DEVELOPMENT OF THE VEINS. 1037 



the mass of mesoderm, called the septum transversum, which lies in the cranial margin of the 

 umbilical orifice and at the caudal end of the pericardium. The septum transversum afterwards 

 takes part in the formation of the diaphragm, the liver, the falciform ligament of the liver and 

 the lesser omentum. As the liver is formed from it the cardiac ends of the vitelline and lateral 

 umbilical veins become enclosed in the liver substance, and pour their blood into a number of 

 freely communicating channels or sinusoidal spaces, which form by far the greater portion of the 

 liver in the early stages of its development. 



Whilst the formation of the sinusoidal spaces is occurring in the liver, the parts of the fore- 

 gut have been denned, and the viteliine veins, as they approach the growing liver, pass along 

 the sides of the duodenum and become connected, around it, by three transverse anastomoses, two 

 of which lie ventral and one dorsal to the duodenum. Cranialwards of these anastomoses each 

 vitelline vein is broken up by the formation of the sinusoidal channels in the liver substance, 

 into a caudal part, the vena advehens, which enters the liver substance, and a cranial part, the 

 vena revehens, which passes from the liver to the heart. After a time the left vena revehens 

 loses its direct connexion with the heart, moves across towards the right, and opens into the 

 cranial end of the right vena revehens. When this change has occurred all the blood passing to 

 the liver by the vitelline veins reaches the heart by the cranial extremity of the right vena 

 revehens, which now becomes the upper end of the inferior vena cava. This also receives the 

 ductus venosus a new channel, which is evolved from the sinusoidal spaces, and carries the major 

 part of the blood from the left lateral umbilical vein to the upper end of the inferior vena cava. 



In the meantime degeneration takes place in the ventral and caudal parts of the vitelline veins 

 and the loops formed by the three transverse anastomoses between them. The ventral parts of the 

 veins disappear with the degeneration of the yolk-sac, and the right half of the caudalward and 

 the left part of the more cranialward situated loops also disappear. Simultaneously the superior 

 mesenteric vein, which has been evolved in association with the formation of the intestine from 

 the mid-gut, opens into the left vitelline vein, caudal to the dorsal transverse anastomosis, and, 

 a little later, the splenic vein enters at the same point. The final result is the formation of the 

 permanent vena portse, which is formed from (1) the cephalic end of the left limb of the caudal 

 loop between the vitelline veins ; (2) the dorsal anastomosis between the vitelline veins ; (3) 

 the right limb of the cephalic loop formed by the vitelline veins. The right branch of the portal 

 vein is the right vena advehens. The left branch of the portal vein is formed from the left vena 

 advehens, and the most cranialward of the two ventral anastomoses between the vitelline veins. It 

 is connected with the ligamentum teres of the liver, because the left lateral umbilical vein, which 

 opened at one time into the left horn of the sinus venosus of the heart, and afterwards into the 

 sinusoids of the liver, finally becomes connected with the left vena advehens, at the level of the 

 cranialward ventral anastomosis between the two vitelline veins ; and it is connected with the 

 ductus venosus so that a channel may exist by which the blood from the placenta can pass to the 

 right vena revehens without much admixture with the venous blood passing to the liver through 

 the left branch of the portal vein and the left vena advehens. Therefore the ductus venosus 

 is developed from the sinusoidal spaces of the liver when the left lateral umbilical vein is trans- 

 ferred to the left vitelline vein. 



The venae revehentes, which transfer the blood from the liver to the heart, are the cranial 

 ends of the primitive vitelline veins. The left vena revehens, as already stated, eventually loses 

 its connexion with the heart and ends in the right vena revehens, which receives the ductus venosus 

 also. The right vena revehens thus becomes the only channel by which blood is returned to the 

 heart from the alimentary canal and from the placenta : that is, it becomes the upper or cranial 

 end of the inferior vena cava. The stems of the right and left venae revehentes become the right 

 and left hepatic veins which convey to the inferior vena cava the blood which was carried from 

 the alimentary canal to the liver by the portal vein and its branches. 



The sinusoidal spaces become reduced to the blood capillaries of the liver, and the ductus 

 venosus which, during foetal life, conveyed the greater part of the placental blood to the inferior 

 vena cava becomes reduced, after birth, to the ligamentum venosum, which connects the left 

 branch of the portal vein with the upper end of the inferior vena cava. 



As the cranial part of the right vitelline vein is transformed from the right vena revehens 

 into the upper end of the inferior vena cava, an outgrowth passes caudally from it, along the 

 dorsal aspect of the liver ; this becomes connected, at its caudal end, with the right subcardinal 

 vein, and it forms that part of the inferior vena cava which lies in the groove on the dorsal aspect 

 of the right lobe of the liver. 



The Umbilical Veins. In the earliest stages of development there are three umbilical veins, 



the vena umbilicalis impar and the left and right lateral umbilical veins. The vena umbilicalis 



impar and the left lateral vein persist until birth, and a remnant of the latter is found, in the 



adult, as the ligamentum teres of the liver ; the right lateral vein disappears entirely at an 



. early stage of development. 



The vena umbilicalis impar passes from the placenta to the caudal boundary of the umbilical 

 i orifice, where it terminates in the left and right lateral umbilical veins. Each of the latter 

 unites, for a time, with the corresponding vitelline vein ; then it becomes directly connected with 

 the corresponding cornu of the sinus venosus of the heart, and still later with sinusoidal spaces 

 of the liver. The right lateral umbilical vein has also a temporary secondary connexion with 

 the right vitelline vein, but at an early period it undergoes atrophy and all parts of it completely 

 disappear. 



The left lateral umbilical vein, which is connected first with the left vitelline vein, next with 

 ; the heart, still later with the liver, and finally with the left vitelline again, at the point where 





