1907.] AZYGOS VEIXS IX MAMMALS. 209 



an anterior branch fi-om the right jugular. This latter bi-anch 

 divides soon aftei' issuing from the jugular (to reverse for the 

 ■convenience of description the direction of the blood-flow) into 

 two, one anterior and the other posterior. Each of these supplies 

 .several intercostal spaces, the anterior two and the posterior 

 three. The azygos propei- does not bi'anch until after it has 

 passeil the I'egion supplied by the branch referred to. Its first 

 twig supplies two intercostal spaces. The rest are single branches. 

 On the left side of the body is a single vessel supplying only one 

 space. In the other specimen I observed no bi-anch on the left 

 side of the body. On the right side the first branch of the azygos 

 runs forward and supplies on each side of the body three inter- 

 costal spaces. It obviously corresponds to the anterior branch of 

 the jugular of the last individual. 



In Ccdogenys paca I have observed rather different arrange- 

 ments of these veins in two specimens. This animal, unlike 

 Dolichotis, has two j agulars. In one indi\ddual (both were males) 

 the azygos was developed on both sides of the body though con- 

 siderably longer- on the i-ight. On this side of the body a branch 

 is given off early in the course of the azygos which supplies 

 sevei'al intercostal sjaaces ; the main trvmk of the vein runs of 

 com'se parallel with this, and gives off no branches until the last 

 of those supplied by the large branch referi'ed to. In this speci- 

 men the inferior intercostal veins do not open independently into 

 the jugular, but into the azygos before it communicates with the 

 jugular. In the second individual both right and left superior 

 intercostals were quite independent of the azygos of their side, 

 entering each jugular separately. The azygos vein of the left 

 side of the body although present was very slightly developed, 

 4ipparently collecting blood from only one intercostal space. 



In Dolichotis 'patachonica the conditions of the azygos were as 

 follows : — The trunk of the right side is the chief one to be 

 developed. It is, however, as in some other forms (e. g. Macropus 

 dorscdis*), derived from two branches which arise separately from 

 the jugular. The fii-st is a small branch which runs in a forward 

 •direction. The second trunk is the main azygos. This at once 

 gives off a branch which divides into three twigs, the rest of the 

 branches are single in theix- origin from the azygos. On the left 

 .side of the body a vein arises three or four ribs further forward. 

 Before reaching the parietes it gives off a slender twig which 

 runs backwards along the carotid and the aorta. I have not 

 traced this vessel out to its end ; it is evidently to be compared 

 with a similar vessel in Lemur 'macaco. When it has reached 

 the parietes this azygos divides into two vessels which run in the 

 ,same straight line and are continuous fore and aft along the body- 

 wall. Two intercostal veins aiise from the anterior section and 

 three fi-om the posteiior section. I did not detect any connection 

 •of this left-hand vessel with the subclavian vein anteriorly, 



* But ill tliis case it is the left azygos which shows the peculiarity mentioned. 



Proc. Zool. Soc— 1907, No. XIV. 14 



