THE PORTAL SYSTEM. 



87 



THE SECOND OR PORTAL SYSTEM OR CIRCULATION. 



The portal by 

 some physiologists, 

 is considered the 

 primary circula- 

 tion. 



Meckel remarks, 

 "we are deficient 

 in exact observa- 

 tions relatively to 

 what parts of the 

 vascular system are 

 formed first, either 

 in man or in the 

 mammalia. Never- 

 theless, we may ad- 

 mit, as almost cer- 

 tain, that the veins 

 appear before the 

 arteries, and that 

 the first are those of the umbilical vesicle for it is proved 

 in birds that the vitelline veins, and particularly the om- 

 phalo-mesenteric, are soonest developed. Now, the umbili- 

 cal vesicle, in man, corresponds exactly with the vitelline 

 sac of birds." 



"As to the mode of development of the vessels," continues 

 this Anatomist, "we learn the following from what occurs 

 in the egg. When at some distance from the embryo, we 

 see in the membrane of the yolk, which is at first homoge- 

 neous, certain rounded, circumscribed rents, which are filled 

 with a mass more fluid. These rents are at first entirely 

 separated from each other, and appear like islands in the 



FIG. 6. The vena-portarum a inferior mesenteric vein. 6 The pancreas. 

 c The splenic vein, d The spleen. Gastric veins uniting with the splenic. 

 / Superior mesenteric vein, g Descending portion of duodenum, h Transverse 

 portion, i Vena portarum. j Hepatic artery, k Ductus communis, chole- 

 dochus. I Division of vessels and duct at the transverse fissure of the lever. 

 m Cystic duct. 



