x.] THE CIRCULATING SYSTEM. 427 



Thus the blood, instead of being propelled in a double 

 circuit as in man, may, as in Fishes and young Batrachians, 

 make but a single great circuit, not returning to the heart till 

 the whole round has been completed. 



In this case the heart propels venous, unaerated blood 

 only, which, quitting that organ by the bulbus aortae, passes 

 to the gills, where it undergoes aeration, not by decomposition 

 of the water and extraction of its oxygen, but by the recep- 

 tion of that gas from the particles of air mechanically mixed 

 up with the water which the creature inhabits. Having been 

 thus oxygenated, the blood passes to the great dorsal de- 

 scending aorta, and is thence distributed over the body. 



Again, more or less blood may return to the heart before 

 being distributed to the body generally, and thus there may 

 be two circulations, as in man, and in all air-breathing 

 Vertebrates. 



In many animals which possess this double circulation, 

 both venous and arterial blood may be more or less mixed up 

 in the heart itself, and thus an impure fluid may be propelled 

 by the aortic arches. This condition exists in all Batrachians 

 and Reptiles, except the Crocodiles, though (owing to com- 

 plex conditions with regard to the valves of the chambers ot 

 the heart and of the aorta) the mixture is much less com- 

 plete than might be supposed, the blood from the lungs being 

 almost entirely forced into that aortic arch which distributes 

 its contents to the anterior region of the body. This is the 

 case even in so low an animal as the common Frog. 



The two states of blood may be as strictly divided between 

 the two sides of the heart as in man, and yet a certain im- 

 purity in the circulation may exist. This is so in Crocodiles, 

 where the two aortic arches, coming respectively from the 

 venous and arterial sides of the heart, communicate. 



That subordinate sub-division of the larger (or systemic) 

 circulation which is called the portal system may, as we have 

 seen, receive blood from more extensive sources than in man ; 

 and it may be, as we have also seen, not the" only secondary 

 circulation of the kind for another like it may at the same 

 time exist in each kidney, as is notably the case in 

 Batrachians. 



Rhythmical contractility, instead of being confined to 

 the heart, as in man, may be widely distributed, as in the 

 Lancelet ; and this is especially the case w r ith regard to the 

 proximal ends of the main arteries and veins, which may be 

 dilated respectively (as we have seen) into a bulbiis artcriosits 



