ALIMENTARY CANAL 309 



B. Internal Structure of the Alimentary Canal. 



1. The stomach. 



Remove the stomach from the body, cutting across the 

 esophagus and duodenum close to it, and taking care not to 

 injure the vessels of the liver. Slit up the stomach along its 

 posterior surface : wash, and examine it under water. 



The wall of the stomach consists of an outer 

 peritoneal investment ; a middle muscular layer, 

 thickened at the pyloric end ; and an inner layer of 

 mucous membrane, which is raised into irregular 

 longitudinal folds. The pyloric orifice is narrowed 

 by a prominent muscular rirn. 



2. The small intestine. 



Cut out a small piece from the wall, and examine its inner 

 surface under water. 



The villi are minute tag-like processes of the 

 mucous membrane, covering its surface, and giving it 

 a velvety appearance. 



Peyer's patches present a honeycombed appear- 

 ance, the depressions being occupied by smooth convex 

 lymph-follicles, bordered by villi. 



3. The caecum. 



Remove the ccscum from the body, cutting across the small 

 intestine and the colon about an inch from it. Take the 

 c&cum to the sink, and slit it open longitudinally : wash 

 freely, and examine under water. 



The inner surface is papillose, and raised along 

 the line of the external constriction into a prominent 

 spiral fold, a quarter of an inch or more in width. 

 The opening from the sacculus rotundus is small and 

 circular ; that into the colon is much larger. 



Cut the vermiform appendix across to see the thickness of 

 its u-all ; and slit it open to examine its inner surf ace, which is 

 similar to that of Peyer's patches, but with smaller follicles. 



