170 SPECIAL MUSCULAR MECHANISMS 



the viscus at a rate of 1 mm. per second, and may be excited 

 by mechanical stimulation of the mucous membrane by food 

 or otherwise. Such movements, arising at any point, do not 

 pass the ileocecal valve. Intestinal movements of this kind 

 are intimately dependent upon a definite mechanism within 

 the intestinal wall, for if a portion of the tube be resected, 

 turned about and resutured in its place, peristaltic waves 

 cannot pass the operated area. This mechanism is looked 

 upon as involving Auerbach's plexus, for all progressive peri- 

 stalses are abolished by poisons like cocaine and nicotine. 



A second variety of movement of the small intestine is the 

 rhythmic type. Here the intestines, when examined with 

 Rontgen rays, after suitably mixing the food with bismuth 

 subnitrate, may be seen to separate the intestinal contents 

 into a series of segments by circular, stationary constrictions. 

 These segments are in turn divided by other constrictions 

 and the divided halves of neighboring segments fuse together 

 to form a new set of segments. This process, in the cat, is 

 repeated at the rate of 30 per minute. The result is a thor- 

 ough mixing of the intestinal contents with the intestinal 

 secretions. After possibly a thousand segmentations a peri- 

 staltic wave moves the segments to another loop of the intestine, 

 where the rhythmic process is repeated. These movements 

 are differently affected by cocaine and nicotine than are the 

 peristaltic movements. They may even become increased 

 in vigor. They too, however, are also dependent upon Auer- 

 bach's plexus, for if the circular muscle coat is stripped off 

 it loses its automaticity while the longitudinal coat still in con- 

 nection with the plexus retains its spontaneous movements. 

 Under special conditions there may be a reversal of the direc- 

 tion in which the intestinal contents move. This occurs in 

 obstruction of the intestine, and, experimentally, substances 

 introduced into the rectum have later been found in the 

 stomach. 



Movements of the Large Intestine. The musculature in the 

 large intestine has the same general arrangement as in the 

 small and the law of intestinal peristalsis holds as well, i. e., 

 a local excitation causes a constriction above and a dilatation 

 below the point stimulated. A study of the large intestine 

 by means of .r-rays has shown that it is functionally divisible 



