62 NUTRITIONAL PHYSIOLOGY 



discharges. It probably acts as a lubricant in the digestive 

 tract and also protects the lining cells from harsh contacts, 

 both physical and chemical. The mucous membrane is 

 depressed to form the recesses of the glands, and in the 

 small intestine is raised into the microscopic prominences 

 referred to as villi. Underlying it blood-vessels and nerve - 

 fibers run thickly. 



Between the mucous layer and the serous coat on the 

 outside of the intestine there is a development of muscle 

 of the order described in a previous chapter as characteristic 

 of most internal organs, that is to say, slow acting, more or 

 less automatic, and much given to tone changes. In the 

 small intestine there are two distinct muscular coats: the 

 inner and thicker has its fibers at right angles to the axis 

 of the canal, while the outer has them set parallel to this 

 axis. The inner coat is hence spoken of as circular and the 

 outer as longitudinal. There is no doubt of the superior 

 prominence of the circular coat in the production of intes- 

 tinal movements. In the stomach the muscular organiza- 

 tion is less simple and there are oblique elements in addi- 

 tion to those which can be classed as circular and longi- 

 tudinal. The colon has the circular coat, but instead of a 

 complete covering of longitudinal muscle it has three 

 bands of contractile tissue extending along its wall. 



At any point along the course of the intestine temporary 

 closure may be effected by the contraction of the circular 

 muscle. But there are certain places where such closure 

 is far more frequent or, indeed, the usual condition. 

 Where the esophagus joins the stomach an irritable band, 

 the cardiac sphincter, is much of the time firmly contracted. 

 There is, similarly, a pyloric sphincter guarding the open- 

 ing between the stomach and the duodenum. Where 

 the ileum enters the cecum a valve exists which is adapted 

 to prevent the reflux of material from the colon to the small 

 intestine. This, the ileocecal valve, is probably reinforced 

 in its mechanical action by muscular support. Finally, 

 the short anal canal is closed by an inner sphincter which 

 is essentially a thickening of its own wall, and an external 

 one composed of skeletal muscle. 



