INTESTINAL DIGESTION. 79 



mixed with bismuth. A column of food can be seen to divide into 

 several segments, each of which in a few seconds breaks into two 

 the neighboring halves then joining together, and the process 

 repeating itself. 



Two varieties of peristaltic waves are usually described, both, 

 of which are characterized by a marked constriction preceded by 

 a distinct dilatation of the gut, which may extend for a consid- 

 erable distance down it (two feet). The one variety of wave 

 travels slowly (y 2 cm. per minute), and has the function of car- 

 rying along the food ; the other travels very rapidly (peristaltic 

 rush), and is evidently for the purpose of hurrying along irri- 

 tating substances. 



Besides being set up by the presence of food in the intestine, 

 these waves may be influenced through the nervous system ; stim- 

 ulation of the vagus excites them, whereas stimulation of the 

 sympathetic brings about a marked inhibition, in which the whole 

 gut becomes profoundly relaxed with the exception of the ileo- 

 colic sphincter, which contracts. This influence of the splanch- 

 nic may be excited reflexly, as by pain or fear. 



The movements of the large intestine are more difficult to study 

 than those of the small intestine. They vary considerably in 

 different animals, as indeed is to be expected when we remember 

 that the function of this part of the alimentary tract depends 

 upon the nature of the food. In herbivora, for example, food 

 may lie in the capacious caecum for days, and even in carnivora, 

 in which this part of the gut is rudimentary, it may remain for 

 twenty-four hours. In man the conditions seem to be intermedi- 

 ate between those in the herbivora and carnivora, and the move- 

 ments are believed to be as follows: As the semi-fluid food en- 

 ters the ca?cum through the ileo-caecal valve and collects in the 

 caecum and proximal colon, it excites the occurrence of waves of 

 constriction, which start probably about the hepatic flexure and 

 travel back towards the caecum, thus forcing the food into this 

 sac and tending to cause recurring axial currents to be set up. 



Occasionally the arrival of the wave at the caecum starts a 

 true peristaltic wave, which travels distally getting feebler as it 

 proceeds, and which may carry some of the contents into the 



