222 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



branchial arteries. On passing through the gill filaments the blood is aerated and becomes 

 arterial blood. It is then collected by the efferent branchial arteries and passed into the 

 dorsal aorta which distributes it to all parts of the body. The venous blood is returned to 

 the heart by means of the systemic veins and the two portal systems. There is thus in the 

 elasmobranchs (and all fishes) a single circulation through the heart. Since the heart contains 

 only venous blood, the heart must obtain arterial blood from some outside source. This is 

 accomplished by means of the coronary artery which arises from the efferent branchial vessels 

 and returns to the heart. As we shall see the coronary artery originates in a different manner 

 in fish than in other vertebrates. 



9. The pericardio-peritoneal canals. Inspect the posterior wall of the peri- 

 cardial cavity after the removal of the heart. In the dogfishes, a large opening 

 will be found in the posterior wall dorsal to the previous attachment of the 

 sinus venosus. On probing into it, it will be found to lead into a canal, the 

 pericardio-peritoneal canal, situated along the ventral wall of the esophagus inter- 

 nal to the visceral peritoneum of the esophagus. Eventually the canal opens 

 into the pleuroperitoneal cavity by a small slit. In the skate there is an open- 

 ing of moderate size in the center of the posterior wall of the sinus venosus. 

 On probing into this it will be found to lead into a canal, the pericardia-peritoneal 

 canal, which passes through the center of the hepatic sinus. On probing into this 

 it will be found to fork into two canals lying on the ventral wall of the esopha- 

 gus internal to its serosa. They open into the pleuroperitoneal cavity by minute 

 slits. The pericardio-peritoneal canals serve to connect the pericardial and 

 pleuroperitoneal cavities, and arise through the failure of the transverse septum 

 to close completely across the coelom. 



C. THE CIRCULATORY SYSTEM OF NECTURUS 



i. The chambers of the heart. The pericardial cavity has already been 

 exposed; if a new specimen is provided, the pericardial and pleuroperitoneal 

 cavities are to be opened as before. The parts of the heart visible in ventral 

 view are the ventricle, the auricles, and the conus arteriosus. The ventricle is 

 thick walled and conical in form. Anterior to the ventricle on either side is a 

 thin-walled auricle. Springing from the base of the ventricle and passing for- 

 ward between the two auricles is the tubular conus arteriosus. Anteriorly the 

 conus passes into the enlarged beginning of the ventral aorta; this muscular 

 expansion of the base of the ventral aorta is named the bulbus arteriosus. Lift 

 the apex of the ventricle and note the sinus venosus situated dorsad to the 

 ventricle. The sinus venosus receives two large venous channels lying in the 

 dorsal wall of the pericardial cavity. These are formed by the union of the two 

 hepatic sinuses with the two common cardinal veins. The hepatic sinuses are the 

 two large veins which emerge from the transverse septum and pass into the 

 sinus. The common cardinal veins join the hepatic sinuses on their lateral 

 surfaces. 



