670 PHYSIOLOGY OF DIGESTION AND SECRETION. 



may be produced also by shutting off the blood-supply, and again 

 temporarily when the supply is re-established. A condition of 

 dyspnea may also start movements in the intestines or in some 

 cases inhibit movements which are already in progress, the stimu- 

 lus in this case seeming to act upon the central nervous system and 

 to stimulate both the motor and the inhibitory fibers. Oxygen gas 

 within the bowels tends to suspend the movements of the intes- 

 tine, while CO 2 , CH 4 , and H 2 S act as stimuli, increasing the move- 

 ments. Organic acids, such as acetic, propionic, formic, and 

 caprylic, which may be formed normally within the intestine as 

 the result of bacterial action, act also as strong stimulants. 



Movements of the Large Intestine. The musculature in the 

 large intestine has the same general arrangement as in the small, 

 and the usual view has been that the movements are similar, 

 although more infrequent, so that the material received from the 

 small intestine is slowly moved along while becoming more and 

 more solid from the absorption of water, until in the form of feces 

 it reaches the sigmoid flexure and rectum. Bayliss and Starling 

 state that their law of intestinal peristalsis holds in this portion of 

 the intestine, that is, local excitation causes a constriction above 

 and a dilatation below the point stimulated. Cannon,* however, 

 from his studies of the normal movements in cats, as seen by the 

 Roentgen rays, comes to the conclusion that the movements in the 

 large intestine show a marked peculiarity previously overlooked. 

 He divides the large intestine into two parts; in the second, cor- 

 responding roughly to the descending colon the food is moved 

 toward the rectum by peristaltic waves. A number of constrictions 

 may be seen simultaneously within a length of some inches. In 

 the ascending and transverse colon and cecum, on the contrary, the 

 most frequent movement is that of antiperistalsis. The food in this 

 portion of the canal is more or less liquid and its presence sets up 

 running waves of constriction, which, beginning somewhere in the 

 colon, pass toward the ileocecal valve. These waves occur in groups 

 separated by periods of rest. The presence of the ileocecal valve 

 prevents the material from being forced back into the small in- 

 testine. The value of this peculiar reversal of the normal move- 

 ment of the bowels at this particular point would seem to lie in the 

 fact that it delays the passage of the material toward the rectum 

 and by thoroughly mixing it gives increased opportunities for the 

 completion of the processes of digestion and absorption. As the 

 colon becomes rilled some of the material penetrates into the 

 descending part where the normal peristalsis carries it toward the 

 rectum. 



The large intestine particularly the descending colon and 

 rectum receives its nerve supply from two sources: (1) Fibers 



* Cannon, loc. cit. 



