—Vein split open to show the valves.

*Why the Arteries are Elastic.*—The elasticity of the arteries serves a twofold purpose. It keeps the arteries from bursting when the blood is forced into them from the ventricles, and it is a means of supplying pressure to the blood while the ventricles are in a condition of relaxation. The latter purpose is accomplished as follows:

Contraction of the ventricles fills the arteries overfull, causing them to swell out and make room for the excess of blood. Then while the ventricles are resting and filling, the stretched arteries press upon the blood to keep it flowing into the capillaries. In this way they cause the intermittent flow from, the heart to become a steady stream in the capillaries.

The swelling of the arteries at each contraction of the ventricle is easily felt at certain places in the body, such as the wrist. This expansion, known as the "pulse," is the chief means employed by the physician in determining the force and rapidity of the heart's action.

*Purpose of the Valves in the Veins.*—The valves in the veins are not used for directing the general flow of the blood, the valves of the heart being sufficient for this purpose. Their presence is necessary because of the pressure to which the veins are subjected in different parts of the body. The contraction of a muscle will, for example, close the small veins in its vicinity and diminish the capacity of the larger ones. The natural tendency of such pressure is to empty the veins in two directions—one in the same direction as the regular movement of the blood, but the other in the opposite direction. The valves by closing cause the contracting muscle to push the blood in one direction only—toward the heart. The valves in the veins are, therefore, an economical device for enabling variable pressure in different parts of the body to assist in the circulation. Veins like the inferior vena cava and the veins of the brain, which are not compressed by movements of the body, do not have valves.

*Purposes of the Muscular Coat.*—The muscular coat, which is thicker in the arteries than in the veins and is more marked in small arteries than in large ones, serves two important purposes. In the first place it, together with the elastic tissue, keeps the capacity of the blood vessels equal to the volume of the blood. Since the blood vessels are capable of holding more blood than may be present at a given time in the body, there is a liability of empty spaces occurring in these tubes. Such spaces would seriously interfere with the circulation, since the heart pressure could not then reach all parts of the blood stream. This is prevented by the contracted state, or "tone," of the blood vessels, due to the muscular coat.

In the second place, the muscular coat serves the purpose of regulating the amount of blood which any given organ or part of the body receives. This it does by varying the caliber of the arteries going to the organ in question. To increase the blood supply, the muscular coat relaxes. The arteries are then dilated by the blood pressure from within so as to let through a larger quantity of blood. To diminish the supply, the muscle contracts, making the caliber of the arteries less, so that less blood can flow to this part of the body. Since the need of organs for blood varies with their activity, the muscular coat serves in this way a very necessary purpose.

[Fig. 21]

Fig. 21—Diagram of network of capillaries between a very small artery and a very small vein. Shading indicates the change of color of the blood as it passes through the capillaries. S. Places between capillaries occupied by the cells.

*Capillaries.*—The capillaries consist of a network of minute blood vessels which connect the terminations of the smallest arteries with the beginnings of the smallest veins (Fig. 21). They have an average diameter of less than one two-thousandth of an inch (12 mu) and an average length of less than one twenty-fifth of an inch (1 millimeter). Their walls consist of a single coat which is continuous with the lining of the arteries and veins. This coat is formed of a single layer of thin, flat cells placed edge to edge (Fig. 22). With a few exceptions, the capillaries are found in great abundance in all parts of the body.

[Fig. 22]

Fig. 22—*Surface of capillary* highly magnified, showing its coat of thin cells placed edge to edge.

*Functions of the Capillaries.*—On account of the thinness of their walls, the capillaries are able to serve a twofold purpose in the body:

1. They admit materials into the blood vessels.

2. They allow materials to pass from the blood vessels to the surrounding tissues.

When it is remembered that the blood, as blood, does not escape from the blood vessels under normal conditions, the importance of the work of the capillaries is apparent. To serve its purpose as a carrier, there must be places where the blood can load up with the materials which it is to carry, and places also where these can be unloaded. Such places are supplied by the capillaries.

The capillaries also serve the purpose of spreading the blood out and of bringing it very near the individual cells in all parts of the body (Fig. 21).

*Functions of Arteries and Veins.*—While the capillaries provide the means whereby materials may both enter and leave the blood, the arteries and veins serve the general purpose of passing the blood from one set of capillaries to another. Since pressure is necessary for moving the blood, these tubes must connect with the source of the pressure, which is the heart. In the arteries and veins the blood neither receives nor gives up material, but having received or given up material at one set of capillaries, it is then pushed through these tubes to where it can serve a similar purpose in another set of capillaries (Fig. 23).

*Divisions of the Circulation.*—Man, in common with all warm-blooded animals, has a double circulation, a fact which explains the double structure of his heart. The two divisions are known as the pulmonary and the systemic circulations. By the former the blood passes from the right ventricle through the lungs, and is then returned to the left auricle; by the latter it passes from the left ventricle through all parts of the body, returning to the right auricle.

The general plan of the circulation is indicated in Fig. 23. All the blood flows continuously through both circulations and passes the various parts in the following order: right auricle, tricuspid valve, right ventricle, right semilunar valve, pulmonary artery and its branches, capillaries of the lungs, pulmonary veins, left auricle, mitral valve, left ventricle, left semilunar valve, aorta and its branches, systemic capillaries, the smaller veins, superior and inferior venae cavae, and then again into the right auricle.

In the pulmonary capillaries the blood gives up carbon dioxide and receives oxygen, changing from a dark red to a bright red color. In the systemic capillaries it gives up oxygen, receives carbon dioxide and other impurities, and changes back to a dark red color.

In addition to the two main divisions of the circulation, special circuits are found in various places. Such a circuit in the liver is called the portal circulation, and another in the kidneys is termed the renal circulation. To some extent the blood supply to the walls of the heart is also outside of the general movement; it is called the coronary circulation.

[Fig. 23]

Fig. 23—*General scheme of the circulation*, showing places where the blood takes on and gives off materials. 1. Body in general. 2. Lungs. 3. Kidneys. 4. Liver. 5. Organs of digestion. 6. Lymph ducts. 7. Pulmonary artery. 8. Aorta.

*Blood Pressure and Velocity.*—The blood, in obedience to physical laws, passes continuously through the blood vessels, moving always from a place of greater to one of less pressure. Through the contraction of the ventricles, a relatively high pressure is maintained in the arteries nearest the heart.(21) This pressure diminishes rapidly in the small arteries, becomes comparatively slight in the capillaries, and falls practically to nothing in the veins. Near the heart in the superior and inferior venae cavae, the pressure at intervals is said to be negative. This means that the blood from these veins is actually drawn into the right auricle by the expansion of the chest walls in breathing.(22)

The velocity of the blood is greatest in the arteries, less in the veins, and much less in the capillaries than in either the arteries or the veins. The slower flow of the blood through the capillaries is accounted for by the fact that their united area is many times greater than that of the arteries which supply, or the veins which relieve, them. This allows the same quantity of blood, flowing through them in a given time, a wider channel and causes it to move more slowly. The time required for a complete circulation is less than one minute.

*Summary of Causes of Circulation.*—The chief factor in the circulation of the blood is, of course, the heart. The ventricles keep a pressure on the blood which is sufficient to force it through all the blood tubes and back to the auricles. The heart is aided in its work by the elasticity of the arteries, which keeps the blood under pressure while the ventricles are in a state of relaxation. It is also aided by the muscles and elastic tissue in all of the blood vessels. These, by keeping the blood vessels in a state of "tone," or so contracted that their capacity just equals the volume of the blood, enable pressure from the heart to be transmitted to all parts of the blood stream. A further aid to the circulation is found in the valves in the veins, which enable muscular contraction within the body, and variable pressure upon its surface, to drive the blood toward the heart. The heart is also aided to some extent by the movements of the chest walls in breathing. The organs Of circulation are under the control of the nervous system (Chapter XVIII).



HYGIENE OF THE CIRCULATION

*Care of the Heart.*—The heart, consisting largely of muscle, is subject to the laws of muscular exercise. It may be injured by over-exertion, but is strengthened by a moderate increase in its usual work.(23) It may even be subjected to great exertion without danger, if it be trained by gradually increasing its work. Such training, by giving the heart time to gain in size and strength, prepares it for tasks that could not at first be accomplished.

In taking up a new exercise requiring considerable exertion, precautions should be observed to prevent an overstrain of the heart. The heart of the amateur athlete, bicyclist, or mountain climber is frequently injured by attempting more than the previous training warrants. The new work should be taken up gradually, and feats requiring a large outlay of physical energy should be attempted only after long periods of training.

Since the heart is controlled by the nervous system, it frequently becomes irregular in its action through conditions that exhaust the nervous energy. Palpitations of the heart, the missing of beats, and pains in the heart region frequently arise from this cause. It is through their effect upon the nervous system that worry, overstudy, undue excitement, and dissipation cause disturbances of the heart. In all such cases the remedy lies in the removal of the cause. The nervous system should also be "toned up" through rest, plenty of sleep, and moderate exercise in the open air.

*Effect of Drugs.*—A number of substances classed as drugs, mainly by their action on the nervous system, produce undesirable effects upon the organs of circulation. Unfortunately some of these are extensively used, alcohol being one of them. If taken in any but small quantities, alcohol is a disturbing factor in the circulation. It increases the rate of the heart beat and dilates the capillaries. Its effect upon the capillaries is shown by the "bloodshot" eye and the "red nose" of the hard drinker. Another bad effect from the use of much alcohol is the weakening of the heart through the accumulation of fat around this organ and within the heart muscle. The use of alcohol also leads in many cases to a hardening of the walls of the arteries, such as occurs in old age. This effect makes the use of alcohol especially dangerous for those in advanced years.

Tobacco contains a drug, called nicotine, which has a bad effect upon the heart in at least two ways: 1. When the use of tobacco is begun in early life, it interferes with the growth of the heart, leading to its weakness in the adult. 2. When used in considerable quantity, by young or old, it causes a nervous condition both distressing and dangerous, known as "tobacco heart."

Tea and coffee contain a drug, called caffeine, which acts upon the nervous system and which may, on this account, interfere with the proper control of the heart. In some individuals the taking of a very small amount of either tea or coffee is sufficient to cause irregularities in the action of the heart. Tea is considered the milder of the two liquids and the one less liable to injure.

*Effect of Rheumatism.*—The disease which affects the heart more frequently than any other is rheumatism. This attacks the lining membrane, or endocardium, and causes, not infrequently, a shrinkage of the heart valves. The heart is thus rendered defective and, to perform its function in the body, must work harder than if it were in a normal condition. Rheumatic attacks of the heart do most harm when they occur in early life—the period when the valves are the most easily affected. Any tendency toward rheumatism in children has, therefore, a serious significance and should receive the attention of the physician. Any one having a defective heart should avoid all forms of exercise that demand great exertion.

*Strengthening of the Blood Vessels.*—Disturbances of the circulation, causing too much blood to be sent to certain parts of the body and an insufficient amount to others, when resulting from slight causes, are usually due to weakness of the walls of the blood vessels, particularly of the muscular coat. Such weakness is frequently indicated by extreme sensitiveness to heat or cold and by a tendency to "catch cold." From a health standpoint the preservation of the normal muscular "tone" of the blood vessels is a problem of great importance. Though the muscles of the blood vessels cannot be exercised in the same manner as the voluntary muscles, they may be called actively into play through all the conditions that induce changes in the blood supply to different parts of the body. The usual forms of physical exercise necessitate such changes and indirectly exercise the muscular coat. The exposure of the body to cold for short intervals, because of the changes in the circulation which this induces, also serves the same purpose. A cold bath taken with proper precautions is beneficial to the circulation of many and so also is a brisk walk on a frosty morning. Both indirectly exercise and strengthen the muscular coat of the blood vessels. On the other hand, too much time spent indoors, especially in overheated rooms, leads to a weakening of the muscular coat and should be avoided.

*Checking of Flow of Blood from Wounds.*—The loss of any considerable quantity of blood is such a serious matter that every one should know the simpler methods of checking its flow from wounds. In small wounds the flow is easily checked by binding cotton or linen fiber over the place. The absorbent cotton, sold in small packages at drug stores, is excellent for this purpose and should be kept in every home. A simple method of checking "nosebleed" is that of drawing air through the bleeding nostril, while the other nostril is compressed with the finger.(24) Another method is to "press with the finger (or insert a small roll of paper) under the lip against the base of the nose." (25) Where the bleeding is persistent, the nostril should be plugged with a small roll of clean cotton or paper. When this is done, the plug should not be removed too soon because of the likelihood of starting the flow afresh.

In dealing with large wounds the services of a physician are indispensable. But in waiting for the physician to arrive temporary aid must be rendered. The one who gives such aid should first decide whether an artery or a vein has been injured. This is easily determined by the nature of the blood stream, which is in jets, or spurts, from an artery, but flows steadily from a vein. If an artery is injured, the limb should be tightly bandaged on the side of the wound nearest the heart; if a vein, on the side farthest from the heart. In addition to this, the edges of the wound should be closed and covered with cotton fiber and the limb should be placed on a support above the level of the rest of the body. A large handkerchief makes a convenient bandage if properly applied. This should be folded diagonally and a knot tied in the middle. Opposite ends are then tied, making a loose-fitting loop around the limb. The knot is placed directly over the blood vessel to be compressed and a short stick inserted in the loop. The necessary pressure is then applied by twisting the handkerchief with the stick. Time must not be lost, however, in the preparation of a suitable bandage. The blood vessel should be compressed with the fingers while the bandage is being prepared.

*Summary.*—The blood, to serve as a transporting agent, must be kept continually moving through all parts of the body. The blood vessels hold the blood, supply the channels and force necessary for its circulation, and provide conditions which enable materials both to enter and to leave the blood stream. The heart is the chief factor in propelling the blood, although the muscles and the elastic tissue in the walls of the arteries and the valves in the veins are necessary aids in the process. In the capillaries the blood takes on and gives off materials, while the arteries and veins serve chiefly as tubes for conveying the blood from one system of capillaries to another.

*Exercises.*—1. Of what special value in the study of the body was the discovery of the circulation of the blood?

2. State the necessity for a circulating liquid in the body.

3. Show by a drawing the general plan of the heart, locating and naming the essential parts. Show also the connection of the large blood vessels with the cavities of the heart.

4. Compare the purpose served by the chordae tendineae to that served by doorstops (the strips against which the door strikes in closing).

5. Explain how the heart propels the blood. To what class of pumps does it belong? What special work is performed by each of its divisions?

6. Define a valve. Of what use are the valves in the heart? In the veins?

7. By what means is pressure from contracting muscles in different parts of the body made to assist in the circulation?

8. Of what advantage is the elasticity of the arteries?

9. How is blood forced from the capillaries back to the heart?

10. Why should there be a difference in structure between the two sides of the heart?

11. Following Fig. 23, trace the blood through a complete circulation, naming all the divisions of the system in the order of the flow of the blood.

12. If the period of rest following the period of contraction of the heart be as long as the period of contraction, how many hours is the heart able to rest out of every twenty-four?

13. State the functions of the capillaries. Show how their structure adapts them to their work.

14. What kind of physical exercise tends to strengthen the heart? What forms of exercise tend to injure it? State the effects of alcohol and tobacco on the heart.

15. How may rheumatism injure the heart?

16. Give directions for checking the flow of blood from small and from large blood vessels.



PRACTICAL WORK

In showing the relations of the different parts of the heart, a large dissectible model is of great service (Fig. 24). Indeed, where the time of the class is limited, the practical work may be confined to the study of the heart model, diagrams of the heart and the circulation, and a few simple experiments. However, where the course is more extended, the dissection of the heart of some animal as described below is strongly advised.

*Observations on the Heart.*—Procure, by the assistance of a butcher, the heart of a sheep, calf, or hog. To insure the specimen against mutilation, the lungs and the diaphragm must be left attached to the heart. In studying the different parts, good results will be obtained by observing the following order:

1. Observe the connection of the heart to the lungs, diaphragm, and large blood vessels. Inflate the lungs and observe the position of the heart with reference to them.

2. Examine the sac surrounding the heart, called the pericardium. Pierce its lower portion and collect the pericardial fluid. Increase the opening thus made until it is large enough to slip the heart out through it. Then slide back the pericardium until its connection with the large blood vessels above the heart is found. Observe that a thin layer of it continues down from this attachment, forming the outer covering of the heart.

3. Trace out for a short distance and study the veins and arteries connected with the heart. The arteries are to be distinguished by their thick walls. The heart may now be severed from the lungs by cutting the large blood vessels, care being taken to leave a considerable length of each one attached to the heart.

[Fig. 24]

Fig. 24—Model for demonstrating the heart.

4. Observe the outside of the heart. The thick, lower portion contains the cavities called ventricles; the thin, upper, ear-shaped portions are the auricles. The thicker and denser side lies toward the left of the animal's body and is called the left side of the heart; the other is the right side. Locate the right auricle and the right ventricle; the left auricle and the left ventricle.

5. Lay the heart on the table with the front side up and the apex pointing from the operator. This places the left side of the heart to his left and the right side to his right. Notice the groove between the ventricles, called the inter-ventricular groove. Make an incision half an inch to the right of this groove and cut toward the base of the heart until the pulmonary artery is laid open. Then, following within half an inch of the groove, cut down and around the right side of the heart. The wall of the right ventricle may now be raised and the cavity exposed. Observe the extent of the cavity, its shape, its lining, its columns of muscles, its half columns of muscles, its tendons (chordae tendineae), the tricuspid valve from the under side, etc. Also notice the valve at the beginning of the pulmonary artery (the right semilunar) and the sinuses, or depressions, in the artery immediately behind its divisions.

6. Now cut through the middle of the loosened ventricular wall from the apex to the middle of the right auricle, laying it open for observation. Observe the openings into the auricle, there being one each for the vena cava superior, the vena cava inferior, and the coronary vein. Compare the walls, lining, shape, size, etc., with the ventricle below.

7. Cut off the end of the left ventricle about an inch above the apex. This will show the extension of the cavity to the apex; it will also show the thickness of the walls and the shape of the cavity. Split up the ventricular wall far enough to examine the mitral valve and the chordae tendineae from the lower side.

8. Make an incision in the left auricle. Examine its inner surface and find the places of entrance of the pulmonary veins. Examine the mitral valve from above. Compare the two sides of the heart, part for part.

9. Separate the aorta from the other blood vessels and cut it entirely free from the heart, care being taken to leave enough of the heart attached to the artery to insure the semilunar valve's being left in good condition. After tying or plugging up the holes in the sides of the artery, pour water into the small end and observe the closing of the semilunar valve. Repeat the experiment until the action of the valve is understood. Sketch the artery, showing the valve in a closed condition.

*To illustrate the Action of a Ventricle.*—Procure a syringe bulb with an opening at each end. Connect a rubber tube with each opening, letting the tubes reach into two tumblers containing water. By alternately compressing and releasing the bulb, water is pumped from one vessel into the other. The bulb may be taken to represent one of the ventricles. What action of the ventricle is represented by compressing the bulb? By releasing the pressure? Show by a sectional drawing the arrangement of the valves in the syringe bulb.

[Fig. 25]

Fig. 25—Illustrating elasticity of arteries.

*To show the Advantage of the Elasticity of Arteries.*—Connect the syringe bulb used in the last experiment with a rubber tube three or four feet in length and having rather thin walls. In the opposite end of the rubber tube insert a short glass tube which has been drawn (by heating) to a fine point (Fig. 25). Pump water into the rubber tube, observing:

1. The swelling of the tube (pulse) as the water is forced into it. (This is best observed by placing the fingers on the tube.)

2. The forcing of water from the pointed tubs during the interval when no pressure is being applied from the bulb. Compare with the action of the arteries when blood is forced into them from the ventricles.

Repeat the experiment, using a long glass tube terminating in a point instead of the rubber tube. (In fitting the glass tube to the bulb use a very short rubber tube.) Observe and account for the differences in the flow of water through the inelastic tube.

*To show the Advantage of Valves in the Veins.*—Attach an open glass tube one foot in length to each end of the rubber tube used in the preceding experiment and fill with water (by sucking) to within about six inches of the end. Lay on the table with the glass tubes secured in an upright position (Fig. 26). Now compress the tube with the hand, noting that the water rises in both tubes, being pushed in both directions. This effect is similar to that produced on the blood when a vein having no valves is compressed.

[Fig. 26]

Fig. 26.—*Simple apparatus* for showing advantage of valves in veins.

Now imitate the action of a valve by clamping the tube at one point, or by closing it by pressure from the finger, and then compressing with the hand some portion of the tube on the table. Observe in this instance that the water is *all* pushed in the same direction. The movement of the water is now like the effect produced on the blood in veins having valves when the veins are compressed.

*To show the Position of the Valves in the Veins.*—Exercise the arm and hand for a moment to increase the blood supply. Expose the forearm and examine the veins on its surface. With a finger, stroke one of the veins toward the heart, noting that, as the blood is pushed along on one side of the finger the blood follows on the other side. Now stroke the vein toward the hand. Places are found beyond which the blood does not follow the finger. These mark the positions of valves.

*To show Effect of Exercise upon the Circulation.*—1. With a finger on the "pulse" at the wrist or temple, count the number of heart beats during a period of one minute under the following conditions: (a) when sitting; (b) when standing; (c) after active exercise, as running. What relation, if any, do these observations indicate between the general activity of the body and the work of the heart?

2. Compare the size of the veins on the backs of the hands when they are placed side by side on a table. Then exercise briskly the right hand and arm, clenching and unclenching the fist and flexing the arm at the elbow. Place the hands again side by side and, after waiting a minute, observe the increase in the size of the veins in the hand exercised. How is this accounted for?

*To Show the Effect of Gravity on the Circulation.*—Hold one hand high above the head, at the same time letting the other hand hang loosely by the side. Observe the difference in the color of the hands and the degree to which the large veins are filled. Repeat the experiment, reversing the position of the hands. What results are observed? In what parts of the body does gravity aid in the return of the blood to the heart? In what parts does it hinder? Where fainting is caused by lack of blood in the brain (the usual cause), is it better to let the patient lie down flat or to force him into a sitting posture?

*To study the Circulation in a Frog's Foot* (Optional).—A compound microscope is needed in this study and for extended examination it is best to destroy the frog's brain. This is done by inserting some blunt-pointed instrument into the skull cavity from the neck and moving it about. A small frog, on account of the thinness of its webs, gives the best results. It should be attached to a thin board which has an opening in one end over which the web of the foot may be stretched. Threads should extend from two of the toes to pins driven into the board to secure the necessary tension of the web, and the foot and lower leg should be kept moist. Using a two-thirds-inch objective, observe the branching of the small arteries into the capillaries and the union of the capillaries to form the small veins. The appearance is truly wonderful, but allowance must be made for the fact that the motion of the blood is magnified, as well as the different structures, and that it appears to move much faster than it really does. With a still higher power, the movements of the corpuscles through the capillaries may be studied.

NOTE.—To perform this experiment without destroying the brain, the frog is first carefully wrapped with strips of wet cloth and securely tied to the board. The wrapping, while preventing movements of the frog, must not interfere with the circulation.



CHAPTER VI - THE LYMPH AND ITS MOVEMENT THROUGH THE BODY

[Fig. 27]

Fig. 27—*Diagram showing position of the lymph* with reference to the blood and the cells. The central tube is a capillary. The arrows indicate the direction of slight movements in the lymph.

The blood, it will be remembered, moves everywhere through the body in a system of closed tubes. These keep it from coming in contact with any of the cells of the body except those lining the tubes themselves. The capillaries, to be sure, bring the blood very near the cells of the different tissues; still, there is need of a liquid to fill the space between the capillaries and the cells and to transfer materials from one to the other. The lymph occupies this position and does this work. The position of the lymph with reference to the capillaries and the cells is shown in Fig. 27.

*Origin of the Lymph.*—The chief source of the lymph is the plasma of the blood. As before described, the walls of the capillaries consist of a single layer of flat cells placed edge to edge. Partly on account of the pressure upon the blood and partly on account of the natural tendency of liquids to pass through animal membranes, a considerable portion of the plasma penetrates the thin walls and enters the spaces occupied by the lymph.

The cells themselves also help to form the lymph, since the water and wastes leaving the cells add to its bulk. These mix with the plasma from the blood, forming the resultant liquid which is the lymph. A considerable amount of the material absorbed from the food canal also enters the lymph tubes, but this passes into the blood before reaching the cells.

*Composition and Physical Properties of the Lymph.*(26)—As would naturally be expected, the composition of the lymph is similar to that of the blood. In fact, nearly all the important constituents of the blood are found in the lymph, but in different proportions. Food materials for the cells are present in smaller amounts than in the blood, while impurities from the cells are in larger amounts. As a rule the red corpuscles are absent from the lymph, but the white corpuscles are present and in about the same numbers as in the blood.

The physical properties of the lymph are also similar to those of the blood. Like the blood, the lymph is denser than water and also coagulates, but it coagulates more slowly than does the blood. The most noticeable difference between these liquids is that of color, the lymph being colorless. This is due to the absence of red corpuscles. The quantity of lymph is estimated to be considerably greater than that of the blood.

*Lymph Vessels.*—Most of the lymph lies in minute cavities surrounding the cells and in close relations with the capillaries (Figs. 27 and 30). These are called lymph spaces. Connecting with the lymph spaces on the one hand, and with certain blood vessels on the other, is a system of tubes that return the lymph to the blood stream. The smallest of these, and the ones in greatest abundance, are called lymphatics. They consist of slender, thin-walled tubes, which resemble veins in structure, and, like the veins, have valves. They differ from veins, however, in being more uniform in size and in having thinner walls.

[Fig. 28]

Fig. 28—*Diagram of drainage system for the lymph.* 1. Thoracic duct. 2. Right lymphatic duct. 3. Left subclavian vein. 4. Right subclavian vein. 5. Superior vena cava. 6. Lacteals. 7. Lymphatic glands. The small tubes connecting with the lymph spaces in all parts of the body are the lymphatics.

The lymphatics in different places gradually converge toward, and empty into, the two main lymph tubes of the body. The smaller of these tubes, called the right lymphatic duct, receives the lymph from the lymphatics in the right arm, the right side of the head, and the region of the right shoulder. It connects with, and empties its contents into, the right subclavian vein at the place where it is joined by the right jugular vein (Fig. 28).

The larger of the lymph tubes is called the thoracic duct. This receives lymph from all parts of the body not drained by the right lymphatic duct, and empties it into the left subclavian vein. Connection is made with the subclavian vein on the upper side at the place where it is joined by the left jugular vein. The thoracic duct has a length of from sixteen to eighteen inches, and is about as large around as a goose quill. The lower end terminates in an enlargement in the abdominal cavity, called the receptacle of the chyle. It is provided with valves throughout its course, in addition to one of considerable size which guards the opening into the blood vessel.

The lymphatics which join the thoracic duct from the small intestine are called the lacteals (Fig. 28). These do not differ in structure from the lymphatics in other parts of the body, but they perform a special work in absorbing the digested fat (Chapter XI).

*Lymphatic Glands.*—The lymphatic glands, sometimes called lymph nodes, are small and somewhat rounded bodies situated along the course of the lymphatic tubes. They vary in size, some of them being an inch or more in length. The lymph vessels generally open into them on one side and leave them on the other (Figs. 28 and 30). They are not glands in function, but are so called because of their having the general form of glands. They provide favorable conditions for the development of white corpuscles (page 29). They also separate harmful germs and poisonous wastes from the lymph, thereby preventing their entrance into the blood.

*Relations of the Lymph, the Blood, and the Cells.*—While the blood is necessary as a carrying, or transporting, agent in the body, the lymph is necessary for transferring materials from the blood to the cells and vice versa. Serving as a physiological "go between," or medium of exchange, the lymph enables the blood to minister to the needs of the cells. But the lymph and the blood, everything considered, can hardly be looked upon as two separate and distinct liquids. Not only do they supplement each other in their work and possess striking similarities, but each is made in its movements to pass into the vessels occupied by the other, so that they are constantly mixing and mingling. For these and other reasons, they are more properly regarded as two divisions of a single liquid—one which, by adapting itself to different purposes,(27) supplies all the conditions of a nutrient fluid for the cells.

*Movements of the Lymph.*—As compared with the blood, the lymph must be classed as a quiet liquid. But, as already suggested, it has certain movements which are necessary to the purposes which it serves. A careful study shows it to have three well-defined movements as follows:

1. A movement from the capillaries toward the cells.

2. A movement from the cells toward the capillaries.

3. A movement of the entire body of lymph from the lymph spaces into the lymphatics and along these channels to the ducts through which it enters the blood.

By the first movement the cells receive their nourishment. By the second and third movements the lymph, more or less laden with impurities, is returned to the blood stream. (See Figs. 28 and 30.)

*Causes of the Lymph Movements.*—Let us consider first the movement through the lymph tubes. No pump, like the heart, is known to be connected with these tubes and to supply the pressure necessary for moving the lymph. There are, however, several forces that indirectly aid in its flow. The most important of these are as follows:

1. Blood Pressure at the Capillaries.—The plasma which is forced through the capillary walls by pressure from the heart makes room for itself by pushing a portion of the lymph out of the lymph spaces. This in turn presses upon the lymph in the tubes which it enters. In this way pressure from the heart is transmitted to the lymph, forcing it to move.

2. Variable Pressure on the Walls of the Lymph Vessels.—Pressure exerted on the sides of the lymph tubes by contracting muscles tends to close them up and to push the lymph past the valves, which, by closing, prevent its return (Fig. 29). Pressure at the surface of the body, provided that it is variable, also forces the lymph along. The valves in the lymph vessels serve the same purpose as those in the veins.

[Fig. 29]

Fig. 29—*Diagram* to show how the muscles pump lymph. A. Relaxed muscle beside which is a lymphatic tube. B. Same muscle in state of contraction.

3. The Inspiratory Force.—When the thoracic cavity is enlarged in breathing, the unbalanced atmospheric pressure is exerted from all directions towards the thoracic space. This not only causes the air to flow into the lungs (Chapter VII), but also causes a movement of the blood and lymph in such of their tubes as enter this cavity. It will be noted that both of the large lymph ducts terminate where their contents may be influenced by the respiratory movements. (See Practical Work.)

*Where the Lymph enters the Blood.*—The fact that the lymph is poured into the blood at but two places, and these very close to each other, requires a word of explanation. As a matter of fact, it is impossible for the lymph to flow into blood vessels at most places on account of the blood pressure. This would force the blood into the lymph vessels, instead of allowing the lymph to enter the blood. The lymph can enter only at some place where the blood pressure is less than the pressure that moves the lymph. Such a place is found in the thoracic cavity. As already pointed out (page 54), the blood pressure in the veins entering this cavity becomes, with each expansion of the chest, negative, i.e., less than the pressure of the atmosphere on the outside of the body. This, as we have seen, aids in the flow of the blood into the right auricle. It also aids in the passage of lymph into the blood vessels. The lymph is said to be "sucked in," which means that it is forced in by the unbalanced pressure of the atmosphere.(28) Some advantage is also gained by the lymph duct's entering the subclavian vein on the upper side and at its union with the jugular vein. Everything considered, it is found that the lymph flows into the blood vessels where it can be "drawn in" by the movements of breathing and where it meets with no opposition from the blood stream itself (Fig. 30).

[Fig. 30]

Fig. 30—*Diagram* showing general movement of lymph from the place of relatively high pressure at the lymph spaces to the place of relatively low pressure in the thoracic cavity.

*Lymph Movements at the Cells.*—The double movement of the lymph from the capillaries toward the cells and from the cells toward the capillaries is not entirely understood. Blood pressure in the capillaries undoubtedly has much to do in forcing the plasma through the capillary walls, but this tends to prevent the movement of the lymph in the opposite direction. Movements between the blood and the lymph are known to take place in part according to a general principle, known as osmosis, or dialysis.

[Fig. 31]

Fig. 31—*Vessel* with an upright membranous partition for illustrating osmosis.

*Osmosis.*—The term "osmosis" is used to designate the passage of liquids through some partition which separates them. Thus, if a vessel with an upright membranous partition be filled on the one side with pure water and on the other with water containing salt, an exchange of materials will take place through the membrane until the same proportion of salt exists on the two sides (Fig. 31). The cause of osmosis is the motion of the molecules, or minute particles, that make up the liquid substance. If the partition were not present, this motion would simply cause a mixing of the liquids.

*Conditions under which Osmosis occurs.*—Osmosis may be shown by suitable experiments (see Practical Work) to take place under the following conditions:

1. The liquids on the two sides of the partition must be unlike either in density or in composition. Since the effect of the movement is to reduce the liquids to the same condition, a difference in density causes the flow to be greater from the less dense toward the denser liquid, than in the opposite direction; while a difference in composition causes the substances in solution to move from the place of greater abundance toward places of less abundance.

2. The liquids must be capable of wetting, or penetrating, the partition. If but one of the liquids penetrates the partition, the flow will be in but one direction.

3. The liquids on the two sides of the partition must readily mix with each other.

*Osmosis at the Cells.*—In the body osmosis takes place between the blood and the lymph and between the lymph and the cells, the movements being through the capillary walls and the membranes inclosing the cells (Fig. 27). Oxygen and food materials, which are found in great abundance in the blood, are less abundant in the lymph and still less abundant in the cells. According to the principle of osmosis, the main flow of oxygen and food is from the capillaries toward the cells. On the other hand, the wastes are most abundant in the cells where they are formed, less abundant in the lymph, and least abundant in the blood. Hence the wastes flow from the cells toward the capillaries.

*Solutions.*—Neither the blood plasma nor the lymph, as already shown, are simple liquids; but they consist of water and different substances dissolved in the water. They belong to a class of substances called solutions. The chief point of interest about substances in solution is that they are very finely divided and that their little particles are free to move about in the liquid that contains them. Both the motion and the finely divided condition of the dissolved substances are necessary to the process of osmosis. All substances, however, that appear to be in solution are not able to penetrate membranes, or take part in osmosis.

*Kinds of Solutions in the Body.*—The substances in solution in the body liquids are of two general kinds known as colloids and crystalloids. The crystalloids are able to pass through membranous partitions, while the colloids are not. An example of a colloid is found in the albumin of an egg, which is unable to penetrate the membrane which surrounds it. Examples of crystalloids are found in solutions of salt and sugar in water. The inability of a colloid to penetrate a membrane is due to the fact that it does not form a true solution. Its particles (molecules), instead of being completely separated, still cling together, forming little masses that are too large to penetrate the membrane. Since, however, it has the appearance, on being mixed with water, of being dissolved, it is called a colloidal solution. The crystalloid substance, on the other hand, completely separates in the water and forms a true solution—one which is able to penetrate the partition or membrane.

*Osmosis not a Sufficient Cause.*—The passage of materials through animal membranes, according to the principle of osmosis, is limited to crystalloid substances. But colloid substances are also known to pass through the various partitions of the body. An example of such is found in the proteids of the blood which, as a colloidal solution, pass through the capillary walls to become a part of the lymph. Perhaps the best explanation offered as yet for this passage is that the colloidal substances are changed by the cells lining the capillaries into substances that form true solutions and that after the passage they are changed back again to the colloidal condition.

*Summary.*—Between the cells and the capillaries is a liquid, known as the lymph, which is similar in composition and physical properties to the blood. It consists chiefly of escaped plasma. The vessels that contain it are connected with the system for the circulation of the blood. By adding new material to the lymph and withdrawing waste material from it, the blood keeps this liquid in a suitable condition for supplying the needs of the cells. Supplementing each other in all respects, the blood and the lymph together form the nutrient cell fluid of the body. The interchange of material between the blood and the lymph, and the lymph and the cells, takes place in part according to the principle of osmosis.

*Exercises.*—1. Explain the necessity for the lymph in the body.

2. Compare lymph and water with reference to density, color, and complexity of composition.

3. Compare lymph and blood with reference to color, composition, and movement through the body.

4. Show how blood pressure in the capillaries causes a flow of the lymph.

5. Show how contracting muscles cause the lymph to move. Compare with the effect of muscular contraction upon the blood in the veins.

6. Trace the lymph in its flow from the right hand to where it enters the blood; from the feet to where it enters the blood.

7. What conditions prevail at the cells to cause a movement of food and oxygen in one direction and of waste materials in the opposite direction?

8. What part does water play in the exchanges at the cells?

9. Show that the blood and the lymph together fulfill all the requirements of a nutrient cell fluid in the body.



PRACTICAL WORK

*To illustrate the Effect of Breathing upon the Flow of Lymph.*—Tightly holding one end of a glass tube between the lips, let the other end extend into water in a tumbler on a table. In this position quickly inhale air through the nostrils, noting that with each inhalation there is a slight movement of the water up the tube. (No sucking action should be exerted by the mouth.) Apply to the movements in the large blood and lymph vessels entering the thoracic cavity.

*To illustrate Osmosis.*—1. Separate the shell from the lining membrane at one end of an egg, over an area about one inch in diameter. To do this without injuring the membrane, the shell must first be broken into small pieces and then picked off with a pair of forceps, or a small knife blade. Fit a small glass tube, eight or ten inches long, into the other end so that it will penetrate the membrane and pass down into the yolk. Securely fasten the tube to the shell by melting beeswax around it, and set the egg in a small tumbler partly filled with water. Examine in the course of half an hour. What evidence now exists that the water has passed through the membrane?

2. Tie over the large end of a "thistle tube" (used by chemists) a thin animal membrane, such as a piece of the pericardium or a strip of the membrane from around a sausage. Then fill the bulb and the lower end of the tube with a concentrated solution of some solid, such as sugar, salt, or copper sulphate. Suspend in a vessel of water so that the liquid which it contains is just on a level with the water in the vessel. Examine from time to time, looking for evidence of a movement in each direction through the membrane. Why should the movement of the water into the tube be greater than the movement in the opposite direction? (If the thistle tube has a very slender stem, it is better to fill the bulb before tying on the membrane. The opening in the stem may be plugged during the process of filling.)

[Fig. 32]

Fig. 32—An osmosometer.

NOTE.—With a special piece of apparatus, known as an osmosometer, the principle of osmosis may be more easily illustrated than by the method in either of the above experiments (Fig. 32). This apparatus may be obtained from supply houses.



CHAPTER VII - RESPIRATION

Through the movements of the blood and the lymph, materials entering the body are transported to the cells, and wastes formed at the cells are carried to the organs which remove them from the body. We are now to consider the passage of materials from outside the body to the cells and vice versa. One substance which the body constantly needs is oxygen, and one which it is constantly throwing off is carbon dioxide. Both of these are constituents of

*The Atmosphere.*—The atmosphere, or air, completely surrounds the earth as a kind of envelope, and comes in contact with everything upon its surface. It is composed chiefly of oxygen and nitrogen,(29) but it also contains a small per cent of other substances, such as water-vapor, carbon dioxide, and argon. All of the regular constituents of the atmosphere are gases, and these, as compared with liquids and solids, are very light. Nevertheless the atmosphere has weight and, on this account, exerts pressure upon everything on the earth. At the sea level, its pressure is nearly fifteen pounds to the square inch. The atmosphere forms an essential part of one's physical environment and serves various purposes. The process by which gaseous materials are made to pass between the body and the atmosphere is known as

*Respiration.*—As usually defined, respiration, or breathing, consists of two simple processes—that of taking air into special contrivances in the body, called the lungs, and that of expelling air from the lungs. The first process is known as inspiration; the second as expiration. We must, however, distinguish between respiration by the lungs, called external respiration, and respiration by the cells, called internal respiration.

The purpose of respiration is indicated by the changes that take place in the air while it is in the lungs. Air entering the lungs in ordinary breathing parts with about five per cent of itself in the form of oxygen and receives about four and one half per cent of carbon dioxide, considerable water-vapor, and a small amount of other impurities. These changes suggest a twofold purpose for respiration:

1. To obtain from the atmosphere the supply of oxygen needed by the body.

2. To transfer to the atmosphere certain materials (wastes) which must be removed from the body.

The chief organs concerned in the work of respiration are

*The Lungs.*—The lungs consist of two sac-like bodies suspended in the thoracic cavity, and occupying all the space not taken up by the heart. They are not simple sacs, however, but are separated into numerous divisions, as follows:

1. The lung on the right side of the thorax, called the right lung, is made up of three divisions, or lobes, and the left lung is made up of two lobes.

2. The lobes on either side are separated into smaller divisions, called lobules (Fig. 33). Each lobule receives a distinct division of an air tube and has in itself the structure of a miniature lung.

[Fig. 33]

Fig. 33—*Lungs and air passages* seen from the front. The right lung shows the lobes and their divisions, the lobules. The tissue of the left lung has been dissected away to show the air tubes.

3. In the lobule the air tube divides into a number of smaller tubes, each ending in a thin-walled sac, called an infundibulum. The interior of the infundibulum is separated into many small spaces, known as the alveoli, or air cells.

The lungs are remarkable for their lightness and delicacy of structure.(30) They consist chiefly of the tissues that form their sacs, air tubes, and blood vessels; the membranes that line their inner and outer surfaces; and the connective tissue that binds these parts together. All these tissues are more or less elastic. The relation of the different parts of the lungs to each other and to the outside atmosphere will be seen through a study of the

*Air Passages.*—The air passages consist of a system of tubes which form a continuous passageway between the outside atmosphere and the different divisions of the lungs. The air passes through them as it enters and leaves the lungs, a fact which accounts for the name.

[Fig. 34]

Fig. 34—*Model of section through the head*, showing upper air passages and other parts. 1. Left nostril. 2. Pharynx. 3. Tongue and cavity of mouth. 4. Larynx. 5. Trachea. 6. Esophagus.

The incoming air first enters the nostrils. These consist of two narrow passages lying side by side in the nose, and connecting with the pharynx behind. The lining of the nostrils, called mucous membrane is quite thick, and has its surface much extended by reason of being spread over some thin, scroll-shaped bones that project into the passage. This membrane is well supplied with blood vessels and secretes a considerable quantity of liquid. Because of the nature and arrangement of the membrane, the nostrils are able to warm and moisten the incoming air, and to free it from dust particles, preparing it, in this way, for entrance into the lungs (Fig. 34).

The nostrils are separated from the mouth by a thin layer of bone, and back of both the mouth and the nostrils is the pharynx. The pharynx and the mouth serve as parts of the food canal, as well as air passages, and are described in connection with the organs of digestion (Chapter X). Air entering the pharynx, either by the nostrils or by the mouth, passes through it into the larynx. The larynx, being the special organ for the production of the voice, is described later (Chapter XXI). The entrance into the larynx is guarded by a movable lid of cartilage, called the epiglottis, which prevents food particles and liquids, on being swallowed, from passing into the lower air tubes. The relations of the nostrils, mouth, pharynx, and larynx are shown in Fig. 34.

From the larynx the air enters the trachea, or windpipe. This is a straight and nearly round tube, slightly less than an inch in diameter and about four and one half inches in length. Its walls contain from sixteen to twenty C-shaped, cartilaginous rings, one above the other and encircling the tube. These incomplete rings, with their openings directed backward, are held in place by thin layers of connective and muscular tissue. At the lower end the trachea divides into two branches, called the bronchi, each of which closely resembles it in structure. Each bronchus separates into a number of smaller divisions, called the bronchial tubes, and these in turn divide into still smaller branches, known as the lesser bronchial tubes (Fig. 33). The lesser bronchial tubes, and the branches into which they separate, are the smallest of the air tubes. One of these joins, or expands into, each of the minute lung sacs, or infundibula. Mucous membrane lines all of the air passages.

*General Condition of the Air Passages.*—One necessary condition for the movement of the air into and from the lungs is an unobstructed passageway.(31) The air passages must be kept open and free from obstructions. They are kept open by special contrivances found in their walls, which, by supplying a degree of stiffness, cause the tubes to keep their form. In the trachea, bronchi, and larger bronchial tubes, the stiffness is supplied by rings of cartilage, while in the smaller tubes this is replaced by connective and muscular tissue. The walls of the larynx contain strips and plates of cartilage; while the nostrils and the pharynx are kept open by their bony surroundings.

[Fig. 35]

Fig. 35—*Ciliated epithelial cells.* A. Two cells highly magnified. c. Cilia, n. Nucleus. B. Diagram of a small air tube showing the lining of cilia.

The air passages are kept clean by cells especially adapted to this purpose, known as the ciliated epithelial cells. These are slender, wedge-shaped cells which have projecting from a free end many small, hair-like bodies, called cilia (Fig. 35). They line the mucous membrane in most of the air passages, and are so placed that the cilia project into the tubes. Here they keep up an inward and outward wave-like movement, which is quicker and has greater force in the outward direction. By this means the cilia are able to move small pieces of foreign matter, such as dust particles and bits of partly dried mucus, called phlegm, to places where they can be easily expelled from the lungs.(32)

[Fig. 36]

Fig. 36—*Terminal air sacs.* The two large sacs are infundibula; the small divisions are alveoli. (Enlarged.)

*The Alveoli.*—The alveoli, or air cells, are the small divisions of the infundibula (Fig. 36). They are each about one one-hundredth of an inch (1/4 mm.) in diameter, being formed by the infolding of the infundibular wall. This wall, which has for its framework a thin layer of elastic connective tissue, supports a dense network of capillaries (Fig. 37), and is lined by a single layer of cells placed edge to edge. By this arrangement the air within the alveoli is brought very near a large surface of blood, and the exchange of gases between the air and the blood is made possible. It is at the alveoli that the oxygen passes from the air into the blood, and the carbon dioxide passes from the blood into the air. At no place in the lungs, however, do the air and the blood come in direct contact. Their exchanges must in all cases take place through the capillary walls and the layer of cells lining the alveoli.

[Fig. 37]

Fig. 37—*Inner lung surface (magnified)*, the blood vessels injected with coloring matter. The small pits are alveoli, and the vessels in their walls are chiefly capillaries.

[Fig. 38]

Fig. 38.—*Diagram to show the double movement of air and blood through the lungs.* The blood leaves the heart by the pulmonary artery and returns by the pulmonary veins. The air enters and leaves the lungs by the same system of tubes.

[Fig. 39]

Fig. 39—*Diagram to show air and blood movements in a terminal air sac.* While the air moves into and from the space within the sac, the blood circulates through the sac walls.

*Blood Supply to the Lungs.*—To accomplish the purposes of respiration, not only the air, but the blood also, must be passed into and from the lungs. The chief artery conveying blood to the lungs is the pulmonary artery. This starts at the right ventricle and by its branches conveys blood to the capillaries surrounding the alveoli in all parts of the lungs. The branches of the pulmonary artery lie alongside of, and divide similarly to, the bronchial tubes. At the places where the finest divisions of the air tubes enter the infundibula, the little arteries branch into the capillaries that penetrate the infundibular walls (Figs. 38 and 39). From these capillaries the blood is conveyed by the pulmonary veins to the left auricle.

The lungs also receive blood from two (in some individuals three) small arteries branching from the aorta, known as the bronchial arteries. These convey to the lungs blood that has already been supplied with oxygen, passing it into the capillaries in the walls of the bronchi, bronchial tubes, and large blood vessels, as well as the connective tissue between the lobes of the lungs. This blood leaves the lungs partly by the bronchial veins and partly by the pulmonary veins. No part of the body is so well supplied with blood as the lungs.

[Fig. 40]

Fig. 40—*The pleurae.* Diagram showing the general form of the pleural sacs as they surround the lungs and line the inner surfaces of the chest (other parts removed). A, A'. Places occupied by the lungs. B, B'. Slight space within the pleural sacs containing the pleural secretion, a, a'. Outer layer of pleura and lining of chest walls and upper surface of diaphragm. b, b'. Inner layer of pleura and outer lining of lungs. C. Space occupied by the heart. D. Diaphragm.

*The Pleura.*—The pleura is a thin, smooth, elastic, and tough membrane which covers the outside of the lungs and lines the inside of the chest walls. The covering of each lung is continuous with the lining of the chest wall on its respective side and forms with it a closed sac by which the lung is surrounded, the arrangement being similar to that of the pericardium. Properly speaking, there are two pleurae, one for each lung, and these, besides inclosing the lungs, partition off a middle space which is occupied by the heart (Fig. 40). They also cover the upper surface of the diaphragm, from which they deflect upward, blending with the pericardium. A small amount of liquid is secreted by the pleura, which prevents friction as the surfaces glide over each other in breathing.

*The Thorax.*—The force required for breathing is supplied by the box-like portion of the body in which the lungs are placed. This is known as the thorax, or chest, and includes that part of the trunk between the neck and the abdomen. The space which it incloses, known as the thoracic cavity, is a variable space and the walls surrounding this space are air-tight. A framework for the thorax is supplied by the ribs which connect with the spinal column behind and with the sternum, or breast-bone, in front. They form joints with the spinal column, but connect with the sternum by strips of cartilage. The ribs do not encircle the cavity in a horizontal direction, but slope downward from the spinal column both toward the front and toward the sides, this being necessary to the service which they render in breathing.

*How Air is Brought into and Expelled from the Lungs.*—The principle involved in breathing is that air flows from a place of greater to a place of less pressure. The construction of the thorax and the arrangement of the lungs within it provide for the application of this principle in a most practical manner. The lungs are suspended from the upper portion of the thoracic cavity, and the trachea and the upper air passages provide the only opening to the outside atmosphere. Air entering the thorax must on this account pass into the lungs. As the thorax is enlarged the air in the lungs expands, and there is produced within them a place of slightly less air pressure than that of the atmosphere on the outside of the body. This difference causes the air to flow into the lungs.

[Fig. 41]

Fig. 41—*Diagram illustrating the bellows principle in breathing.* A. The human bellows. B. The hand bellows. Compare part for part.

When the thorax is diminished in size, the air within the lungs is slightly compressed. This causes it to become denser and to exert on this account a pressure slightly greater than that of the atmosphere on the outside. The air now flows out until the equality of the pressure is again restored. Thus the thorax, by making the pressure within the lungs first slightly less and then slightly greater than the atmospheric pressure, causes the air to move into and out of the lungs.

Breathing is well illustrated by means of the common hand bellows, its action being similar to that of the thorax. It will be observed that when the sides are spread apart air flows into the bellows. When they are pressed together the air flows out. If an air-tight sack were hung in the bellows with its mouth attached to the projecting tube, the arrangement would resemble closely the general plan of the breathing organs (Fig. 41). One respect, however, in which the bellows differs from the thorax should be noted. The thorax is never sufficiently compressed to drive out all the air. Air is always present in the lungs. This keeps them more or less distended and pressed against the thoracic walls.

*How the Thoracic Space is Varied.*—One means of varying the size of the thoracic cavity is through the movements of the ribs and their resultant effect upon the walls of the thorax. In bringing about these movements the following muscles are employed:

1. The scaleni muscles, three in number on each side, which connect at one end with the vertebrae of the neck and at the other with the first and second ribs. Their contraction slightly raises the upper portion of the thorax.

2. The elevators of the ribs, twelve in number on each side, which are so distributed that each single muscle is attached, at one end, to the back portion of a rib and, at the other, to a projection of the vertebra a few inches above. The effect of their contraction is to' elevate the middle portion of the ribs and to turn them outward or spread them apart.

3. The intercostal muscles, which form two thin layers between the ribs, known as the internal and the external intercostal muscles. The external intercostals are attached between the outer lower margin of the rib above and the outer upper margin of the rib below, and extend obliquely downward and forward. The internal intercostals are attached between the inner margins of adjacent ribs, and they extend obliquely downward and backward from the front. The contraction of the external intercostal muscles raises the ribs, and the contraction of the internal intercostals tends to lower them.

[Fig. 42]

Fig. 42—*Simple apparatus* for illustrating effect of movements of the ribs upon the thoracic space; strips of cardboard held together by pins, the front part being raised or lowered by threads moving through attachments at 1 and 2. As the front is raised the space between the uprights is increased. The front upright corresponds to the breastbone, the back one to the spinal column, the connecting strips to the ribs, and the threads to the intercostal muscles.

By slightly raising and spreading apart the ribs the thoracic space is increased in two directions—from front to back and from side to side. Lowering and converging the ribs has, of course, the opposite effect (Fig. 42). Except in forced expirations the ribs are lowered and converged by their own weight and by the elastic reaction of the surrounding parts.

*The Diaphragm.*—Another means of varying the thoracic space is found in an organ known as the diaphragm. This is the dome-shaped, movable partition which separates the thoracic cavity from the cavity of the abdomen. The edges of the diaphragm are firmly attached to the walls of the trunk, and the center is supported by the pericardium and the pleura. The outer margin is muscular, but the central portion consists of a strong sheet of connective tissue. By the contraction of its muscles the diaphragm is pulled down, thereby increasing the thoracic cavity. By raising the diaphragm the thoracic cavity is diminished.

The diaphragm, however, is not raised by the contraction of its own muscles, but is pushed up by the organs beneath. By the elastic reaction of the abdominal walls (after their having been pushed out by the lowering of the diaphragm), pressure is exerted on the organs of the abdomen and these in turn press against the diaphragm. This crowds it into the thoracic space. In forced expirations the muscles in the abdominal walls contract to push up the diaphragm.

*Interchange of Gases in the Lungs.*—During each inspiration the air from the outside fills the entire system of bronchial tubes, but the alveoli are largely filled, at the same time, by the air which the last expiratory effort has left in the passages. By the action of currents and eddies and by the rapid diffusion of gas particles, the air from the outside mixes with that in the alveoli and comes in contact with the membranous walls. Here the oxygen, after being dissolved by the moisture in the membrane, diffuses into the blood. The carbon dioxide, on the other hand, being in excess in the blood, diffuses toward the air in the alveoli. The interchange of gases at the lungs, however, is not fully understood, and it is possible that other forces than osmosis play a part.

[Fig. 43]

Fig. 43—*Diagram* illustrating lung capacity.

*Capacity of the Lungs.*—The air which passes into and from the lungs in ordinary breathing, called the tidal air, is but a small part of the whole amount of air which the lungs contain. Even after a forced expiration the lungs are almost half full; the air which remains is called the residual air. The air which is expelled from the lungs by a forced expiration, less the tidal air, is called the reserve, or supplemental, air. These several quantities are easily estimated. (See Practical Work.) In the average individual the total capacity of the lungs (with the chest in repose) is about one gallon. In forced inspirations this capacity may be increased about one third, the excess being known as the complemental air (Fig. 43).

[Fig. 44]

Fig. 44—*Diagram* illustrating internal respiration and its dependence on external respiration. (Modified from Hall.) (See text.)

*Internal, or Cell, Respiration.*—The oxygen which enters the blood in the lungs leaves it in the tissues, passing through the lymph into the cells (Fig. 44). At the same time the carbon dioxide which is being formed at the cells passes into the blood. An exchange of gases is thus taking place between the cells and the blood, similar to that taking place between the blood and the air. This exchange is known as internal, or cell, respiration. By internal respiration the oxygen reaches the place where it is to serve its purpose, and the carbon dioxide begins its movement toward the exterior of the body. This "breathing by the cells" is, therefore, the final and essential act of respiration. Breathing by the lungs is simply the means by which the taking up of oxygen and *the* giving off of carbon dioxide by the cells is made possible.



HYGIENE OF RESPIRATORY ORGANS

The liability of the lungs to attacks from such dread diseases as consumption and pneumonia makes questions touching their hygiene of first importance. Consumption does not as a rule attack sound lung tissue, but usually has its beginning in some weak or enfeebled spot in the lungs which has lost its "power of resistance." Though consumption is not inherited, as some suppose, lung weaknesses may be transmitted from parents to children. This, together with the fact, now generally recognized, that consumption is contagious, accounts for the frequent appearance of this disease in the same family. Consumption as well as other respiratory affections can in the majority of cases be prevented, and in many cases cured, by an intelligent observation of well-known laws of health.

*Breathe through the Nostrils.*—Pure air and plenty of it is the main condition in the hygiene of the lungs. One necessary provision for obtaining pure air is that of breathing through the nostrils. Air is the carrier of dust particles and not infrequently of disease germs.(33) Partly through the small hairs in the nose, but mainly through the moist membrane that lines the passages, the nostrils serve as filters for removing the minute solid particles (Fig. 45). While it is important that nose breathing be observed at all times, it is especially important when one is surrounded by a dusty or smoky atmosphere. Otherwise the small particles that are breathed in through the mouth may find a lodging place in the lungs.

[Fig. 45]

Fig. 45—*Human air filter.* Diagram of a section through the nostrils; shows projecting bones covered with moist membrane against which the air is made to strike by the narrow passages. 1. Air passages. 2. Cavities in the bones. 3. Front lower portion of the cranial cavity.

In addition to removing dust particles and germs, other purposes are served by breathing through the nostrils. The warmth and moisture which the air receives in this way, prepare it for entering the lungs. Mouth breathing, on the other hand, looks bad and during sleep causes snoring. The habit of nose breathing should be established early in life.(34)

*Cultivate Full Breathing.*—Many people, while apparently taking in sufficient air to supply their need for oxygen, do not breathe deeply enough to "freely ventilate the lungs." "Shallow breathing," as this is called, is objectionable because it fails to keep up a healthy condition of the entire lung surface. Portions of the lungs to which air does not easily penetrate fail to get the fresh air and exercise which they need. As a consequence, they become weak and, by losing their "power of resistance," become points of attack in diseases of the lungs.(35) The breathing of each individual should receive attention, and where from some cause it is not sufficiently full and deep, the means should be found for remedying the defect.

*Causes of Shallow Breathing.*—Anything that impedes the free movement of air into the lungs tends to cause shallow breathing A drooping of the back or shoulders and a curved condition of the spinal column, such as is caused by an improper position in sitting, interfere with the free movements of the ribs and are recognized causes. Clothing also may impede the respiratory movements and lead to shallow breathing. If too tight around the chest, clothing interferes with the elevation of the ribs; and if too tight around the waist, it prevents the depression of the diaphragm. Other causes of shallow breathing are found in the absence of vigorous exercise, in the leading of an indoor and inactive life, in obstructions in the nostrils and upper pharynx, and in the lack of attention to proper methods of breathing.

To prevent shallow breathing one should have the habit of sitting and standing erect. The clothing must not be allowed to interfere with the respiratory movements. The taking of exercise sufficiently vigorous to cause deep and rapid breathing should be a common practice and one should spend considerable time out of doors. If one has a flat chest or round shoulders, he should strive by suitable exercises to overcome these defects. Obstructions in the nostrils or pharynx should be removed.

*Breathing Exercises.*—In overcoming the habit of shallow breathing and in strengthening the lungs generally, the practicing of occasional deep breathing has been found most valuable and is widely recommended. With the hands on the hips, the shoulders drawn back and down, the chest pushed upward and forward, and the chin slightly depressed, draw the air slowly through the nostrils until the lungs are completely full. After holding this long enough to count three slowly, expel it quickly from the lungs. Avoid straining. To get the benefit of pure air, it is generally better to practice deep breathing out of doors or before an open window.

By combining deep breathing with simple exercises of the arms, shoulders, and trunk much may be done towards straightening the spine, squaring the shoulders, and overcoming flatness of the chest. Though such movements are best carried on by the aid of a physical director, one can do much to help himself. One may safely proceed on the principle that slight deformities of the chest, spine, and shoulders are corrected by gaining and keeping the natural positions, and may employ any movements which will loosen up the parts and bring them where they naturally belong.(36)

*Serious Nature of Colds.*—That many cases of consumption have their beginning in severe colds (on the lungs) is not only a matter of popular belief, but the judgment also of physicians. Though the cold is a different affection from that of consumption, it may so lower the vitality of the body and weaken the lung surfaces that the germs of consumption find it easy to get a start. On this account a cold on the chest which does not disappear in a few days, but which persists, causing more or less coughing and pain in the lungs, must be given serious consideration.(37) The usual home remedies failing to give relief, a physician should be consulted. It should also be noted that certain diseases of a serious nature (pneumonia, diphtheria, measles, etc.) have in their beginning the appearance of colds. On this account it is wise not only to call a physician, but to call him early, in severe attacks of the lungs. Especially if the attack be attended by difficult breathing, fever, and a rapid pulse is the case serious and medical advice necessary.

*Ventilation.*—The process by which the air in a room is kept fresh and pure is known as ventilation. It is a double process—that of bringing fresh air into the room and that of getting rid of air that has been rendered impure by breathing (38) or by lamps. Outdoor air is usually of a different temperature (colder in winter, warmer in summer) from that indoors, and as a consequence differs from it slightly in weight. On account of this difference, suitable openings in the walls of buildings induce currents which pass between the rooms and the outside atmosphere even when there is no wind. In winter care must be taken to prevent drafts and to avoid too great a loss of heat from the room. A cold draft may even cause more harm to one in delicate health than the breathing of air which is impure. To ventilate a room successfully the problem of preventing drafts must be considered along with that of admitting the fresh air.

[Fig. 46]

Fig. 46—Window adjusted for ventilation without drafts.

The method of ventilation must also be adapted to the construction of the building, the plan of heating, and the condition of the weather. Specific directions cannot be given, but the following suggestions will be found helpful in ventilating rooms where the air is not warmed before being admitted:

1. Introduce, the air through many small openings rather than a few large ones. If the windows are used for this purpose, raise the lower sash and drop the upper one slightly for several windows, varying the width to suit the conditions (Fig. 46). By this means sufficient air may be introduced without causing drafts.

2. Introduce the air at the warmest portions of the room. The air should, if possible, be warmed before reaching the occupants.

3. If the wind is blowing, ventilate principally on the sheltered side of the house.

Ample provision should be made for fresh air in sleeping rooms, and here again drafts must be avoided. Especially should the bed be so placed that strong air currents do not pass over the sleeper. In schoolhouses and halls for public gatherings the means for efficient ventilation should, if possible, be provided in the general plan of construction and method of heating.

[Fig. 47]

Fig. 47—*Artificial respiration* as a laboratory experiment. Expiration. Prone-posture method of Schaffer.

*Artificial Respiration.*—When natural breathing is temporarily suspended, as in partial drowning, or when one has been overcome by breathing some poisonous gas, the saving of life often depends upon the prompt application of artificial respiration. This is accomplished by alternately compressing and enlarging the thorax by means of variable pressure on the outside, imitating the natural process as nearly as possible. Following is the method proposed by Professor E.A. Schaffer of England, and called by him "the prone-posture method of artificial respiration":

The patient is laid face downward with an arm bent under the head, and intermittent pressure applied vertically over the shortest ribs. The pressure drives the air from the lungs, both by compressing the lower portions of the chest and by forcing the abdominal contents against the diaphragm, while the elastic reaction of the parts causes fresh air to enter (Figs. 47 and 48). "The operator kneels or squats by the side of, or across the patient, places his hands over the lowest ribs and swings his body backward and forward so as to allow his weight to fall vertically on the wrists and then to be removed; in this way hardly any muscular exertion is required.... The pressure is applied gradually and slowly, occupying some three seconds; it is then withdrawn during two seconds and again applied; and so on some twelve times per minute."(39)

[Fig. 48]

Fig. 48—Artificial respiration. Inspiration.

The special advantages of the prone-posture method over others that have been employed are: I. It may be applied by a single individual and fora long period of time without exhaustion. 2. It allows the mucus and water (in case of drowning) to run out of the mouth, and causes the tongue to fall forward so as not to obstruct the passageway. 3. It brings a sufficient amount of air into the lungs.(40)

While applying artificial respiration, the heat of the body should not be allowed to escape any more than can possibly be helped. In case of drowning, the patient should be wrapped in dry blankets or clothing, while bottles of hot water may be placed in contact with the body. The circulation should be stimulated, as may be done by rubbing the hands, feet, or limbs in the direction of the flow of the blood in the veins.

*Tobacco Smoke and the Air* Passages.—Smoke consists of minute particles of unburnt carbon, or soot, such as collect in the chimneys of fireplaces and furnaces. If much smoke is taken into the lungs, it irritates the delicate linings and tends to clog them up. Tobacco smoke also contains the poison nicotine, which is absorbed into the blood. For these reasons the cigarette user who inhales the smoke does himself great harm, injuring his nervous system and laying the foundation for diseases of the air passages. The practice of smoking indoors is likewise objectionable, since every one in a room containing the smoke is compelled to breathe it.

*Alcohol and Diseases of the Lungs.*—Pneumonia is a serious disease of the lungs caused by germs. The attacks occur as a result of exposure, especially when the body is in a weakened condition. A noted authority states that "alcoholism is perhaps the most potent predisposing cause" of pneumonia.(41) A person addicted to the use of alcohol is also less likely to recover from the disease than one who has avoided its use, a result due in part to the weakening effect of alcohol upon the heart. The congestion of the lungs in pneumonia makes it very difficult for the heart to force the blood through them. The weakened heart of the drunkard gives way under the task.

The statement sometimes made that alcohol is beneficial in pulmonary tuberculosis is without foundation in fact. On the other hand, alcoholism is a recognized cause of consumption. Some authorities claim that this disease is more frequent in heavy drinkers than in those of temperate habits, in the proportion of about three to one, and that possibly half of the cases of tuberculosis are traceable to alcoholism.(42)

*The Outdoor Cure for Lung Diseases*—Among the many remedies proposed for consumption and kindred diseases, none have proved more beneficial, according to reports, than the so-called "outdoor" cure. The person having consumption is fed plentifully upon the most nourishing food, and is made to spend practically his entire time, including the sleeping hours, out of doors. Not only is this done during the pleasant months of summer, but also during the winter when the temperature is below freezing. Severe exposure is prevented by overhead protection at night and by sufficient clothing to keep the body warm. The abundant supply of pure, cold air toughens the lungs and invigorates the entire body, thereby enabling it to throw off the disease.

The success attending this method of treating consumptives suggests the proper mode of strengthening lungs that are not diseased, but simply weak. The person having weak lungs should spend as much time as he conveniently can out of doors. He should provide the most ample ventilation at night and have a sleeping room to himself. He should practice deep breathing exercises and partake of a nourishing diet. While avoiding prolonged chilling and other conditions liable to induce colds, he should take advantage of every opportunity of exposing himself fully and freely to the outside atmosphere.

*Summary.*—The purpose of respiration is to bring about an exchange of gases between the body and the atmosphere. The organs employed for this purpose, called the respiratory organs, are adapted to handling materials in the gaseous state, and are operated in accordance with principles governing the movements of the atmosphere. By alternately increasing and diminishing the thoracic space, air is made to pass between the outside atmosphere and the interior of the lungs. Finding its way into the smallest divisions of the lungs, called the alveoli, the air comes very near a large surface of blood. By this means the carbon dioxide diffuses out of the blood, and the free oxygen enters. Through the combined action of the organs of respiration and the organs that move the blood and the lymph, the cells in all parts of the body are enabled to exchange certain gaseous materials with the outside atmosphere.

[Fig. 49]

Fig. 49—Model for demonstrating the lungs.

*Exercises.—*1. How does air entering the lungs differ in composition from air leaving the lungs? What purposes of respiration are indicated by these differences?