CHAP, xm.] CIRCULATION. 199 



acid. The case is naturally the same with the mammifers ; in a 

 cetacean, the dugong, the two hearts exist, but they are almost 

 entirely separate. In proportion as the cardiacal cavities take 

 distinct shape they subdivide each into two secondary cavities by 

 means of membranous valves not contractile. Other valves of 

 analogous structure are found at the orifice of the large vessels 

 which go from the heart or debouch into it. 



The circulatory system of the vertebrates forms an ensemble 

 of closed canals, containing a special liquid. There are no 

 longer any lacunae, or at least they are merely exceptional and 

 partial (Fig. 21). From the heart goes a system of efferent or 

 arterial canals, while to the heart comes another afferent or 

 venous system. Lastly, these two systems of canals, which in 

 the superior vertebrates have a different structure, are connected 

 with each other by very fine canalicules, called capillary vessels, 

 with very thin walls. These capillary vessels can be classed in 

 three principal networks, among the superior vertebrates. One 

 of these networks extends on the surface of the respiratory 

 organs : it is the one which gives issue outwardly to the carbonic 

 acid : it is the one which gives at the same time access to the 

 oxygen : it is the network of the respiratory capillaries. The 

 second grand network of capillary canals exists in the liver : it 

 springs from the subdivision of the intestinal venous system. 

 The vasculary ramuscules which constitute it intertwine round 

 the glycogenical cells of the liver, then unite anew to form 

 veins (hepatic veins), which pour their contents into the general 

 venous system. The group of the capillaries of the liver may be 

 called the network of the glycogenical capillaries. The ensemble 

 of all the other capillaries, belonging neither to respiration nor 

 to glycogenesis, may be denominated the network of the nutritive 

 capillaries. The capillaries of this last network plunge into the 

 mechanism of all the organs, even into that of the respiratory 

 organs and of the liver; it is through their walls that the 

 nutritive exchange is effected, that the anatomical elements 

 receive their assimilable substances, and reject their disassimi- 



