196 



THE VITAL FUNCTIOlSrS. 



the Sepia, constitutes a step in the transition from Mollusea 

 to Fishes. In this latter class of animals, the two lateral 

 hearts have united into a single central heart, while the 

 aortic heart has entirely disappeared; and thus the position 

 of the heart with respect to the two circulations is just the 

 reverse of that which it has in the invertebrated classes. 



The plan in Fishes is shown in the 

 diagram, Fig. 356, where the cen- 

 tral organs are seen to consist of 

 four cavities, c, d, e, f, opening 

 successively the one into the other. 

 The heart belongs exclusively to 

 the gills; and there proceeds from 

 it, not the aorta, but the trunk of 

 those branchial arteries (f,) which 

 convey the whole of the blood to 

 the respiratory organs (g, h.) This 

 blood, after being there aerated, is 

 collected by the branchial veins 

 (i,) which unite into a single trunk 

 (a,) passing down the back, and performing, without any 

 intermediate heart, the office of an aorta; that is, it divides 

 into innumerable branches, and distributes the blood to 

 every part of the system.* The blood is then reconveyed 

 to the heart by the ordinary veins, which form a large vena 

 cava (c.) This vein is generally considerably dilated at 

 its termination, or just before it opens into the auricle, con- 

 stituting what has been termed a venous sinus (s.) This, 

 then, is followed by the auricle (d) and the ventricle (e;) 

 but, besides these cavities, there is also a fourth (f,) formed 

 by a dilatation of the beginning of the branchial artery, and 

 termed the biilbus arterlosnSy contributing, doubtless, to 



• The caudal brancli of the aorta is protected by the roots of the inferior 

 spinous processes, joining- to form arches through which it passes; and fre- 

 quently the artery is contained in a bony channel, formed by the bodies of 

 the vertebr?e, which effectually secures it from all external pressure- In the 

 sturgeon even the abdominal aorta is thus protected, being- entirely concealed 

 within this bony canal. 



