234 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 attachment 

 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. 



a. The spiral valve. 



Open the intestine along its whole length by a longitudinal 

 incision along its right side. Washout its contents. Carefully 

 cut away ivith scissors the ventral wall from the several turns 



