242 THE DOG-FISH. 



IV. DISSECTION OF THE DIGESTIVE SYSTEM. 



In dissecting the alimentary canal be careful not to injure 

 the blood-vessels in relation with it : these lie for the most part 

 in the mesentery. 



A. The Alimentary Canal. 



1. The mouth and pharynx. The detailed examination of 



these parts is best deferred until the circulatory 

 system is dissected. 



2. The oesophagus is a short wide tube, which enters the 



abdominal cavity at its anterior end and expands 

 almost at once into the stomach. 



Pass the handle of a scalpel or seeker through the mouth 

 and down the oesophagus into the stomach. 



3. The stomach is U-shaped. Its proximal limb, which is 



very wide and directly continuous with the oeso- 

 phagus, extends back nearly to the hinder end of the 

 abdominal cavity ; it then turns sharply forwards 

 to form the distal limb, which is shorter and much 

 narrower. 



4. The intestine is separated from the distal limb of the 



stomach by a slight pyloric thickening. Its proximal 

 part is about equal in width to the stomach, alongside 

 which it lies ; but it soon dilates to form the wide 

 ileum, the surface of which is marked spirally by 

 blood-vessels, corresponding to the line of attach- 

 ment of the spiral valve. At its hinder end the 

 intestine narrows to form the rectum, which, after 

 a course of about an inch and a half, opens to the 

 exterior at the cloacal aperture. 

 The spiral valve. 

 Open the intestine along its whole length by a longitudinal 

 incision along its right side. Wash out its contents. Carefully 

 cat away with scissors the ventral wall from the several turns 



