carbon dioxide. In the rabbit this exchange takes place as the blood is passing through the capillaries of the lungs. There the blood is separated from the air by a membrane so delicate that gases can readily pass through it; and, hence, on leaving the lungs the blood has got rid of the waste carbon dioxide, and its red corpuscles are laden with fresh oxygen. This exchange of useless carbon dioxide for oxygen constitutes respiration or breathing. The mechanism of respiration.—In active animals the air inside the lungs soon becomes vitiated, unless there is some means of changing it. Under ordinary conditions a man changes the air in his lungs from thirteen to fifteen times a minute. He does this quite automatically, [Pg 243] and without thinking about it. Every four seconds or so a set of muscles contracts and pulls his ribs upwards and outwards; another muscular contraction pulls down the floor of his chest at the same time. As a consequence the cavity is much enlarged. The lungs follow the movements of the walls of the chest, and some thirty cubic inches of air are sucked in. Immediately the ribs fall back to their former position, the chest-floor rises, and air is driven out. Then after a short pause the process is repeated. It should be noticed that only about thirty cubic inches of air are changed at each respiration, although the capacity of the human lungs averages about 230 cubic inches. All mammals breathe in much the same way. Plants and animals.—There are considerably more points of similarity than of difference between plants and animals. In every case the vital activities are accompanied by an oxidation of living substance, and from this fact arises the necessity for food and oxygen. The breathing of plants is essentially like that of animals, and consists in taking in oxygen and giving out carbon dioxide; though the mechanism of respiration is—except in the lowest plants and animals—entirely different in the two cases. It is in the sources from which they obtain their food that plants and animals are most unlike. An animal must be supplied with food which has already been prepared by some other living thing; and it is obvious that the food even of carnivorous animals can ultimately be traced back to plants, for the flesh-eater preys upon the vegetarian. Animals are therefore entirely dependent upon plants for food. In this sense the saying “all flesh is grass” is full of significance.