THE PERCH 49 



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 

 by the veins from the various tissues to the sinus venosus, from 

 which it flows into the auricle, and from this into the ventricle. 



The ventricle sends the blood forward into the bulbus arteriosus, 

 —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 18. Draw a semidiagrammatic view of the pericardial cavity 

 with the heart and the bulbus arteriosus. 



The Inner Structure of the Heart. Cut open the ventricle and 

 the bulbus arteriosus by a lateral incision carried along the left 

 side of both. Turn the flap, which consists of the entire ventral 

 wall of the ventricle and bulbus, to the right and expose their 

 interior. Note the thick walls of the ventricle, and the small cav- 

 ity in the center; in its dorsal wall is the auriculo-ventricular 

 opening into the auricle, a slit guarded on each side by a valve. 

 Use the blowpipe to bring this opening into view. Note also 

 the thick walls of the bulbus; by using the blowpipe find, at 

 the opening of the ventricle into the bulbus, the two valves which 

 prevent the blood from flowing backward. Open the auricle by a 

 lateral incision; blow into it and note its thin walls and large 

 cavity. Find the opening into the sinus venosus, which is guarded 

 by a single valve. 



Exercise 19. Draw a diagram of the heart, showing the structure 

 of its interior. 



The Arteries. 1 The forward continuation of the bulbus arte- 

 riosus forms the aorta ascendens, or ventral aorta. This ves- 



1 The aorta ascendens 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. 



