Animal life and intelligence . which pour theircontents into the left receiver. Thence it passes on intothe left force-pump, by which it is propelled, through a The Process of Life. 23 great arterial vessel and the numerous branches it givesoff, to the head and brain, to the body and limbs, to theabdominal viscera; in short, to all parts of the bodyexcept the lungs. In all the parts thus supplied, thevessels at length break up into a delicate capillary net-work, so that the blood-fluid is se-parated from the tissue-cells onlyby the delicate organic film of thecapillary walls. Then the bloodbeg
Animal life and intelligence . which pour theircontents into the left receiver. Thence it passes on intothe left force-pump, by which it is propelled, through a The Process of Life. 23 great arterial vessel and the numerous branches it givesoff, to the head and brain, to the body and limbs, to theabdominal viscera; in short, to all parts of the bodyexcept the lungs. In all the parts thus supplied, thevessels at length break up into a delicate capillary net-work, so that the blood-fluid is se-parated from the tissue-cells onlyby the delicate organic film of thecapillary walls. Then the bloodbegins to re-collect into larger andlarger veins. But a change hastaken place; the blood-discs havedelivered up to the tissues theirfreight of oxygen; the stream inwhich they float has been chargedwith carbonic acid gas. The veinsleading from various parts of thebody converge upon the heart andpour their contents into the rightreceiver; thence the blood passesinto the right force-pump, by whichit is propelled, by arteries, to the. Fig. 6, -Diagram of circu-lation. , left auricle of the heart;, left ventricle; H., capillaryplexus of the head; B., capillarylungS. There the blood-disCS are plexus of the body ;, alimentary7 . canal; Lr., liver; , right auricle again laden with oxveen, the stream of the heart; E-v- risflt ventricle; . 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 ofcirculation, with which it is so closely associated, in otherforms of life, we shall find many differences in the bird, for example, the mechanism of respiration isdifferent. There is no diaphragm, and the lungs arescarcely distensible. There are, however, large air-sacs inthe abdomen, in the thoracic region, in the fork of themerry-thought, and elsewhere. These are distensible, andto reach them the air has to pass through the lungs, andas i
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