168 VERTEBRATES. 



Observe two ducts entering the intestine together, near 

 its origin, appearing to come directly from the liver. 

 One of these comes from the gall bladder, and the other 

 from the pancreas. The former may be recognized, if dis- 

 tended with bile forced into it by squeezing the gall 

 bladder. The other should be traced from its origin by 

 dissection. 



Posterior to the pancreas is a conspicuous dark-red 

 organ, abundantly supplied with blood vessels, the 

 spleen. 



Remove the reproductive organs. Cut through the 

 bones of the pectoral arch. Open wide this incision by 

 pressing downward on both pectoral spines at once. 

 Wash out any blood that may have flowed from the 

 last incision. Turn backward the anterior lobes of the 

 liver, and observe that the peritoneum rises at this point 

 to meet the pectoral arch, and is perforated by the esoph- 

 agus, and by the blood vessels extending forward from 

 the liver. Trace the blood vessels through it to the 

 heart. In the heart observe three principal divisions: 

 (1) a postero-lateral, irregular, thin-walled part, the 

 auricle ; (2) a larger, oval, muscular, strongly contractile, 

 central part, the ventricle; (3) a smaller, oval, anterior 

 portion of lighter color, and noncontractile, the arterial 

 bulb. This is continued forward into the branchial artery, 

 which soon divides, sending one branch (aortic arch) to 

 each gill. 



Cut off the branchial artery at its origin, and remove 

 the heart. Cut off the esophagus close to the pharynx. 

 Notice how muscular its walls are. Remove the digestive 

 tract entire, beginning with the esophagus, and proceed- 

 ing posteriorly, cutting only the peritoneum where it 

 meets the body wall. Observe other internal organs 

 lying dorsal to the peritoneum. Central among these is 

 a large, white bag filled with air, the air bladder or swim- 



