THE VENOUS SYSTEM. 



657 



V- - DO 



FIG. 370. ANTERIOR 

 PORTION OF THE VENOUS 

 SYSTEM OF AN EMBRYONIC 

 SNAKE. (From Gegenbaur; 

 after Rathke.) 



vc. posterior cardinal 

 vein; vj. jugular vein; DC. 

 ductus Cuvieri ; z>. allan- 

 toic vein ; v. ventricle ; ba. 

 truncus arteriosus ; a. vis- 

 ceral clefts ; /. auditory 

 vesicle. 



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 heart ; and the 

 section of the posterior cardinals opening into 

 the ductus Cuvieri gradually atrophies, their 

 posterior parts remaining however on the outer 

 border of the kidneys as the venae renales 

 advehentes 1 . 



While the front part of the posterior cardinal 

 veins is undergoing atrophy, the intercostal veins, 

 which originally poured their blood into the 

 posterior cardinal veins, become also connected 

 with two longitudinal veins the posterior ver- 

 tebral veins which are homologous with the 

 azygos and hemiazygos veins of Man ; and bear 

 the same relation to the anterior vertebral veins 

 that the anterior and posterior cardinals do to 

 each other. 



These veins are at first connected by 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 intercostal veins falls into the 

 posterior vertebral veins. They are united in front with the anterior verte- 

 bral veins, and the common trunk of the two veins on each side falls into 

 the jugular vein. 



The posterior vertebral veins are at first symmetrical, but after becoming 

 connected by transverse anastomoses, the right becomes the more important 

 of the two. 



The vena cava inferior, though considerably later in its development 

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

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

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

 border of the kidneys 2 . 



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. 



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

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

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



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

 the posterior cardinal veins, as it is stated by Gb'tte to do in the Anura. Gbtte 

 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 

 Gbtte's views. 



B. III. 



