46 HtGH SCHOOL ZOOLOGY. 



Ise of the gill-arches, and gives off the vessels to these. These 

 various divisions of the heart are sepai-ated from each other by 

 pocket-like semilunar valves, which prevent the reflux of the 

 blood after the contractions of its cavities. 



62. The blood-supply of the gills is one of the most interesting 

 parts about the vascular system, because, as we find that the 

 embryos of the air-breathing animals have gill-arches and gill- 

 clefts which disappear as development goes on, so we find that 

 gill-vessels are present also, which afterwards, however, become 

 much altered. These vessels, jmrtaking of the shape of the 

 parts they accompany, are called tiie aortic arches, and they 

 are uninterrupted tubes which arch on either side from the 

 arterial trunk below the alimentary canal, to another arterial 

 trunk above it, which from its position is called the dorsal 

 aorta. Before joining the dorsal aorta, some of the anterior 

 arches give off branches which supply arterial blood to the head. 

 A similar condition obtains in the adult fish, only instead of an 

 uninterrupted aortic arch, there are two vessels to every gill- 

 arch, one which distributes the blood to the gill-filaments, the 

 other which collects it from them. These are known as the 

 aflferent and eflferent branchial arteries. It is therefore the 

 latter which unite to form the dorsal aorta, after the foremost 

 one has on each side given off the carotid arteries to the 

 head. In its course backwards underneath the vertebral 

 column, the dorsal aorta gives ofi" various bi-anches both to the 

 contents of the ccelom, and to the other parts of the trunk and 

 tail, but the venous streams, which collect the blood from these 

 various parts, .undergo some delay in their return to the heart. 

 For example, the venovis blood from the tail, is in part subjected 

 to the action of the kidney, before it reaches the heart through 

 the posterior cardijaal veins, (situated immediately underneath 

 the trunk vertebra^,) while the rest of it, with the blood col- 

 lected from the other contents of the coelom, enters the liver 

 through the portal vein, and is there subjected to the action of 



