VASCULAR SYSTEM Iir 



the posterior extremities. To these are added the venae 

 abdominales, which originate in an embryonic venous appara- 

 tus, and the veins belonging to the portal system. 



The same fundamental principle is seen in the two highest 

 classes of the vertebrates. In the birds the vena cava inferior 

 is constructed of two venae renales which receive the veins 

 from the posterior limbs. Besides those branch veins which 

 take their rise in the kidneys two vense hypogastricae perform 

 the office of collecting up the blood. Similarly in mammals a 

 vena cava very early makes its appearance and receives the 

 blood from the kidneys and germinal glands, and later on from 

 the left umbilical vein. Into the lower end of the vena cava 

 flow the pelvic veins and those of the lower extremities. That 

 portion of the vena omphalo-mesenterica which receives the 

 mesenteric veins forms the trunk of the portal vein for the 

 veins of the spleen and intestinal canal. No modification of 

 this system is found in man as compared with the apes and 

 most of the mammals. The only exceptions are the diving 

 mammals (duckbill, weaver, otter, seal, dolphin, etc.), in which 

 the trunk end of the vena cava inferior is considerably en- 

 larged in order to serve as reservoir for the venous blood and 

 thus prevent undue pressure on the heart. 1 



In discussing the circulatory system, the spleen must be 

 mentioned as one of the organs for the production of lymph 

 cells, which, in the spleen, are conveyed direct to the blood- 

 vessels (see Fig. 58). Throughout the whole class of mammals, 

 with the exception of the Primates, the spleen consists of three 

 lobes (lobus anterior, medrus and posterior). In the Primates 

 the lobus posterior has almost disappeared, and in man has been 

 reduced to the protuberance of the margo obtusus. In the 

 gorilla and chimpanzee, the spleen is placed somewhat higher 

 than in the orang, where it is broad and similar to man's in 

 shape. The splenic follicles in man measure 0*35 mm.; in the 

 domestic mammals they are almost of the size of a pin's head. 

 The figures are as follows : 



In the horse . . . 'S ~ '6 rnm. 



cattle ... i 



1 Wiedersheim, loc. cit., p. 204. 



