538 



THE CARDINAL VEINS. 



to the mesonephros (Wolffian 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 (Cyclo- 

 stomata, 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 (sub- 

 clavian) usually unites with the anterior jugular 



FIG. 360. DIA- vein> 

 fntAM OF THE PAIRED The venous system of the Amphibia aud Am- 



VENOUS SYSTEM OF A n i ota a l ways differs from that of Fishes in the 



H TWIT I HV/vnn Clfmf-m J 



.PISH. (From Gegen- 

 baur.) 



presence of a new vessel, the vena cava inferior, 

 /'. jugular vein which replaces the posterior cardinal veins ; the 

 (anterior cardinal latter only being present, in their piscine form, 

 during embryonic life. It further differs from that 

 Fishes, except the Dipnoi, in the presence 



vein); c. posterior 

 cardinal vein ; //.he- 

 patic veins ; sv. sinus 



venosus; dc. ductus of pulmonary veins bringing back the blood directly 

 from the lungs. 



In the embryos of all the higher forms the general characters of 

 the venous system are at first the same as in Fishes, but with the 

 development of the vena cava inferior the front sections of the pos- 

 terior cardinal veins atrophy, and the ductus Cuvieri, remaining 

 solely connected with the anterior cardinals and their derivatives, 

 constitute the superior venae cavas. The inferior cava receives 

 the hepatic veins. 



Apart from the non-development of the subintestinal vein the 

 visceral section of the venous system is very similar to that in Fishes. 



The further changes in the venous system must be dealt with 

 separately for each group. 



Amphibia. In Amphibia (Gotte, No. 296) the anterior and posterior 

 cardinal veins arise as in Pisces. From the former the internal jugular 

 vein arises as a branch ; the external jugular constituting the main stem. 

 The subclaviau with its large cutaneous branch also springs from the 

 system of the anterior cardinal. The common trunk formed by the junc- 

 tion of these three veins falls into the ductus Cuvieri. 



The posterior cardinal veins occupy the same position as in Pisces, and 

 unite behind with the caudal veins, which Gb'fcte has shewn to be originally 

 situated below the postanal gut. The iliac veins unite with the posterior 

 cardinal veins, where the latter fall into the caudal vein. The original 

 piscine condition of the veins is not long retained. It is first of all 

 disturbed by the development of the anterior part of the important un- 

 paired venous trunk which forms in the adult the vena cava inferior. 

 This i* developed independently, but unites behind with the right posterior 

 cardinal. From this point backwards the two cardinal veins coalesce for 



