382 Venous System of Didelphys Marsupialis 
opposite the head of the third rib. Between its point of union with 
the posteava and the middle of the tenth thoracic vertebra the azygos 
iies dorsal to the aorta and, in consequence of this position, the left 
intercostal arteries in this region pass 
Weise 
RL 
Fig. LV. A diagram representing the usual ar- 
rangement of the azygos branches in Didelphys 
marsupialis. Ventral view. 2 
el., clavicle; d. and e., tributaries of the V. cos- 
tovertebralis; do., dorsal portion of venous ring 
about clavicle; g., second intercostal vein; pc., 
posteava; pre., precava; R., rib; S8., intercostal 
space; v., ventral portion of venous ring about 
clavicle; V.a.. V. azygos; V.an., V. anastomotica ; 
V.ey., V. costovertebralis; V.i., V. intercostalis ; 
V.j.e.,V. jugularis externa; V.1., V.lumbalis; V.r., 
V.renalis; V.s., V. subclavia; V.v., V. vertebralis. 
sides, except the first two, are drained 
blood from the lumbar region, on the 
dorsad on the left side of the 
vein. Between the tenth thor- 
acie vertebra and its point of 
union with the left precava the 
azygos vein occupies a position 
dorsolateral to the aorta, and 
the left intercostal arteries 
pass dorsad on the right side 
of the vein. 
The manner in which the 
caudal end of the azygos vein 
joins the postcava was found to 
be quite variable. It may join 
the posteava to the left, which 
is the usual method, or to the 
right of the aorta; or it may 
bifurcate into two branches on 
the ventral circumference of 
the aorta, one of which joins 
the postcava to the right and 
the other to the left of the 
aorta. In the latter case the 
aorta is encircled by a venous 
ring formed by the azygos and 
posteaval veins. 
On account of the numerous 
variations which occur in the 
cranial part of the thoracic 
region it is difficult to estab- 
lish what might be called a 
typical arrangement of the 
tributaries of the azygos vein. 
The arrangement, however, 
which comes nearest to being 
typical is represented by Text 
Fig. IV. Here all of the 
intercostal spaces on _ both 
by the azygos system; also the 
left side, as far caudad as the 
wi. a 
