232 UN THE ANATOMY OF HIPPOPOTAMUS AMPHIBIUS. [Feb. 1, 



This is of itself important, because in the Artiodactyle Ungulates 

 (with the exception only of the Traguliclse — so far as we know at 

 present) the main azygos is on the left side. There is, I believe, 

 no other exception to this generalisation that has been described. 

 In the second place, the azygos must lie very close to, in fact 

 practically upon, the middle line of the centra of the dorsal 

 vertebrae, for the cut ends of the intercostal trunks themselves 

 lay very close indeed to this line, and I passed a probe up the 

 intercostal vessel to its dorsal cut end, which was some way from 

 the actual orifice into the azygos which, as already stated, had 

 been removed. For some way along the ribs at any rate the 

 intercostal veins were simple veins as in other Mammals, and 

 there was no breaking up of the trunks into anything approaching 

 the retia which Gratiolet has described in the case of the other 

 veins. 



In cutting away the aorta for further study the azygos was 

 found to have been removed with it, and was firmly attached 

 to the right side of the line along which the (at first sight) 

 single intercostal arteries emerge from the aorta. It was, there- 

 fore, completely concealed when the various viscera were in 

 position. It is obviously quite easy to determine upon which 

 side of the aorta the azygos lay, since the intercostals must be 

 supposed to emerge along the middle line, as they begin with 

 single arteries which later divide into two branches, one for 

 each side of the body. Judging in this way, the azygos of the 

 Hippopotamus is a right azygos, and in this it agrees with 

 Traguius and differs from, at any rate, most of the Artiodactyles. 

 It is very noteworthy that these two primitive genera should 

 agree with each other and with the Perissodactyles and Hyrax* . 

 It must, however, be recollected that Cervus sika has also a right 

 azygos vein. I have already dealt with the opening of this 

 vein into the right auricle f. The openings of its branches 

 are very obvious, and have the same irregular arrangement 

 anteriorly followed by a regular arrangement posteriorly, that 

 we find in the case of the intercostal and other minor branches 

 of the thoracic aorta. The first branch was a large one, about 

 an inch from the auricular orifice of the vein. Behind this 

 were two successive pairs of veins, of which one vein in each 

 pair was rather larger than the other, and one vein of each pair 

 was rather in advance of the other. Then followed a group 

 of four inequisized venous orifices. At this point the azygos 

 vein ceases from its close association with the aorta and passes 

 straight to the right auricle, so that the veins hitherto considered 

 belong (at any rate nearly all of them) to the visceral (oesophageal 

 and pulmonary) affluents of the azygos. After this point come 

 the regularly paired intercostals which lie on the ventral surface 

 of the azygos just in front of the corresponding intercostal 

 arteries. 



* Beddavd, " On the Azygos Vein of Mammals," P. Z. S. 1907, p. 181. 

 t Supra, p. 226. 



