TllK VENOUS SYSTEM. 



537 



There join the portal system in Myxinoids and many Teleostei 

 a number of veins from the anterior abdominal walls, representing 

 a commencement of the anterior abdcmnnal or epigastric vein of 

 higher types*. 



In the higher Vertebrates the original subintestinal vessel never attains 

 a full developmeut, even in the embryo. It is represented by (I) the 

 ductus veuosus, which, like the true subintestinal vein, gives origin (in 

 the Amniota) to the vitelline veins to the yolk-sack, and (2) by the caudal 

 vein. Whether the partial atrophy of the subintestinal vessel was primi- 

 tively caused by the development of the cardinal veins, or for some other 

 reason, it is at any rate a fact that in all existing Fishes the cardinal veins 

 form the main venous channels of the trunk. 



Their later development than the subintestinal vessel as w^ell as their 

 absence in Amphioxus, pro- 

 bably indicate that they be- 

 came evolved, at any rate in 

 their present form, within 

 the Vertebrate phylum. 



The embryonic condi- 

 tion of the venous system, 

 with a single large subin- 

 testinal vein is, as has been 

 stated, always modified by 

 the development of a 

 paired system of vessels, 

 known as the cardinal 

 veins, which bring to the 

 heart the greater part of 

 the blood froni the trunk. 



The cardinal veins ap- 

 pear in Fishes as four 

 paired longitudinal trunks 

 (figs. 363 and 369), two 

 anterior ( j) and two pos- 

 terior (c). They unite into 

 two transverse trunks on 

 either side, known as the 

 ductus Cuvieri {dc), which 

 fall into the sinus venosus, 

 passing from the body wall 

 to the sinus by a lateral 

 mesentery of the heart already spoken of (p. 515, fig. 352). The 

 anterior pair, known as the anterior cardinal or jugular veins, bring 

 to the heart the blood from the head and neck. They are 

 [)laced one on each side above the level of the branchial arches 

 (fig. 29S), a.cv). The posterior cardinal veins lie immediately dorsal 



■mp 



-cl.ul 



Fig. 368. Four sections theough the posr- 



ANAL PABT OF THE TAIL OF AN EMBKYO OF THE SAME 

 AGE AS FIG. 28 F. 



A is the posterior section. 

 nc. neural canal; al. post-anal gut; alv. caudal 

 vesicle of post-anal gut; x. subnotochord rod ; m.'p. 

 muscle-plate; ch. notocliord; cl.al. cloaca; ao. 

 aorta; v.cau. caudal vein. 



Stannius, Venjlcich. Aiiat., p. 251. 



