468 SCHULTE, SEI WHALE. 



The azygos depends developmentally upon the appearance in mammalia of the supracardi- 

 nal veins, a neomorphic longitudinal system, intermediate in position between the postcardinals 

 and the vessels of the spinal canal, with both of which it communicates at frequent intervals. 

 The fact that the azygos as well as a large portion of the postcava, is of supracardinal origin 

 offers a satisfactory explanation of the differences obtaining in these systems as between mam- 

 mals and reptiles, in which latter the supracardinals are not developed, and in the second line 

 affords a standpoint for the interpretation of the variations in the azygos and spinal collateral 

 circulation within the class mammalia. As regards the latter, communications between the post- 

 cava and the spinal plexus are so universally found when looked for, though not always of large 

 size, that their omission in the description of a specific type of circulation, is far from being 

 evidence of their absence. In my judgment they are to be looked upon as a general, though not 

 always conspicuous, feature of the mammalian plan of circulation. Their common rostral drain- 

 age is by way of the vertebral veins. Large thoracic taps are infrequent, but an approach to 

 the condition present in the Cetacea exists in the usual large spinal tap of the artiodactyl cer- 

 vico-costal vein, which in our experience is normally present. In these communications we have 

 a persistence of a part of the far greater system of anastomoses of the embryo mammal, as well 

 as in its definitive pattern a retention of the reptilian type. With the appearance of the azy- 

 gos the thoracic supracardinal and the development of its prevertebral anastomoses with 

 the postcava, a collateral equilibratory line is established, which is evidently favorably placed 

 with regard to the mechanics of circulation in most mammals, for its presence as a rule is associ- 

 ated with a reduction, not suppression, of the spinal taps and of the longitudinal anastomosis of 

 the abdominal wall. As a rare variant it may even supplant the suprahepatic postcava which 

 then discharges by way of the azygos major into the precaval trunk. 1 In the case of forms such 

 as the one under consideration we are of course in doubt from lack of knowledge of the embryo, 

 whether we are dealing with a rudimentary supracardinal system, or whether the supracardinals 

 have here developed and undergone a subsequent reduction in the embryo. Their evident role 

 in the formation of the postcava favors the latter supposition. We have here an illustration of 

 the difficulties inherent in the comparison of adult types of the venous system, when their develop- 

 ment is not known, for in view of the multiplicity of the embryonic vessels, almost any variant 

 is easily explainable as a retention of some portion of the primitive network which is usually 

 lost, and convergence may be achieved by differing developmental processes. 



The postcava has but a minimal prehapetic segment on account of the dorso-ventral orien- 

 tation of the heart, its lack of rotation, and the close adherence of the pericardium to the dia- 

 phragm. Its hepatic segment lies at the bottom of a deep caval fissure, the apposition of the 

 walls of which conceals it from view. It receives just before its emergence proximad a large right 

 and left hepatic vein, and separately on rather a more dorsal plane the ductus venosus. It 

 leaves the liver caudad beside a very rudimentary caval lobe, and descends with an inclination 

 to the left to the level of the renal veins. In this portion of its course it is entirely retroperitoneal, 

 having the portal vein and sagittal portion of the pancreas ventral, and both adherent to it. In 

 this portion of its course it receives the right adrenal vein and communicates by a large dorsal 

 branch with the vessels of the spinal cord. This anastomosis permits of the blood conveyed 

 . . _ * . 



1 Cf. McClure, C. F. W. A contribution to the anatomy and development of the venous system of Didelphys marsupialis (L.). 

 Pt. 2. Development. Am. Jour. Anat., Vol. 5, 1908. McCallum, W. G. Anomaly of the inferior vena cava with thrombosis. N. Y. 

 Path. Soc., Vol. XII, 1912. 



