VEINS OF VERTEBEATA. 589 



in tho Amphibia, the Art. coeliaco-mesenterica arises from the end 

 of the left aortic arch in the Reptilia (Saurii, Chelonii) ; this arch 

 is only connected by a narrow tract with the right one ; or there 

 are several visceral arteries (some Sanrii) ; these are especially 

 nnmerons in the Ophidii, in consequence of the elongated form of 

 their body. In the Crocodilinii, also, independent mesenteric arteries 

 are given off from the unpaired aorta in company with the arteries 

 from the left aortic arch. 



In Birds, where the left aorta disappears, the aortic trunk is the 

 sole vessel from which the visceral arteries are given off. 



In Mammals the coeliac and superior mesenteric artery are tho 

 chief arteries of the enteric canal. In the placental forms the inferior 

 mesenteric is also a large vessel. 



The large number of renal arteries found in Fishes are found also 

 in the Amphibia and in most Reptiles ; in Birds also there are several 

 renal arteries, the middle one being given off from the ischiac artery. 

 It is very rarely that there are a number of these arteries in the 

 Mammalia. 



The arteries of the hind-limbs are not direct branches of the 

 posterior aorta until these parts are very largely developed. Tho 

 two chief trunks (iliac arteries) of this region are not always tho 

 same. As is clear from their topographical relations to the pelvis, 

 different branches may supply the area of these arteries. In tho 

 Sauropsida the ischiac are the chief trunks of the hinder extremities. 

 In the Mammalia the chief trunks are formed by the crural artery, 

 and there are numerous modifications in its more special characters 

 which are of less importance. 



Venous System. 



§ 437. 



Tho venous system of the Vertebrata exhibits no less important 

 phamomena in the various modifications that obtain in it, as we pass 

 from the Fishes to the Mammalia, than does the arterial portion of 

 the circulatory system. Our knowledge is in many points as yet 

 incomplete. The blood which returns to the heart is, in Fishes, 

 collected into four longitudinal trunks, two anterior and two 

 posterior. Those of either side pass into a transverse trunk (ductus 

 Cuvieri, Fig 334, dc), which opens with that of the opposite side into 

 a sinus (sv) which is placed behind the auricle of the heart. The 

 anterior pair, which chiefly collects the venous blood of the head, 

 forms the jugular veins (J), which are placed above the branchial 

 arches ; the hinder pair, which receives the blood from the walls 

 of the trunk and from the renal and generative organs, forms the 

 cardinal veins (c); an unpaired caudal vein runs below the artery in 

 the caudal canal ; in the Cyclostomata, Selachii, and some Teleostei, 

 this divides into two branches which are continued into the cardinal 



