VENOUS SYSTEM 221 



ing into the body-cavity, which may be double, is 

 smaller. 



Find the pericardial opening in the middle line, imme- 

 diately dorsal to the sinus venosus. Insert a seeker, and pass 

 it down the canal into the body-cavity. 



B. The Venous System, 



The venous system of the dog-fish consists of three great 

 divisions ; (1) the veins opening into the sinus venosus, which 

 return blood from all parts of the body to the heart ; (2) the 

 hepatic portal system, which conveys to the liver the blood 

 from the alimentary canal, pancreas, and spleen ; (3) the renal 

 portal system, which conveys to the kidneys the blood from 

 the part of the body behind the cloaca. 

 1 . The veins opening into the sinus venosus. 



These veins are most readily traced by following 

 them back from the sinus venosus, and they will be 

 described in this way, though it must be remembered 

 that the flow of blood in them during life is towards 

 the sinus venosus. They are chiefly remarkable for 

 their enormous size in places, the dilated portions 

 being spoken of as sinuses. 



To dissect the veins, slit open the ventral wall of the sinus 

 venosus transversely, and wash out the contained blood : then 

 follow the veins with a seeker, sliiting them open along their 

 ventral walls, and washing out the blood. If preferred, they 

 may be injected with plaster of Paris, or with a coloured solu- 

 tion such as Prussian blue suspended in water ; or they may 

 be inflated with air. 



a. The hepatic sinuses open into the sinus venosus 

 by a pair of apertures in its posterior wall, close 

 to the median plane and hear the dorsal surface. 

 The apertures lead into a pair of wide sinuses, 

 separated from each other by an imperfect septum, 

 and placed immediately behind the pericardium, 

 between it and the liver, and ventral to the 

 oesophagus. 



