1907.] 



AZYGOS VEINS IN MAMMALS. 215 



actually observe the arteries here) ; later on it could be distinctly 

 seen to lie within that area. The intercostal arteries could be 

 observed to lie outside of the vein. We can now consider what is 

 the nature of these several veins. There seem to me to be only 

 three possible views, one of which is so improbable thatit may be 

 shortly considered first of all and then dismissed _ This view is 

 that the paired vessels are two azygos veins which have been 

 formed in a way not indicated in the specimens from the post- 

 <;ardinals, and that the single more median vein represents the 

 oesophageal branch which is at least very commonly deve oped 

 from the azygos and often leaves it near to its connection with the 

 precaval If this is the case the vessel must degenerate very imicli 

 in maturity, and its numerous cross connections^ with the paired 

 veins are not easy to understand. It seems m fact unlikely that 

 such unimportant vessels as the visceral branches of the azygos 

 should have so portentous a beginning and so impotent a conclu- 

 sion The other alternatives are mutually exclusive. _ Jiither the 

 paired veins are the postcardinals and the unpaired vein is a single 

 ,azvR-os or the single vein is the one remaining postcardinal and 

 the paired veins are azygos veins which have been derived from 

 that and another vanished postcardinal. The first of these views 

 seems to me to be fairly obviously the correct one; and tor the 

 following reasons. In the first place, paired postcardinals and an 

 unpaired azygos are more likely than the reverse. This is, ot 

 <;oiu-se, not conclusive, for one ecc hypothesi postcardinal might 

 have disappeared. Secondly, the presumed azygos corresponds 

 with the adult Myopotamus where there is only one azygos, and, 

 as is so general with the azygos, it lies partly outside and partly 

 inside the intercostal arteries. The median position too of the 

 presumed azygos is like that of undoubted azygos veins, while the 

 lateral position of the presumed postcardinals is again m consonance 

 with that view of their nature. In the next place, Dr. McOhire * 

 has pointed out that the azygos arises from the postcardinal m the 

 Opossum to the median side of that vein, which is precisely the 

 position occupied by the presumed azygos in the young Myopotamus 

 Furthermore, the break between the thoracic and the abdominal 

 re-ions of the paired veins is in accord with the view that they 

 really are the postcardinals; for such a break has been described 

 in the course of the development of those veins. 



It is obvious that these facts throw some light upon the azygos 

 veins in general. It seems highly probable that we shall have to 

 distino-uish in future between real azygos veins or vein and 

 persistent postcardinals. It is even possible that the true azygos 

 vein is always a single vein, and that when there are apparently 

 two— one on each side— it is a case either of both postcardinals 

 ner^istinc- or of a single azygos with one persistent postcardinal. 

 That the?e should be such large difi-erences even in allied mammals 

 surprising; but it is by no means impossible even on 



.seems 



* Lne. cif. (see p. 183). 



