THE DIGESTIVE SYSTEM 51 



Covering the outside of the tube and giving it 

 its smooth, shining appearance, is a thin serous 

 tunic derived from the peritoneum (Fig. 24). 

 Under this is the muscular coat in which three 

 strata can be recognised. The outermost and 

 innermost strata consist of muscular fibres 

 disposed in a longitudinal direction ; whereas 

 the middle stratum, much thicker than the 

 other two, contains fibres arranged circularly. 



Strictly speaking the innermost longitudinal 

 layer should not be included in the proper 

 muscular coat of the gut ; but should be 

 regarded as belonging to the mucous membrane. 

 That is, it is muscularis mucosae, and lies close 

 to the circular stratum of the muscular tunic 

 because of the sparsity of submucous tissue. 



The thick, soft, and highly vascular mucous 

 membrane which lines the intestine has a 

 velvety appearance to the naked eye, especially 

 if a piece of gut be sunk under water. This 

 appearance is due to the presence of innumer- 

 able long, thin projections known as villi, 

 actively concerned in the absorption of the 

 nutritive part of the food. Each villus is 

 covered by a single layer of epithelial cells, 

 some of which are columnar in form, and others 

 shaped like goblets (Fig. 25). The more 



