[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 s.p.a.ce 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 pa.s.sed 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, pa.s.sing 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 s.p.a.ce 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._ s.p.a.ce 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, part.i.tion off a middle s.p.a.ce 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 s.p.a.ce which it incloses, known as the thoracic cavity, is a _variable_ s.p.a.ce and the walls surrounding this s.p.a.ce 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 pa.s.sages provide the only opening to the outside atmosphere. Air entering the thorax must on this account pa.s.s 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 ill.u.s.trating 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 ill.u.s.trated 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 s.p.a.ce 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 ill.u.s.trating effect of movements of the ribs upon the thoracic s.p.a.ce; 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 s.p.a.ce 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 s.p.a.ce 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 s.p.a.ce is found in an organ known as the diaphragm. This is the dome-shaped, _movable part.i.tion_ 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 s.p.a.ce. 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 pa.s.sages. 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* ill.u.s.trating lung capacity.

*Capacity of the Lungs.*-The air which pa.s.ses 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 quant.i.ties 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* ill.u.s.trating 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, pa.s.sing through the lymph into the cells (Fig. 44). At the same time the carbon dioxide which is being formed at the cells pa.s.ses 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 pa.s.sages, 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 pa.s.sages. 1. Air pa.s.sages. 2. Cavities in the bones. 3. Front lower portion of the cranial cavity.