1907]. AZYGOS VEINS IN MAMMALS. 193 



with it for some little distance as a longitudinal trunk, and receives 

 branches from the second to the fifth intercostal spaces inclusive. 

 The main trunk of the azygos commences with an affluent from 

 the sixth intercostal space. There is, it Avill be observed, an 

 obvious likeness here to certain Rodents also ; for instance, 

 Ccelogenys*. I may also observe that the vein lettered joosi.c. in 

 text- fig. 65 also probably corresponds to this additional azygos, for 

 the nature of which see p. 213 below. 



In Cerviis aristotelis the azygos on the right side of the body 

 pours blood into the single precaval from five affluents, which 

 collect blood from the first five intercostal spaces. On the left 

 side there is a corresponding vein which, however, flows into the 

 precaval rather in front of the right azygos, between the second 

 and third ribs. This collects blood from the first and a few sub- 

 sequent intercostal spaces. I think that it is continuous with the 

 usual Artiodactyle left azygos, which enters the heart opposite to 

 the sixth rib and extends back to the diaphragm. 



Of Perissodactyle Ungulates I have only examined Equus 

 chap'mani. The arrangement here was precisely as in the Horse. 

 The azygos was present ujjon the right side only. The first 

 affluent contributing to the vein arose from the interspace 

 between the fifth and sixth ribs, and the vessel appeared to end 

 posteriorly after the affluent from the thirteenth rib. 



Hyrax capensis. — This animal shows affinities to the Ungulates 

 in the structure of the azygos veins combined with a good many 

 differences from such Ungulates as I have been able to examine. 

 It is noteworthy that this " Subungulate" agrees with the Horse 

 and Tragulxis rather than with most Artiodactyles in the much 

 greater size of the right azygos. In view of other comparisons 

 that ha,ve been made between Hyrax and the Perissodactyle 

 Ungulates, this additional fact is of interest. In the Rhinoceros, 

 according to Owen t, the right azygos is the principal azygos. This 

 feature alone is, however, obviously not enough to establish an 

 affinity with the Perissodactyle section of the Ungulata, for many 

 mammals possess only the right azj^gos, with or without traces of 

 the left. It is the characters of the azygos on the left side which 

 indicate the Ungulate affinities of this animal. In one of two 

 examples of Hyrax capensis which I have dissected, the first two 

 costal interspaces were occupied by veins which united to form a 

 single trunk opening into the vena cava anterior. These represent 

 the superior intercostal vein of other mammals. On the right side 

 of the body the corresponding veins were present, but each opened 

 separately into the vena cava. The next two ribs on the left side, 

 i. e. Nos. 3 and 4, were supplied by a vein each, the two veins lying 

 both of them in the same intercostal space, between ribs 3 and 4. 

 These united to form a slender trunk which passed backwards 



* See p. 209. 



f " On the Anatomy of the Indian Ehinocevos," Trans. Z. S. iv. p. 46. The Tapir 

 also has been stated to possess a large right azygos. 



Proc. Zool. Soc— 1907, No. XIII. 13 



