140 DIGESTION 



divide, adjacent halves of neighbouring segments uniting 

 together. The new segments again divide, with a return 

 to the first. These changes occur in man at the rate of 

 about seven per minute. 



These movements favour both digestion and absorption ; 

 digestion by effecting a thorough mixture of the food with 

 the digestive juices, absorption by affording the maximum 

 exposure of the products of digestion to the intestinal 

 mucosa. Moreover, the alternate constriction and dilata- 

 tion of the intestinal blood-vessels facihtate the flow of 

 blood, while at each constriction of the vilh the material 

 which has been absorbed is pumped into the portal vein 

 and thoracic duct. 



Accompanying rhythmic segmentation are the pendular 

 movements, which consist in a lateral swaying without 

 alteration in the size of the lumen. 



The above movements, being unaffected when the local 

 nerve plexus is paralysed by means of nicotine, are myogenic 

 in origin. 



Neither segmentation nor pendular movement causes 

 any propulsion of the intestinal contents. 



After continuing for a period of a half to two hours 

 segmentation ceases. A peristaltic wave then moves the 

 whole mass forward to a fresh section of gut where segmen- 

 tation is renewed. A peristaltic wave consists of a wave 

 of contraction preceded by a wave of relaxation. Its 

 continuation after section of the vagi and splanchnics on 

 the one hand, and its abolition after the apphcation of 

 nicotine on the other, prove it to be due to the local nerve 

 centres — to Auerbach's plexus. When the gut is distended 

 at any part there occurs contraction above and relaxation 

 below the point of contact. 



Two kinds of peristalsis are recognised — distinguished 

 by their rapidity and by the length of intestine which 

 they traverse. The more freqvient is slow peristalsis, 

 which travels at the rate of about 1 cm. per second, and 

 after propelling the contents a short way, dies out. Its 



