. Animal life and intelligence. Biology; Animal intelligence; Psychology, Comparative; Evolution. The Process of Life. 23 great arterial vessel and the numerous branches it gives off, to the head and brain, to the body and limbs, to the abdominal viscera; in short, to all parts of the body except the lungs. In all the parts thus supplied, the vessels at length break up into a delicate capillary net- work, so that the blood-fluid is se- parated from the tissue-cells only by the delicate organic film of the capillary walls. Then the blood begins to re-collect into larger and larger veins. But a
. Animal life and intelligence. Biology; Animal intelligence; Psychology, Comparative; Evolution. The Process of Life. 23 great arterial vessel and the numerous branches it gives off, to the head and brain, to the body and limbs, to the abdominal viscera; in short, to all parts of the body except the lungs. In all the parts thus supplied, the vessels at length break up into a delicate capillary net- work, so that the blood-fluid is se- parated from the tissue-cells only by the delicate organic film of the capillary walls. Then the blood begins to re-collect into larger and larger veins. But a change has taken place; the blood-discs have delivered up to the tissues their freight of oxygen; the stream in which they float has been charged with carbonic acid gas. The veins leading from various parts of the body converge upon the heart and pour their contents into the right receiver; thence the blood passes into the right force-pump, by which. Fig. 6.—Diagram of circu- lation. , left auricle of the heart; it IS propelled, by arteries, tO the , left ventricle; H., capillary „ . plexus of the head; B., capillary lungS. There the blood-disCS are plexus of the body ;, alimentary _ , canal; Lr„ liver; , right auricle again laden with oxvgen, the stream °f the heart; R- v-> risnt ventricle; m ° . J ° ' Lu., lungs. is again purified of its carbonic acid gas, and the blood proceeds on its course, to renew the cycle of its circulation. Now, if we study the process of respiration and that of circulation, with which it is so closely associated, in other forms of life, we shall find many differences in detail. In the bird, for example, the mechanism of respiration is different. There is no diaphragm, and the lungs are scarcely distensible. There are, however, large air-sacs in the abdomen, in the thoracic region, in the fork of the merry-thought, and elsewhere. These are distensible, and to reach them the air has to pass through the lungs, and as it thus
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