654 



THE CARDINAL VEINS. 



venosus, 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 primitively 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 well as their 

 absence in Amphioxus, probably indicate that they became evolved, at any 

 rate in their present form, within the Vertebrate phylum. 



The embryonic condition of the venous system, with a single 

 large subintestinal 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 from the trunk. 



The cardinal veins appear in Fishes as four paired longi- 

 tudinal trunks (figs. 363 and 369), two anterior (/) and two 

 posterior (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. 627, 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 placed one on each side above the level 

 of the branchial arches (fig. 299, a.cv). The 

 posterior cardinal veins lie immediately dor- 

 sal to the mesonephros (Wolfifian body), and 

 are mainly supplied by the blood from this 

 organ and from the walls of the body (fig. 

 275, c,a.v). In many forms (Cyclostomata, 

 Elasmobranchii and many Teleostei) they 

 unite posteriorly with the caudal veins in 

 the manner already described, and in a large 

 number of instances the connecting branch 

 between the two systems, in its passage through 

 the mesonephros, breaks up into a capillary 

 network, and so gives rise to a renal portal 

 system. 



The vein from the anterior pair of fins 

 (subclavian) usually unites with the anterior 

 jugular vein. 



FIG. 369. DIA- 



OF A FISH. (From 

 Gegenbaur.) 



j. jugular vein 

 (anterior cardinal 

 vein) ; c. posterior 

 cardinal vein; h. he- 

 patic veins ; sv. sinus 

 venosus ; dc. ductus 

 Cuvieri. 



