39 



Type D was found by Darrach ('07) in twenty-one (3.4 

 per cent) of his series of 605 adult cats. Also, it has been 

 described in the cat by Hunt ('19). With the exception of a 

 very doubtful case mentioned by Lawrence in 1897, Type D 

 has not thus far been found in man. 



16. Persistence of embryonic supracardinal system of veins 

 to form a continuous system of vessels in the lumbar 

 and the thoracic regions, by means of which blood from 

 the iliac region can be conveyed directly to the heart, 

 without passing through the liver 



(B and C, and supracardinal (azygos), dext. and sin., in fig. 1). Figures 54, 

 man (fig. 5, Fawcett, '98) ; 55, man (fig. 1, Lauber, '01) ; 56, man (fig. 3, 

 Kollmann, '93); 57, man (Hochstetter, '93), and 58, cat (Princeton Morpho- 

 logical Museum, no. 1040) 



This type of variant has been designated by previous 

 writers as ' absence of vena cava posterior' and as 'persist- 

 ence of posterior cardinal veins.' From the standpoint of 

 its development, this variant in both cat and man is an 

 example of the persistence of the supracardinal complex of 

 veins which, in the embryo, extend as continuous channels 

 through the lumbar and the thoracic regions. In both cat and 

 man, the bilaterally symmetrical supracardinal veins of the 

 embryo subsequently anastomose freely with each other, dor- 

 sal to the aorta, in both the thoracic and the lumbar regions, 

 though in the latter, in man less freely than in the cat 

 (McClure and Butler, '25). From this complex are typically 

 derived in its lumbar segment (postrenal) the normal adult 

 cava and, in the thoracic region, the azygos vein (Huntington 

 and McClure, '20). When the supracardinal veins persist in 

 the adult as a continuous system throughout the lumbar and 

 the thoracic regions, blood from the iliac veins can be returned 

 to the heart without passing through the liver. ( See B and C 

 and supracardinal (azygos) in fig. 1.) 



As thus far observed in the adult cat and in man, all cases 

 of atypical vena cava posterior, which result from the persist- 

 ence in the lumbar and the thoracic regions of a continuous 



