Chap, v.] ARTERIES OF VERTEBRATES. 203 



From this cone or bulb cf the heart there pro- 

 ceeds a vessel which soon breaks up into a number 

 of arches (Fig. 87); in Fishes the number of these 

 is in correspondence with that of the gill clefts. 

 Within the substance of the gill plate the artery 

 (branchial artery) breaks up into a plexus of 

 tine capillaries, and these become collected into a 

 common trunk on either side which passes forwards 

 to the brain and backwards to the rest of the 

 body ; behind the heart, the two trunks unite into 

 a single median and dorsal aorta, whence vessels 

 (arteries) are given off to the different organs and 

 regions of the body. 



When, as in the Dipnoi, a pair of lung sacs 

 become developed, one of the branchial vessels (the 

 fourth) gives off on its way from the gills a large 

 trunk which passes directly to the lungs, whence 

 the blood is returned directly to the left side of 

 the heart. When the gills are lost altogether the 

 branchial capillaries lose their function, and, for the 

 greatest part, become aborted, though the frog re- 

 tains in its so-called carotid gland the plexiform 

 arrangement of the capillaries which was of use to 

 it in its gill-bearing tadpole stage. As the arterial 

 cone is retained by the Amphibia, the general re- 

 lation of the great vessels to the ventricle is the same 

 as in Fishes, and the only differences that obtain are 

 such as are due to the differences in function of differ- 

 ent vessels, which influence their size and distribution. 



In the Beptilia, as has been already explained, 

 the orifices of the great vessels, which are ordinarily 

 guarded by merely two semilunar valves, are brought 

 into closer relation with certain parts of the ventricle; 

 the arterial cone (Fig. 88 ; tr) becomes shorter, and is 

 divided internally by septa. 



In the lizard (Fig. 88) three arches arise from 

 the heart ; the two anterior are aortic, the third 



