6 VERTEBRATE ZOOLOGY 



Exercise 4. Draw an outline of the abdominal cavity and the organs 

 which appear in the ventral aspect. 



The Pericardial Cavity and the Heart. The organs just exam- 

 ined lie in the abdominal cavity ; the pericardial cavity contains 

 the heart. This organ in fishes stands in close relation to the gills 

 and lies in the midventral plane between them. 



Cut away the skin covering the median portion of the pectoral 

 girdle and the interbranchial muscles just in front of it. Cut away 

 and remove the girdle and the muscles and expose the pericardial 

 chamber. Study the heart, which lies in it. 



Note the shape of the pericardial chamber. Note also that its 

 posterior wall lies against the anterior wall of the abdominal cham- 

 ber and that the two walls form the false diaphragm. The heart 

 consists of a single ventricle, which is a muscular organ occupying 

 a large part of the pericardial chamber ; a single auricle, which is 

 the large, triangular, thin-walled sac lying dorsal to (back of) the 

 ventricle and appearing at its sides ; a conus arteriosus, a muscular, 

 cylindrical prolongation of the anterior end of the ventricle ; and 

 the sinus venosus, a thin-walled tube stretching transversely across 

 the hinder part of the pericardial chamber, to the dorsal wall of 

 which it is attached. The sinus will be seen by pressing the ven- 

 tricle forward or lifting it a little. Blood is brought from the tis- 

 sues and organs of the body to the sinus venosus, from which it 

 enters the auricle. From this vessel it is transferred to the ven- 

 tricle, by which it is pumped through the conus arteriosus to the 

 gills. The heart contains only venous blood. 



The pericardial cavity is placed in communication with the ab- 

 dominal cavity by the pericardio-peritoneal canal, a median pore 

 in the false diaphragm dorsal to the liver. 



Exercise 5. Draw an outline of the ventral aspect of the pericardial 

 cavity and the organs in it. 



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

 conus arteriosus by a lateral incision carried along the left side 

 of both. Turn the flap, which will constitute the entire ventral 

 wall of the heart, to one side, exposing its interior ; wash this 

 out thoroughly. The ventricle will be seen to be a thick-walled 



