656 VEINS OF THE SNAKE. 



dinal vessel on the outer border of the kidneys, which receives transverse 

 vertebral veins and transmits their blood to the kidneys, thus forming a 

 renal portal system. The anterior limbs of the rhomboid formed by the 

 cardinal veins soon atrophy, so that the blood from the hind limbs can only 

 pass to the inferior vena cava through the renal portal system. The 

 posterior parts of the two cardinal veins (uniting in the Urodela directly 

 with the unpaired caudal vein) still persist. The iliac veins also become 

 directly connected with a new vein, the anterior abdominal vein, which 

 has meanwhile become developed. Thus the iliac veins become united 

 with the system of the vena cava inferior through the vena renalis advehens 

 on the outer border of the kidney, and with the anterior abdominal veins by 

 the epigastric veins. 



The visceral venous system begins with the development of two vitelline 

 veins, which at first join the sinus venosus directly. They soon become 

 enveloped in the liver, where they break up into a capillary system, which 

 is also joined by the other veins from the viscera. The hepatic system has 

 in fact the same relations as in Fishes. Into this system the anterior 

 abdominal vein also pours itself in the adult. This vein is originally 

 formed of two vessels, which at first fall directly into the sinus venosus, 

 uniting close to their opening into the sinus with a vein from the truncus 

 arteriosus. They become prolonged backwards, and after receiving the 

 epigastric veins above mentioned from the iliac veins, and also veins from 

 the allantoic bladder, unite behind into a single vessel. Anteriorly the 

 right vein atrophies and the left continues forward the unpaired posterior 

 section. 



A secondary connection becomes established between the anterior abdo- 

 minal vein and the portal system ; so that the blood originally transported 

 by the former vein to the heart becomes diverted so as to fall into the liver. 

 A remnant of the primitive connection is still retained in the adult in the 

 form of a small vein, the so-called vena bulbi posterior, which brings the 

 blood from the walls of the truncus arteriosus directly into the anterior 

 abdominal vein. 



The pulmonary veins grow directly from the heart to the lungs. 



For our knowledge of the development of the venous system of the 

 Amniota we are mainly indebted to Rathke. 



Reptilia. As an example of the Reptilia the Snake may be selected, 

 its venous system having been fully worked out by Rathke in his important 

 memoir on its development (No. 300). 



The anterior (external jugular) and posterior cardinal veins are formed in 

 the embryo as in all other types (fig. 370, vj and vc] ; and the anterior 

 cardinal, after giving rise to the anterior vertebral and to the cephalic veins, 

 persists with but slight modifications in the adult ; while the two ductus 

 Cuvieri constitute the superior venos cavas. 



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. 



