Physiology and Hygiene for Secondary Schools. Francis M. Walters
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—The pleuræ. 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[pg 085] which the lung is surrounded, the arrangement being similar to that of the pericardium. Properly speaking, there are two pleuræ, 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[pg 086] 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—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[pg 087] 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 vertebræ 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—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.
[pg 088]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