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 

 it is propelled, by arteries, to the 



Fig. 6, 



-Diagram of circu- 

 lation. 



L.A., left auricle of the heart; 

 L.V., left ventricle; H., capillary 

 plexus of the head ; B., capillary 

 lungS. There the blood-disCS are plexus of the body ;A.C, alimentary 

 7 . canal ; Lr., liver ; R.A., right auricle 



again laden with oxveen, the stream of the heart ; E - v - ri s flt 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 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 passes through the delicate tubes of the lungs, it 

 supplies the blood with oxygen and takes away carbonic 



