DIGESTIVE SYSTEM 215 



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



very wide and directly continuous with the oesophagus, 

 extends back nearly to the hinder end of the abdo- 

 minal 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 

 colon, 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. Wash out its contents. Carefully 



cut away ivith scissors the ventral wall from the several turns 



>of the spiral valve, and turn it over to the left side, so as to 



hilly expose the cavity of the intestine. Wash freely. 



The spiral valve is a membranous fold, attached 

 along one edge to the inner surface of the intes- 

 tine, round which it runs spirally. The fold is 

 about an inch and a half wide at the anterior 

 end, but gradually diminishes in width towards 

 the posterior end, and disappears at the com- 

 mencement of the rectum. The first turn of the 

 spiral is a long one, but the succeeding ones, of 

 which there are usually seven or eight, are only 

 about a quarter of an inch apart. The general 

 appearance of the valve is that of a series of cones 

 one within another, the apices of the cones being 

 usually directed forwards, but sometimes at the 

 hinder end backwards. 



