CIR CULA TOR Y S YSTEM. 



525 



on the ventral surface of the skull. Just after the first and second main 

 efferent branches have united, a vertebral is given off, which passes 

 through a hole in the vertebral plate to the spinal cord and brain. 



The dorsal aorta gives off (i) a subclavian to each pectoral fin ; (2) 

 a cceliac to the stomach, duodenum, and liver ; (3) a superior mesen- 

 teric to the intestine, pancreas, and spleen ; (4) spermatic arteries to 

 the reproductive organs ; (5) an inferior mesenteric to the rectum ; 



(6) renal arteries to the kidneys ; 



(7) arteries to the pelvic fins. It 

 ends in the caudal artery. 



At each end of the bow-shaped 

 sinus venosus there is a precaval sinus. 

 This receives venous blood as follows : 

 (a) from the head by a jugular vein ; 

 (/>) from the liver by a hepatic sinus, 

 which runs from one precaval sinus 

 to the other like the string of the bow ; 

 (r) from a large posterior cardinal 

 sinus (between the reproductive 

 organs) by a cardinal vein on each 

 side ; (d) from the hind-fin by an 

 epigastric, with which brachials from 

 the fore-limb unite anteriorly, The 

 great cardinal sinus receives blood 

 from the hind-limbs, the kidneys, and 

 other posterior parts. 



Blood passes info the liver (a) 

 from the cceliac artery, and (l>) by 

 portal veins from the intestine (the 

 hepatic portal system) ; blood leaves 

 the liver by hepatic veins which enter 

 the hepatic sinus. 



Blood passes into the kidneys (a] 

 from the renal arteries, and (b} by 

 renal portal veins from the caudal, 

 pelvic, and lumbar regions (the renal 

 portal system) ; blood leaves the 

 kidneys by posterior cardinal veins, 

 which enter the cardinal sinus. 



Into the precaval sinus there also 

 opens the lymphatic trunk. 



The heart lies in a pericardial 

 cavity, which is connected with the 

 abdominal cavity by two fine canals, and is an anterior part of the ccelom. 

 The blood contains, as usual, red blood corpuscles and leucocytes. 



The dark red spleen lies in the curve of the stomach. 

 The red thyroid gland lies just in front of the anterior 

 end of the ventral aorta. The whitish thymus gland is a 

 paired structure lying dorsally above the gills. 



FIG. 253. Urogenital organs 

 of male skate. From a speci- 

 men in Edinburgh Museum of 

 Science and Art. 



T., Testis ; Ep., epididymis ; v.J., 



vas deferens ; K., kidney; v.s., 



seminal vesicle ; s.s., sperm sac ; 



u.g.s., urogenital sinus ; CL, 

 cloaca. 



