540 



VEINS OF THE SXAKE. 



The two posterior cardinals unite behind with the caudal veins. They 

 are placed in the usual situation on the dorsal and outer border of the 

 kidneys. 



With the development of the vena cava inferior, to be described below, 

 the blood from the kidneys becomes mainly 

 transported by this vessel to the lieart; and the 

 section of the posterior cardinals oi)ening into the 

 ductus Cnvieri gradually atiophies, their posterior 

 ]>arts remaining however on the outer border of 

 the kidneys as the vente renales advehentes'. 



While the front jiart of the ])Osterior cardinal 

 veins is underi^oing atrophy, the intercostal veins, 

 which originally poured their blood into the 

 l)Osterior cardinal veins, become also connected 

 with tAVO longitudinal veins- — tlie posterior ver- 

 tebral veins — which are homologous with the 

 azygos and hemiazygos veins of Man ; and bear 

 the same relation to tlie anterior vertebral veins 

 that the anterior and posterior cardinals do to 

 each other. 



These veins are at first connected hy trans- 

 verse anastomoses with the posterior cardinals, 

 but, on the disappearance of the front part of 

 the latter, the whole of the blood from the in- 

 tercostal veins falls into the posterior vertebral 

 veins. They are united in front with the anterior 

 vertebral \eins, and the common trunk of tlie 

 two veins on each side falls into the jugular vein. 



The posterior vertebral veins are at first symmetrical, but after be- 

 coming connected by transverse anastomoses, the right becomes tiio more 

 important of the two. 



The vena cava inferior, though considerably later in its develo})ment 

 than the cardinals, arises fairly early. It constitutes in front an unpaired 

 trunk, at first very small, opening into tlie rigid allantoic vein, close to the 

 heart. Posteriorly it is continuous with two veins placed on the inner 

 border of the kidneys^ 



The vena cava inferior passes through the dorsal part of the liver, and 

 in doing so receives the hepatic veins. 



The portal system is at first constituted by the vitelline vein, which is 

 directly continuous with the venous end of the heart, and at first receives 

 the two ductus Cuvieri, but at a later period unites with the left ductus. 

 It soon receives a mesenteric vein bringing the blood from the viscera, 

 which is small at first but rapidly increases in importance. 



Anteiuoii 

 poktion of the venous 

 system of an embryonic 

 Snake. (From Gegenbaur; 

 after Eathke. ) 



vc. pOHteiior cardinal 

 vein; vj. jugular vein; DC. 

 ductus Cuvieri ; tui. allan- 

 toic vein ; v. ventricle ; ha. 

 truucus arteriosus; a. vis- 

 ceral clefts ; I. auditory 

 vesicle. 



1 Rathke's account of the vena renalis advehens is thus entirely opposed to that 

 which Gi3tte gives for the Frog, but my own observations on the Lizard incline me to 

 accept Rathke's statements, for the Amniota at any rate. 



^ The vena cava inferior does not according to Rathke's account unite behind with 

 the posterior cardinal veins, as it is stated by Gotte to do in the Auura. Gotte 

 questions the accuracy of Rathke's statements on this head, but my own observations 

 are entirely in favour of Rathke's observations, and lend no support whatever to 

 Gotte's views. 



