OLOGY 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 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 clavicle, 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.

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475-494. _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 into 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?

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[Illustration: Fig. 95. 6, Four of the vertebrae, to which are attached
three r