734 PHYSIOLOGY OF DIGESTION AND SECRETION. 



food, broken into separate segments, stretches at one time practi- 

 cally along the whole length of the small intestine. 



Mechanism of the Peristaltic Movement. The means by which 

 the peristaltic movement makes its orderly forward progression 

 have not been determined beyond question. The simplest explana- 

 tion would be to assume that an impulse is conveyed directly from 

 cell to cell in the circular muscular coat, so that a contraction started 

 at any point would spread by direct conduction of the contraction 

 change. This theory, however, does not explain satisfactorily the 

 normal conduction of the wave of contraction always in one direc- 

 tion, nor the fact that the wave of contraction is preceded by a 

 wave of inhibition. Moreover, Bayliss and Starling state that, 

 although the peristaltic movements continue after section of the 

 extrinsic nerves, indeed, become more marked under these con- 

 ditions, the application of cocain or nicotin prevents their oc- 

 currence. Since these substances may be supposed to act on the 

 intrinsic nerves, it is probable that the co-ordination of the move- 

 ment is effected through the local nerve ganglia, but our knowledge 

 of the mechanism and physiology of these peripheral nerve-plexuses 

 is as yet quite incomplete. 



Rhythmical Movements. In addition to the peristaltic wave a 

 second kind of movement may be observed in the small intestines. 

 It consists essentially in a series of local constrictions of the intes- 

 tinal wall, the constrictions occurring rhythmically at those points 

 at which masses of food lie. 



Cannon * has studied these movements most successfully by 



Fig. 286. Diagram to show the effect of the rhythmical constricting movements of 

 the small intestine upon the contained food. A string of food (1) is divided suddenly into 

 a series of segments (2) ; each of the latter is again divided and the process is repeated a 

 number of times (3 and 4). Eventually a peristaltic wave sweeps these segments forward 

 a certain distance and gathers them again into a long string, as in (1). The process of 

 segmentation is then repeated as described above. (Cannon.) 



means of the Roentgen rays. He finds that as a result of these 

 contractions the masses or strings of food lying in the intestine are 

 suddenly segmented, repeatedly and in a definite manner, into a 

 number of small pieces, which move to and fro as the pieces combine 

 and are again separated (see Fig. 286). These segmentations may 

 proceed at the rate of thirty per minute for a certain time, and the 



* Cannon, "American Journal of Physiology," 6, 251, 1902. 



