108 ALLEN 
branch receives a vessel coming from behind the ray. These 
2 branches unite at about the level of the apex of the neural 
spine, forming the neural vein proper. At this point the neural 
vein receives the dorsal lateral vein (fig. 1; D.Lat.V.), return- 
ing the blood from the dorsal region of the 2 neighboring myo- 
tomes, and immediately after receiving this branch the neural 
vein passes obliquely ventrad between the neural lymphatic 
vessel and the neural spine. Then curving forward and out- 
ward it crosses the neural canal, the centrum, the dorsal aorta, 
and empties into the caudal vein. In crossing the vertebral 
column it receives a sfenal vern, coming through the spinal 
foramen from the myel, and the medzan Jateral vern (fig. 1; 
M.Lat.V.), returning the blood from the central region of the 2 
adjacent myotomes. The hema/ vezns pursue a similar course 
from the ventral side of the body. Each of these vessels (fig. 
1; Hez.V.) has its source from the superficial and profundus 
levator and depressor muscles of 2 successive anal rays. In 
the region of the apex of the hzmal spine it receives the ven- 
tral lateral vein (fig. 1; V.Lat.V.), coming from the ventral 
portion of the two adjacent myotomes. Then passing obliquely 
dorsad between the hzemal lymphatic vessel and the hemal 
spine it empties into the caudal vein. This is the normal 
arrangement of a neural or a hemal vein; occasionally, how- 
ever, a neural or a hemal vein may drain the region of 3 or 
even 4 myotomes, and a neural vein may cross either side of 
the vertebral column. Usually between the first and second 
caudal vertebre the caudal vein receives the urinary bladder 
vein (figs. r and 10; Ur.B.V.), coming from the posterior sur- 
face of the bladder. Very often, however, as is shown in fig. 
10, this vein does not empty into the caudal vein, but penetrates 
the posterior ventral end of the kidney and reaches the cardinal 
through the renal veins. 
After passing through the hemal canal of the first caudal 
vertebra the caudal vein curves ventrad and pierces the dorsal 
surface of the kidney and becomes the renal portal vein. 
Shortly after the caudal vein, or renal portal as it really is, 
penetrates the kidney it receives a rather large trunk designated 
as the posterior mesenteric vein (Pl. I, figs. 1 and 10; P.- 
