416 A TEXTBOOK OF PHYSIOLOGY 



neighbouring halves uniting to form new segments. A moment later, 

 the process is repeated, and this ''rhythmic segmentation" of the 

 intestinal contents proceeds incessantly for about half an hour at a 

 rate of about twenty -eight to thirty a minute. During this period, 

 the position of the food within the gut is but slightly changed. Rhyth- 

 mic segmentation has been observed in the cat, white rat, dog, and 

 man. In the rabbit, a rhythmic shifting of the food to and fro has 

 been observed (Fig. 210). Segmentation of the small intestine in man 

 is shown in Fig. 211. 



The effect of rhythmic segmentation is to mix the food in the gut 

 thoroughly, bring it into intimate contact with the mucous membrane, 

 and to pump on the contents of the capillaries and lacteals carrying 

 the absorbed foodstuffs. These movements probably correspond to 

 the gentle, swaying, "pendulum" movements which have been 

 observed by the method of direct observation. The pendular move- 

 ments" are accompanied by rhythmical contractions at a rate of twelve 



FIG. 212. PENDULUM MOVEMENTS OF THE INTESTINE INHIBITED BY EXCITATION 

 OF THE SPLANCHNIC NERVE DURING THE PERIOD MARKED BY THE WHITE LINE. 

 (Starling.) 



to thirteen per minute. These movements are not affected by the 

 application of nicotine or cocaine. They appear, however, to be 

 dependent upon the integrity both of the muscle and the nervous 

 plexus, since strips of the intestinal longitudinal muscle devoid of 

 any nervous plexus do not perform these movements. They are 

 inhibited by excitation of the splanchnic nerve (Fig. 212). The 

 separated muscle has no refractory period, gives summated con- 

 tractions, can be tetanized, and gives no rhythmic response to 

 continued stimulation. Preparations of muscle with Auerbach's 

 plexus attached possess a refractory period to weak stimulation, 

 cannot be summated or tetanized, and exhibit rhythmic contrac- 

 tions to continued stimulation. 



The food is moved onward in the intestine by means of a peristaltic 

 wave, which may be observed in two forms (1) a slowly advancing 

 contraction (2 to 3 centimetres per minute), which moves the nutri- 

 ment but a slight distance (" true peristalsis "); and (2) a swift move- 



