1907.] AZYGOS VEINS IX MAMMALS. 217 



been mentioned, the azygos bends over the vertebi-al coknnn and 

 joins a vein rnnning nearer to the ventral side of the body, which 

 is hidden posteriorly by the kidney and opens into the post- 

 caval near to the entry thereinto of the renal vein. Previously 

 to this the vein has collected tributaries from the lumbar body- 

 w^all. This is precisely what occurs in other and adult mammals. 

 It is frequently' the case that the azygos, after emitting the vein 

 to the last intercostal space, ciu-ves in to the same way over the 

 vertebral column and can be traced back to the kidney, in the 

 neighbourhood of which it debouches into the postcaval vein. 

 It seems to be clear, from the relations of the different sections of 

 the veins concerned, that the bend to the right (of the right 

 azygos) is the equivalent of one of the cross branches, by many 

 of which the azygos is put into communication with the right 

 postcardinal. Thus the complete azygos in such mammals is 

 azygos plus one cross communication with the postcardinal pZ?(s 

 the lumbar section of the postcai-dinal. The azygos, therefore, 

 has as complex a formation as has the postcaval itself. 



This condition of the azygos in many mammals contrasts with 

 the conditions to be observed among the Marsupials. Hei'e, as 

 has been pointed out, it is not infrequent for one azygos, either 

 right or left, to be a vein of very considerable importance, not 

 merely in the thoracic region but also in the abdominal region. 

 In various Diprotodont Marsupials, duly recorded in the pre- 

 ceding pages, I have found a thick vein in the kidney legion, 

 sometimes even extending furthei- back, in fact quite into the 

 pelvic region. This vein, which in the abdominal region runs 

 pai-allel with the postcaval, is connected with that vein by an 

 anastomosis. Fvu^ther forwards it is directly continuous with the 

 azygos vein of its side, which lies in or very nearly in the same 

 straight line with it. Not unfrequently there is a thinning 

 before the two veins meet, but still a continviit}'. This Avas seen 

 for example in a specimen of ^Epyprymnus rufescens. In others 

 there was no such thinning. These cases are, as it appears to 

 me, to be best explained on the assumption that here the " azygos " 

 of the adult is in reality the persistent postcardinal of its side. 

 Hence the absence of the bend such as occurs in Myopotmnus 

 coypu. The thinning I put down to the "break" which has 

 already been referred to as occurring between the thoracic and 

 abdominal parts of the postcardinal in many mammals 



There are two other matters for consideration in view of this 

 suggestion with reference to the diffeiing natvire of the azygos in 

 the adult. Firstly, the variability of these veins becomes of a 

 different aspect, since it is not one but two distinct veins or pairs 

 of veins which vary. The actual variabilit}" is therefore reducible 

 in consequence, and to be considered from different points of 

 view. The A^ariability in the proportions of the two azygos veins 

 in the Diprotodont Marsupials is, if my suggestions be correct, 

 obviously not the same thing as the variability iu the two veins 

 which bear the same name among the holloAv-horned Ilximinants. 



