88 THE PORTAL SYSTEM. 



rest of the mass new lacunas are gradually formed in the 

 substance of the membrane of the yolk, which increase the 

 number of islands, and give rise to a fine net work of ves- 

 sels, which ramify exceedingly these soon contain real 

 blood, instead of the clear, thin fluid which first filled them. 

 This vascular net-work is the commencement of the ompha- 

 lo-mesenteric vein its trunk is not the first portion formed, 

 but the ends of the vessel appear soonest, these gradually 

 unite into branches and finally produce the trunk. When 

 the omphalo-mesenteric vein is thus once formed, the rest of 

 the vascular system produces itself as follows: 



"The vein bends from below upwards, and dilates on the 

 anterior face of the body of the foetus to form the heart. 

 From this the trunk of the arteries of the body arises, 

 which carries the blood to the organs, and after this we 

 see the accompanying veins. The vessel into which the 

 omphalo-mesenteric vein opens, or to speak more exactly, 

 into which it is changed, is the vena portae. This, which 

 at a later epoch finds itself simply enclosed in the general 

 system of the veins of the body, constitutes at present the 

 principal trunk, and at its upper part produces the heart." 



The portal system, in connection with the general and 

 pulmonary, have each a common form, which is compared 

 to a tree consisting of a trunk, branches, twigs, and ramus- 

 cules. The portal, situated entirely within the abdominal 

 cavity, has its trunk about three inches in length, lying 

 between the duodenum and the liver. Its roots are the 

 numerous capillaries arising from the small and large intes- 

 tines, the stomach, pancreas, and spleen, while the almost 

 endless divisions and subdivisions in the liver, are the 

 different branches and twigs of the tree. The liver is 

 viewed as the centre of this circulation. 



The structure of the portal vessels consists of three mem- 

 branes, an external, middle, and internal. The first is a con- 

 densed cellular tissue, not so strong as in the arteries. The 

 second is fibrous and contractile, and by some considered 

 muscular, having its fibres running longitudinally; while 

 the third is a delicate serous membrane, having a smooth 



