272 



THE VITAL FUNCTIONS. 



356. 



The remarkable distribution of the muscular 

 powers, which give an impulse to the circulating 

 fluids, met with in the Sepia, constitutes a step 

 in the transition from MoUusca 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 con- 

 sist of four cavities, (c,d,e, 

 f), opening successively the 

 one into the other. The 

 heart belongs exclusively 

 to the gills ; and there pro- 

 ceeds 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 perform- 

 ing, without any intermediate heart, the office of 

 an aorta ; that is, it divides into innumerable 

 branches, and distributes the blood to every part 



