2IO PHYLUM CHORD ATA 



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

 from the stomach proper and the pyloric appendages, and the 

 pneumatocystic vein from the air bladder. Note the exact ar- 

 rangement of these veins ; note also the branching of the portal 

 vein on the hinder surface of the liver. 



Exercise 12, 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 up, as we have seen, of three parts : the large muscular 

 ventricle, the deep-red auricle, and the sinus venosus, the vessel 

 which lies across the hinder end of the pericardial space. Blood 

 is brought from the various tissues to the sinus venosus by the 

 veins, from which it flows into the auricle, and from it into the 

 ventricle. 



The ventricle sends the blood forward into the bulbus arte- 

 riosus, a thick-walled vessel in front of the ventricle, which is 

 the beginning of the aorta. The muscular walls of the bulbus 

 are highly elastic, and when they are distended they exert a 

 constant pressure upon the blood which is passing through it ; 

 the blood thus flows forward in a constant stream and without 

 pulse beats. 



Exercise 13. Draw a semidiagrammatic view of the pericardial cavity 

 with the heart and the bulbus arteriosus. 



The Arterial System.^ The forward continuation of the bul- 

 bus arteriosus forms the ventral aorta. This vessel sends ofT 

 four pairs of afferent branchial arteries which carry blood to 

 the gills. Four pairs of efferent branchial arteries then run 



1 The ventral aorta and afferent branchial arteries may be injected through the 

 bulbus arteriosus. In order to inject the remainder of the arterial system, cut off 

 the tail a short distance in front of the caudal fin and inject forward in the caudal 

 artery. This is the uppermost of the two vessels which lie in the canal formed by 

 the bony arches on the ventral side of the spinal column. 



