SEC. 5. THE MUSCULAR MECHANISMS OF DIGESTION. 



§ 218. 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. 



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. 



The main part of a peristaltic movement, as seen in the small 

 intestine, is a wave of contraction progressing longitudinally over 

 the circular coat (§ 84). A contraction of the circular coat takes 

 place at some level or other, narrowing the intestine at this level. 

 From thence, the circularly disposed bundles contracting in 

 sequence, the contraction travels as a wave downwards or up- 

 wards or both downwards and upwards. As a rule the wave, 

 when started naturally, travels downwards from a part nearer the 

 mouth to a part nearer the rectum. Thus a narrowing or con- 

 striction of the tube travels onwards as a wave driving the contents 

 of the tube before it ; when a butcher empties the contents of the 

 intestine of a slaughtered animal by squeezing it high up with his 

 hand or his thumb or forefinger, and then carrying the squeezing 

 action downwards along the length of the intestine, he makes the 

 passive intestine do very much what the circular coat does, 

 actively, by contraction, in the living animal. 



This action of the circular coat is further aided by a corre- 

 sponding contraction of the longitudinal coat. When a length 

 of the longitudinal coat is thrown into contraction, that length of 

 the tube is shortened and widened ; the effect is the antagonist 

 of that produced by the contraction of the circular coat. Hence 



