THE VENOUS SYSTEM. 655 



The venous system of the Amphibia and Amniota always 

 differs from that of Fishes in the presence of a new vessel, the 

 vena cava inferior, which replaces the posterior cardinal veins; 

 the latter only being present, in their piscine form, during 

 embryonic life. It further differs from that of all Fishes, except 

 the Dipnoi, in the presence 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 posterior cardinal veins atrophy, and the ductus Cuvieri, 

 remaining solely connected with the anterior cardinals and their 

 derivatives, constitute the superior venas cavae. 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 

 subclavian with its large cutaneous branch also springs from the system of 

 the anterior cardinal. The common trunk formed by the junction 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 Gotte has shewn to be originally 

 situated below the post-anal 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 unpaired venous 

 trunk which forms in the adult the vena cava inferior. This is developed 

 independently, but unites behind with the right posterior cardinal. From 

 this point backwards the two cardinal veins coalesce for some distance, to 

 give rise to the posterior section of the vena cava inferior, situated between 

 the kidneys 1 . The anterior sections of the cardinal veins subsequently 

 atrophy. The posterior part of the cardinal veins, from their junction with 

 the vena cava inferior to the caudal veins, forms a rhomboidal figure. The 

 iliac vein joins the outer angle of this figure, and is thus in direct communi- 

 cation with the inferior vena cava, but it is also connected with a longitu- 



1 This statement of Gotte's is opposed to that of Rathke for the Amniota, and 

 cannot be considered as completely established. 



