48 VERTEBRATE ZOOLOGY 



The Vascular System. This is made up of the following organs : 

 (i) the heart, which receives venous blood from the tissues and 

 forwards it to the gills; (2) the arteries, which carry (a) venous 

 blood from the heart to the gills and (b) arterial blood from the 

 gills to the tissues ; (3) the veins, which carry venous blood to the 

 heart; and (4) the capillaries. 



The Veins. Two distinct systems of veins are present, one of 

 which is composed of the systemic veins, which carry blood di- 

 rectly to the heart ; the other is composed of those which carry 

 blood first to the liver, and is called the portal system. 



We shall first study the latter system. Take a fresh animal 

 and open its body cavity by a midventral incision. Cut away 

 both the right and the left body wall between the incision and 

 the air bladder. The portal system consists of a pair of intestinal 

 veins which lie alongside the intestine, a splenic vein from the 

 spleen, several gastric veins from the stomach, and a pneumato- 

 cystic vein from the air bladder, all of which unite to form the 

 single large portal vein. This vein passes to the hinder surface of 

 the liver, where it breaks into branches which carry the blood to 

 all parts of that organ. 



The veins of this system are often difficult to dissect because of 

 the fat in which they usually lie embedded. First find the two 

 intestinal veins and free them from the fat. Lift up the spleen and 

 the duodenal loop in which it lies and find the point of union of 

 these two veins ; find also the splenic vein, the gastric veins from 

 the stomach proper and the pyloric appendages, and the pneumato- 

 cystic vein from the air bladder. Note the exact arrangement of 

 these veins ; also the branching of the portal vein on the hinder 

 surface of the liver. 



Exercise 17. Draw a semidiagrammatic view of the portal system. 



The Heart and the Pericardial Space. Cut away all the muscles 

 between the gills, being very careful not to injure the heart or 

 the artery which issues from its forward end ; the pericardial cavity 

 will be thus exposed. 



The heart in fishes stands in close relation to the gills and lies in 

 the posteroventral portion of the head between them. It is made 



