I08 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. i ; 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 obliqueh' 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 spinal vein, coming through the spinal 

 foramen from the myel, and the median lateral vein (fig. i ; 

 M.Lat.V.), returning the blood from the central region of the 2 

 adjacent myotomes. The harnal veins pursue a similar course 

 from the ventral side of the body. Each of these vessels (fig. 

 I ; Hee.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 hsemal spine it receives the ven- 

 tral lateral vein (fig. i ; V.Lat.V.), coming from the ventral 

 portion of the two adjacent m3'otomes. Then passing obliquely 

 dorsad between the haemal lymphatic vessel and the haemal 

 spine it empties into the caudal vein. This is the normal 

 arrangement of a neural or a haemal vein ; occasionally, how- 

 ever, a neural or a haemal 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 vertebrge the caudal vein receives the urinary bladder 

 vein (figs, i 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 htemal canal of the first caudal 

 vertebra the caudal vein curves ventrad and pierces the dorsal 

 surface of the kidne}' and becomes tlie 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 tlie posterior mesenteric vein (PI. I, figs, i and 10; P.- 



