226 THE DOG-FISH 



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 two inches behind the cloaca. 

 Find the caudal vein lying in the hcemal 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, from the 

 pylorus, 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 two lobes, 

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

 necessary, but not cutting anything. 



G. 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 purified ; 

 (2) the efferent branchial vessels, which collect ihe purified 

 blood from the gills ; and (3) the dorsal aorta, which is formed 

 by the union of the efferent branchial vessels, and conveys 

 arterial blood to the body generally. 



1. The afferent branchial vessels. 



The afferent branchial vessels are most readily followed by 

 injecting them from the conus arteriosus 3 but this must not be 

 done if the heart is to be dissected in the same specimen. 



Trace the cardiac aorta forwards from the conus arteriosus, 

 and follow its branches on the left side to the gills, removing 

 as much of the muscles of the under surface of the head and 

 of the gills as is necessary to expose the vessels thoroughly. 



