1907.] AZYGOS VEINS IN MAMMALS. 201 



I shall commence with the Kangaroos, of which I have examined 

 a good many different species. 



In j\Iacro2nt,s ualahatus the condition of the azygos veins is the 

 most primitive of that to be seen in Marsupials. There are two 

 veins, one on each side, each, of course, opening into its corre- 

 sponding vena, cava anterior. These veins appear to be exactly 

 equal in length. 



In MacTopus agil'is I have found one of the two opposite 

 extremes. In the single example of this species which I have 

 dissected there was but one azygos, lying upon the right side of the 

 vertebral column. In the preceding species of Macropus the two 

 azygos veins end, or at any rate dwindle to almost nothing, in the 

 neighboui'hood of the last rib behind the diajDhragm. Their last 

 affluent is derived from the body- wall close to the last rib. In 

 the specimen of Mao^opus agUis, on the other hand, the single 

 right azygos is continued as a wide vein for a long distance back- 

 wards. A little in front of the kidney it ceases to be superficial 

 and is imbedded in the doisal musculature. It then emerges and 

 gives ofl' a branch joining the vena cava posterior in the region 

 of the kidney. This vessel is not the renal vein itself, for the 

 latter can be followed from the kidney to the vena cava as a 

 distinct and much thinner vessel. Behind this again the azygos 

 dies away towards the pelvis, i^etaining till its extremity a con- 

 siderable size. 



In Alacrojyus melanops I found in one individual the same state 

 of affairs as in the last species in so far as only a single azygos 

 was present, and that on the right side. This vein, however, was 

 slender and drew blood from only six intercostal spaces, after 

 which it dwindled or entirely disappeared. In a second specimen 

 of the same species there was also a limited azygos on the right 

 side ; but, in addition to this, traces of one belonging to the left 

 side of the body, which, however, was quite small and only drew 

 blood from one intercostal space. 



An example of Macropus bruni showed conditions intermediate 

 between the two extremes already dealt with. In this species 

 there was an azygos on the right side extending as far back as to 

 the diaphragm. On the left side there was a less completely 

 developed vein only dra,wing blood from four intercostal sj)aces. 



A female Macropus dorsalis was not very different. In this 

 Kangaroo the azygos was only properly developed upon tlie right 

 side of the body. It was prolonged, however, for a long way back 

 as in Macropus agilis. In the same way a branch was given off 

 to the vena cava posterior underlying the renal vein. The azygos 

 trunk ended some way behind this. 



On the left side of the body there were traces of a left azygos. 

 This consisted of the azygos proper, being a short vessel gathering 

 blood from one intei-costal space only, and of another trunk in 

 front of this flowing separately into the vena cava anterior of that 

 side of the body, which corresponds to the superior intercostal o£ 

 other mammals. 



