SEC. 7. THE MUSCULAR MECHANISMS OF DIGESTION. 



267. From its entrance into the mouth until such remnant 

 of it as is undigested leaves the body, the food is continually 

 subjected to movements having for their object the trituration of 

 the food as in mastication, or its more complete mixture with the 

 digestive juices, or its forward progress through the alimentary 

 canal. These various movements may briefly be considered in 

 detail. 



Peristaltic Movements. The dominant movement in the ali- 

 mentary canal is of the kind called peristaltic, carried out by 

 means of the circular and longitudinal muscular coats. This is 

 seen in its simplest form in the small intestine, is somewhat modi- 

 fied in other parts, as in the stomach, and at the beginning and end 

 of the canal is replaced or assisted by complicated movements 

 carried out by various muscles. 



We have already, in 92, spoken of these 'peristaltic' move- 

 ments, but it may be well to consider them briefly again under a 

 general aspect, before dwelling on the special movements of the 

 several parts of the alimentary canal. 



The muscular coat of the alimentary canal consists as we have 

 seen of two layers, separated more or less distinctly by a sheet of 

 connective tissue, an outer thinner longitudinal layer, and an 

 inner thicker circular layer ; and a similar arrangement obtains in 

 nearly all the muscular hollow tubes of the body, except the 

 arteries, in which the muscular elements are present not so much 

 for the purpose of driving the blood onward as for the sake of 

 regulating the irrigation. 



The action of the circular coat is fairly simple. A contraction 

 starting at any part travels onwards in the same direction, generally 

 downwards, that is to say from a part nearer the mouth to a part 

 nearer the rectum, for a greater or less distance, the circularly 

 disposed bundles contracting in sequence. The result is a 

 narrowing or constriction of the tube which, travelling more or 

 less slowly along the tube, drives the contents onwards ; when a 

 butcher empties the intestine of a slaughtered animal by squeezing 



