BLOOD-VESSELS. 245 



surface the opening of the genital sinus, which 

 surrounds the reproductive organ. 



2. The renal portal system. 



The blood from the tail is returned by the caudal 

 vein, -which lies below the caudal artery in the 

 haemal arches of the vertebras. Opposite the hinder 

 ends of the kidneys the caudal vein divide into the 

 right and left renal portal veins, which run forwards 

 along the dorsal edges of the right and left kidneys 

 respectively, receiving branches from the body-wall, 

 and entering the kidneys along their whole length. 



The dissection of the renal portal system should be post- 

 poned till after that of the urino-genital system, in order to 

 avoid damaging the cloaca. 



Cut across the tail about tivo inches behind the cloaca. 

 Find the caudal vein lying in the hamal arches of the vertebra, 

 ventral to the caudal artery ; trace the vein forwards with a 

 seeker, or inject it, and then dissect it out. 



3. The hepatic portal system. 



The hepatic portal vein is formed by the union of 

 twO veins, one from the main lobe of the spleen and 

 the other from the intestine. It runs forwards in the 

 mesentery along the dorsal border of the pancreas ; 

 and, receiving veins from the pancreas and from the 

 stomach, enters the liver a little to the right of the 

 median plane. 



Find the hepatic portal vein at the point where it enters 

 the liver, a little to the right of the junction of the tico lobes, 

 and follow it backwards, turning aside the viscera as may be 

 necessary, but not cutting anything. 



C. The Arterial System. 



The arterial system may conveniently be described under 

 three heads : (1) the afferent branchial vessels, which carry 

 the venous blood from the heart to the gills to be aerated ; 



