THE VENOUS SYSTEM. 639 



some distance, to give rise to the posterior section of the vena cava inferior, 

 situated between the kidneys'. The anterior sections of the cardinal veins 

 subsequently atropliy. The posterior part of the cardinal veins, from their 

 junction with the vena cava inferior to the caudal veins, forms a rhom- 

 boidal figure. The iliac vein joins the outer angle of this figure, and is 

 thus in direct communication with the inferior vena cava, but it is also 

 connected with a longitudinal vessel on the outer border of the kidneys, 

 which receives transverse vertebral veins and transmits their blood to the 

 kidnevs, thus forming a renal portal system. The anterior limbs of the 

 rhombo d formed by the cardinal veins soon atrophy, so that the blood 

 fi'om the hind limbs can only pass to the infei-ior vena cava through the 

 renal jjortal 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 bordei" of the 

 kidney, and with the anterior abdominal veins by the epigasti-ic vein«. 



The visceral venous system begins with the develoi)ment 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 o|)euing into the sinus with a vein from the truncus 

 arteriosus. They become j)rolonged backwards, aiid after i-eceiving the 

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

 the allantoic bladdei-, unite behind into a single vessel. Anteriorly the 

 right vein atrophies and the left continues forward the iin]»aired 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 fiall 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 posterioi', 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 venae cavse. 



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

 cannot he considered as completely cstahlished. 



