THE MAMMALIAN VENA CAVA POSTERIOR 13 



longitudinal vessels, there is shown at the iliac level a median 

 vein (C.C.) lying ventral to the aorta which, with contiguous 

 veins, forms venous rings through which pass the umbilical 

 arteries. This has been termed the cardinal collateral vein 

 (C.C., fig. 1), as it is a caudal component of a longitudinal 

 subaortic system of veins, between the iliac and the renal 

 levels, which, though transitory in the cat, in marsupials 

 forms the vena cava posterior (McClure, '06). This subaortic 

 plexus (C.C., cardinal collateral system of veins) reaches the 

 height of its development in the lumbar region of a 29-mm. 

 cat embryo, as is shown in figures 4 and 5, representing, re- 

 spectively, the right and the left sides of a reconstruction. 



The veins at the renal level form a prominent circumaortic 

 venous ring, which we have designated the renal collar 

 (fig. 1). It is in this region that all the early bilaterally sym- 

 metrical longitudinal venous channels drain into the unpaired 

 prerenal division of the posterior vena cava (P.Subc. and 

 P. Hep.) which, from the renal collar, extends craniad through 

 the liver to the right auricle. 



The bilaterally symmetrical periureteric venous rings of 

 early stages, through which pass the ureters, are accurately 

 represented in the diagram (fig. 1). It is to be observed that 

 as each ureter (Ur.) emerges from the kidney, it at first 

 passes dorsal and then farther caudad ventral, to the pos- 

 terior cardinal vein. 



The supracardinal system of veins (Sprc.) is shown in the 

 diagram (fig. 1) as a bilaterally symmetrical system of veins, 

 extending as continuous vessels through the lumbar and the 

 thoracic regions. In the thoracic, as in the lumbar region, the 

 supracardinals anastomose freely with each other dorsal to 

 the aorta, and it is from this intersupracardinal complex 

 (Int.Sprc.Anast.) that the dorsally situated azygos vein of 

 the adult is for the most part developed. 



If the reader will take the trouble to master the simple 

 conditions represented in figure 1, existing amongst the longi- 

 tudinal venous channels and the anastomoses between them 

 at different levels, he will have no difficulty in interpreting 



uu LIBRARY 



