THE VASCULAR SYSTEM 



351 



or unpaired, vein, from a 

 mistaken early notion that 

 it had no mate on the 

 other side; the other, the 

 incomplete one, was named 

 the hemiasygos. What lit- 

 tle applicability these names 

 may possess, however, is 

 confined to Man and allied 

 forms, since in many other 

 mammals quite different 

 results obtain. Thus, in 

 rabbits, the main trunk of 

 the hemiazygos entirely 

 disappears, and the azygos 

 receives the intercostal 

 veins from both sides, 

 while in the pig the re- 

 verse is the case and it is 

 the hemiazygos which per- 

 sists. These relations are 

 extremely variable, even 

 in Man, where the oc- 

 casional, conditions classed 

 as anomalies receive their 

 complete explanation 

 through the morphologi- 

 cal history of the region. 

 As a review of the ven- 

 ous system of Man, with 

 the morphological signifi- 

 cance of the principal 

 parts, there may here be 

 presented Fig. 100, which 

 shows the course of the 

 main trunks in the adult, 

 the older parts which have 



D.int 



FIG 100. Diagram showing the 

 history of the venous system in Man. 

 [Modified from THANE, in QUAIN'S 

 Anatomy.] 



Primitive vessels that become atrophied 

 are marked by cross lines; those secon- 

 darily established are marked by rows 

 of dots. 



