104 THE BODY AT WORK 



oscillate backwards and forwards on their way down the 

 canal. The slower vermicular movement which squeezes the 

 contents forwards is called " peristalsis." It resembles the pro- 

 gressive contraction of an elastic tube which may be effected 

 by drawing it through a ring, but is rather more complicated. 

 At the point at which it is occurring the circular coat is sharply 

 contracted. Above this it is also somewhat contracted ; 

 below it is relaxed. The longitudinal fibres, using the con- 

 stricted portion as a point tfappui, pull up the segment of the 

 intestine which lies immediately below it, drawing it off the 

 contents of the tube as a glove from a finger. 



When food is swallowed, it falls down the oesophagus, aided 

 by slight peristalsis. As soon as sufficient has accumulated on 

 the upper surface of the cardiac valve of the stomach, the 

 valve relaxes ; at the same time a stronger peristalsis of 

 the lower portion of the oesophagus squeezes its contents into 

 the stomach. Food remains in the stomach until it has reached 

 a certain stage of digestion, the chief object of which is its 

 subdivision into small particles. Until this stage is reached, 

 the pyloric valve is firmly closed. The contractions of the wall 

 of the stomach drive its contents round and round down the 

 greater and up the lesser curvature mixing them thoroughly 

 with the gastric juice (cf. p. 124). As the acidity of the mixture 

 increases, the peristaltic contractions of the stomach become 

 more vigorous, until, the pyloric valve relaxing, the food is 

 little by little driven into the duodenum. 



The alimentary canal has an abundant supply of nerves from 

 the vagus and the sympathetic systems. It contains also within 

 its own wall an enormous quantity of nerve-fibres and nerve- 

 cells. They are disposed as two plexuses, one in the sub- 

 mucosa, the other between the circular and longitudinal 

 muscular coats. In a specimen successfully stained with 

 methylene blue, they are so abundant as to give the impression 

 that every plain muscle-cell may have its own separate nerve- 

 twig. Nevertheless, the contraction of the muscle-cells may 

 take place independently of all nerve-influence independently, 

 even, of the local mechanism, the plexus referred to above. 

 Nicotin applied to the wall of the intestine paralyses the local 

 nerves ; yet rhythmic contractions still occur. They are, how- 

 ever, no longer progressive. They do not drive the contents 



