14 CHARLES P. W. MCCLUEE AND GEORGE S. HUNTINGTON 



the variant conditions presented by the veins in the adult cat 

 or in man, to be described in the following pages. 



As longitudinal channels, the subcardinals in the cat lose 

 their identity at a relatively early stage of development, and 

 the chance of their being retained in the adult in their em- 

 bryonic form is negligible. With the exception of the inter 

 subcardinal anastomosis at the renal level (Int.Subc.Anast., 

 fig. 1), of the pars subcardinalis of the vena cava (P.Subc.}, 

 of a segment of the internal spermatic veins, and of the 

 adrenal veins, the subcardinals are not otherwise carried into 

 the adult stage. 



The subaortic plexus in the lumbar region (cardinal col- 

 lateral veins, C.C., in figs. 4 and 5) develops late in ontogeny, 

 in connection with the supracardinals, after the latter have 

 assumed the function of returning practically all blood from 

 the iliac veins to the prerenal division of the cava ; therefore, 

 the chance of these veins being retained in the adult is like- 

 wise very slight. The posterior cardinals, however ( A and Z>, 

 fig. 1), function as important vessels in conveying blood from 

 the iliac region to the prerenal division of the cava until a 

 relatively late stage of development (16- to 17 -nun, cat em- 

 bryo), and then are gradually replaced in performing this 

 function by the supracardinal veins (B and C, in fig. 1). The 

 persistence of the two posterior cardinals and the two supra- 

 cardinals so late in ontogeny as conveyors of blood from the 

 iliac veins to the prerenal division of the cava, accounts for 

 the fact that some one, or else a combination of two or more 

 of them, is often carried into the adult to establish an atypical 

 condition. Most of the atypical forms of cava found in the 

 lumbar region of the adult cat are due to the persistence of 

 the right posterior cardinal, of the left supracardinal, of the 

 left posterior cardinal, or of any one of the possible combina- 

 tions which exist between the two posterior cardinal and the 

 two supracardinal veins. As a matter of fact, there are in the 

 lumbar region fifteen (15) possibilities of the persistence of 

 one or more of these four veins. One of them constitutes the 

 normal type of cava, while fourteen are atypical in character. 



