472. The RIBS are joined to the spinal column at their posterior extremity; and in front, they terminate in cartilages, which unite them to the sternum. They incline downward, from the spinal column to the breast-bone, and form resisting walls that assist in producing the partial vacuum necessary for inspiration.

Fig. 93. A section of the chest when the lungs are inflated. 1, The diaphragm 2, The muscular walls of the abdomen.

Fig. 94. A section of the chest when the lungs are contracted. 1, The diaphragm, in common expiration. 2, 2, The muscular walls of the abdomen. 3, The position of the diaphragm in forced expiration.

These engravings show the diaphragm to be more convex, and the walls of the abdomen more flattened, when the lungs are collapsed, than when they are inflated.

473. The DIAPHRAGM is a flexible circular partition, that separates the respiratory from the digestive organs, and the chest from the abdomen. Its margin is attached to the spinal column, the sternum, and cartilages of the lower ribs. The lungs rest upon its upper surface, while the liver and stomach

are placed below it, (fig. 88.) In a state of repose, its upper surface forms an arch, the convexity of which is toward the chest. In forced expiration, its upper point reaches as high as the fourth rib. In an ordinary inspiration, it is depressed as low as the seventh rib, which increases the capacity of the chest.

474. The RESPIRATORY muscles are, in general, attached at one extremity to the parts about the shoulders, head, and upper portion of the spinal column. From these, they run downward and forward, and are attached, at the opposite extremity, to the sternum, clavicle, and upper rib. Other muscles are attached at one extremity to a rib above, and by the opposite extremity to a rib below. These fill the spaces between the ribs, and, from their situation, are called in-tercost'al muscles.

Observation. 1st. There are several actions of common occurrence, that are intimately connected with respiration; such as hiccough, sneezing, &c. Hiccough is an involuntary contraction of the muscles of respiration, particularly the diaphragm.

2d. Sneezing is a violent, involuntary contraction of the respiratory muscles, as in hiccough. When an acrid stimulant, as snuff, is applied to the mucous membrane of the nose, an irritation is produced which is accompanied by a violent expulsion of air from the lungs. This is owing to the connection between the nasal and respiratory nerves.

What is its form when not in action? 474. Where do the respiratory muscles make their attachment? What name is given to those muscles that fill the places between the ribs? What is hiccough? What is sneezing?

CHAPTER XXIV.

PHYSIOLOGY OF THE RESPIRATORY ORGANS.

475. RESPIRATION, or breathing, is that process by which air is taken into the lungs and expelled from them. The object of respiration is, 1st. To supply the system with oxygen, which is essential to the generation of animal heat; 2d. To convert the chyle into blood. This is done by the oxygen of the inspired air; 3d. To relieve the organs of the body of the principal elements (carbon and hydrogen) that compose the old and useless particles of matter. The organs of the system, as already mentioned, are principally composed of carbon, hydrogen, oxygen, and nitrogen.

476. By the action of the lymphatics and capillary veins, the old and worn-out particles are conveyed into the veins of the systemic circulation. The hydrogen, in form of watery vapor, is easily discharged in the perspiration and other secretions. The nitrogen and oxygen are, or may be, separated from the blood, through the agency of several different organs; but carbon does not escape so readily. It is probable that a part of the surplus carbon of the venous blood is secreted by the liver; but a far greater amount passes to the lungs, and these may be considered as special organs designed to separate this element from the venous blood.

477. An ordinary inspiration may be accomplished by the action of the diaphragm, and a slight elevation of the ribs. In

475-491. Give the physiology of the respiratory organs. 475. What is respiration? What is the principal object in breathing? 476. How are the useless atoms of matter conveyed in:o the veins of the systemic circulation? How may the principal elementary substances be separated from the blood? 477. How may an ordinary inspiration be accomplished?

full inspiration, the diaphragm is not only more depressed, but the ribs are evidently elevated. To produce this effect on the ribs, two sets of muscles are called into action. Those which are attached to the upper rib, sternum, and cavicle, contract and elevate the lower and free extremities of the ribs. This enlarges the cavity of the chest between the spinal column and the sternum. But the lateral diameter, in consequence, is only slightly increased, because the central portion of the ribs sinks lower than their posterior extremities, or their cartilaginous attachment to the sternum.

Fig. 95. 6, Four of the vertebræ, to which are attached three ribs, (7, 7, 7,) with their intercostal muscles, (8, 8.) These ribs, in their natural position, have their anterior cartilaginous extremity at 4, while the posterior extremity is attached to the vertebræ, (6,) which are neither elevated nor depressed in respiration. 1, 1, and 2,2, parallel lines, within which the ribs lie in their natural position. If the anterior extremity of the ribs is elevated from 4 to 5, they will not lie within the line 2, 2, but will reach the line 3, 3. If two bands extend from 1, 1, to 2, 2, they will effectually prevent the elevation of the ribs from 4 to 5, as the line 2, 2, cannot be moved to 3, 3.

478. The central portion of the ribs is raised by the action of intercostal muscles. The first, or upper rib, has but little movement; the second has more motion than the first, while

What effect has a full inspiration on the ribs and diaphragm? How is the chest enlarged between the spinal column and sternum? What is said of the lateral diameter of the chest? Explain fig. 95.

the third has still more than the second. The second rib is elevated by the contraction of the muscles between it and the first. The third rib is raised by the action of two sets of muscles; one lies between the first and second ribs, the other between the second and third. The motion of each succeeding rib is increased, because it is not only acted upon by the muscles that move the ribs above, but by an additional intercostal; so that the movement of the twelfth rib is very free, as it is elevated by the contraction of eleven muscles.

479. The tenth rib is raised eight times as much as the second rib, and the lateral diameter of the lower portion of the chest is increased in a corresponding degree. At the same time, the muscular margin of the diaphragm contracts, which depresses its central portion; and in this way, the chest is enlarged forward, laterally, and downward, simultaneously with the relaxation of the walls of the abdomen.

480. The lungs follow the variations of capacity in the chest, expanding their air-cells when the latter is enlarged, and contracting when the chest is diminished. Thus, when the chest is expanded, the lungs follow, and consequently a vacuum is produced in their air-cells. The air then rushes through the mouth and nose into the trachea and its branches, and fills the vacuum as fast as it is made. This mechanical process constitutes inspiration.

481. After the expansion of the chest, the muscles that elevated the ribs relax, together with the diaphragm. The elasticity of the cartilages of the ribs depresses them, and the cavity of the chest is diminished, attended by the expulsion of a portion of the air from the lungs. At the same time, the muscles that form the front walls of the.

478. Describe the action of the intercostal muscles upon the ribs. 479. How does the elevation of the tenth rib compare with the second? What effect has this elevation upon the lateral diameter of the chest? 480. Describe the process of inspiration. 481. Describe the process by which the air is forced out of the lungs.