590 



COMPARATIVE ANATOMY. 



veins of their own side. In many Teleostei, this caudal vein is 

 continued into the right cardinal by a large, and into the left 



cardinal by a smaller branch ; in this case the 

 left cardinal vein is also smaller than the right 

 one. This leads to the condition in which the 

 whole of the caudal vein passes into the right 

 cardinal ; this has been observed in a number of 

 Teleosteans. 



As the caudal vein sends off branches into 

 the kidneys, which either break up completely or 

 partially in this organ, these branches form the 

 venas renales advehentes, and send their blood 

 into the cardinal veins through the vena3 reve- 

 hentes. In this way the renal portal system 

 is developed. A second vascular apparatus of 

 similar character has its roots on the digestive 

 canal ; its venous blood is carried to the liver by 

 a trunk which is known as the portal vein. It 

 is distributed in this organ, and is carried to the 

 common venous sinus by hepatic veins, which 

 are generally united into several trunks. 

 In this arrangement of the venous system in Fishes we may 

 distinguish the paired, and ordinarily symmetrical, portion from the 

 unpaired portion, which is solely represented by the hepatic veins. 



We will first follow out the former in its 

 changes throughout the Vertebrate series, for 

 in all of them its essential characters, at any 

 rate, may be observed in the early stages of 

 development, as a transmitted arrangement, 

 and, since it is the groundwork of the embry- 

 onic venous system, it furnishes the starting- 

 point for all later metamorphoses. 



Fig. 334. Diagram 

 of the primitive 

 venous system. 

 j Jugular, c Cardinal 

 vein. dc Ductus 

 Cuvieri. h Vena3 

 hepaticee. sv Sinus 

 venosus. 



§ 438. 



Fig. 335. Anterior por- 

 tion of the venous sys- 

 tem of an embryonic 

 Ophidian, v Ventricle. 

 ha Bulbus arteriosus, 

 c Auricle. DO Ductus 

 Cuvieri. vc Cardinal 

 vein, vj Jugular, vu Um- 

 bilical vein. {/Primitive 

 kidney. I Rudiment of 

 the labyrinth (after H. 

 Rathke). 



In the Amphibia and Reptilia the venous 

 sinus receives the two jugular veins, which 

 have the same area of origin as in Fishes. 

 They persist in all the higher Vertebrata, 

 while the hinder pah- — the cardinal veins 

 (Fig. 335, vc) — have only the same characters 

 as in Fishes, during the earliest stages of em- 

 bryonic life. They are the veins of the primi- 

 tive kidneys (U). Their anterior portion is 

 obliterated, while their posterior portion re- 

 ceives veins from other regions, and forms the venas renales adve- 

 hentes. Even before the disappearance of that part of the cardinal 

 veins which opens into the ductus Cuvieri, four other trunks are 

 developed in the Reptilia, which chiefly receive the intercostal veins, 



