THE VASCULAR SYSTEM 1 77 



DEVELOPMENT OF THE VENOUS SYSTEM 



The primary veins of the mammalian embryo are four pairs 

 to which there is early added a fifth pair. These are the pre- 

 cardinals, the postcardinals, the subcardinals, the umbilicals, 

 and later the supracardinals. These primary vessels may be 

 regarded as forming the fundamental ground plan common to 

 all vertebrates. The precardinals and postcardinals join 

 in the heart region to form a duct of Cuvier on each side which 

 open into the sinus venosus. The ducts of Cuvier also receive 

 the umbilicals. The subcardinals open into the postcardinals 

 some distance caudad of the duct of Cuvier. The subcardinals 

 and supracardinals play an important role in the formation of 

 the postcava. From these five pairs of veins are developed 

 the larger venous trunks of the adult. 



A short outline of the history of the development of the post- 

 cava will serve to illustrate the complexity of vascular ontogeny 

 and of the interesting transformations taking place during the 

 embryological development of an animal. The account is 

 based upon the work done by Huntington and McClure on 

 the development of the postcava in the cat. 



After the four primary systems have been differentiated from 

 the general plexus system the first important step is the forma- 

 tion of an anastomosis between the subcardinals in the neighbor- 

 hood of the origin of the omphalomesenteric artery from the 

 aorta, the intersubcardinal anastomosis. (Fig. 89, B.) At 

 about the same level and time there is also formed on each side 

 a subcardino-postcardinal anastomosis. During this stage of 

 development there is a rearrangement of the channels and 

 sinusoids of the liver so as to furnish a more direct route for 

 the blood stream through this organ to the heart so that most 

 of the blood from the posterior regions of the body passes 

 through the liver rather than through the duct of Cuvier. 

 There is thus established in the liver a broad channel which is 

 later to form the hepatic segment of the postcava. However, 

 in order to accomplish this there has been formed in the caval 



