1907.] AZYGOS VEINS IJif MAMMALS. 183 



much the same view. For they remark that in the Pig " the 

 hemiazygos " (which is the left azygos of my nomenclatm-e) "fi-om 

 the i^egion of the heart to the tenth rib is therefore to be 

 regarded as the persistent anterior portion of the left post- 

 cardinal." The rest of the vein is formed, as they think, fi-om 

 "the accessory veins," which appear to be the subcardinals of 

 McClure*. They thus agree with Rathke in holding that the 

 anterior part of the azygos is persistent j)ostcardinal, but differ 

 from him as to the mode of foimation of the j^osterior region of 

 the azygos ; for Rathke held that this I'egion was due to a con- 

 tinual longitudinal anastomosis between intercostal veins, and 

 was thus an entirely new structure. Zumstein f put forward the 

 older view also in the case of man, where that anatomist believed 

 that he had traced the azj'gos and the hemiazygos to the post- 

 cardinals exclusively. In the Guinea-pig, however, he J found 

 that the postcardinals took practically no share in the formation 

 of the azygos. Hochstetter § came to conclusions which were not 

 dissimilar. He allowed in the case of the Rabbit and of the Cat 

 that the azygos of the adult down to about the eighth thoracic 

 segment was the postcardinal, but that thereafter it was a new 

 structure not formed from the postcardinal veins : " von da an 

 caudalwarts aber ist sie eine Neubildung." This region of the 

 postcardinal, in fact, becomes a part of the postcaval. A perfectly 

 difterent origin of the azygos veins is asserted by McClure ]] of 

 Didelphys. Excepting just at their entry into the duct of Cnvier 

 they are quite independent of the postcardinals and of the sub- 

 cardinals, though coiuiected with the former by cross anastomoses. 

 I shall bi'ing forward various facts in the following pages which 

 bear upon this question of the morphological nature of the azygos 

 veins in mammals. 



(2) The Azygos Veins in the TJngulata. 



I have paid special attention to this group since I have par- 

 ticularly favourable opportunities, as compared with those enjoyed 

 by other zoologists, of examining recently dead specimens. These 

 bulky animals obviously cannot be preserved, and must therefore 

 be studied immediately after death. It thus follows that I am 

 able to add a good deal to what is known upon the subject. 

 It will be seen, however, that thei'e is a very geneiYil agreement 

 among the Artiodactyle division of tliat Order as contrasting 

 with the Perissodactyles, but that the division, as shown by 

 the azygos vein, is not absolute. Max Weber, in liis recent text- 

 book (Die Saugetiere, p. 642), uses the condition of the azygos 

 to define the Artiodactyla thus : — " Die S^ena azygos fehlt ; die 



* "Development of Veins oi Didelphys" Amer. Jouvn. Anat. v. 1906, p. 163. 



f " Entwicklung des Venensj'stems des Mensclien," Anat.-Het'te, Bd. vi. 1896. 



+ " Venensj'stem bei dem Meerschweinclien," ihid. Bd. viii. 1897. 



§ ZfOC. cit. p. 574. il Loc. cit. p. 185. 



