646 PHYSIOLOGY OF DIGESTION AND SECRETION. 



is preceded by a wave of relaxation or inhibition. The force of the 

 contraction as measured by Cash* in the dog's intestine is very small. 

 A weight of five to eight grams was sufficient to check the onward 

 movement of the substance in the intestine and to set up violent, 

 colicky contractions which caused the animal evident uneasiness. 

 We may suppose that under normal conditions each injection of 

 chyme from the stomach into the duodenum is followed by a 

 peristalsis that, beginning at the duodenum, passes slowly down- 

 ward over a part or all of the small intestine. According to most 

 observers, the movement is blocked at the ileocecal valve, and the 

 peristaltic movements of the large intestine form an independent 

 group. 



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. 



Pendular Movements. In addition to the peristaltic wave a 

 second kind of movement may be observed in the exposed intestines 

 of a living animal. This movement is characterized by a gentle 

 swinging to and fro of the different loops, whence its name of pendu- 

 lar movement. Mallf has shown that the main feature of this 

 movement is a rhythmical contraction of the circular muscles. 

 He prefers to speak of the movements as rhythmical instead of 

 pendular contractions, and points out that, owing to the arrange- 

 ment of the blood-vessels in the coats of the intestine, the rhythmical 

 contractions should act as a pump to expel the blood from the 

 submucous venous plexus into the radicles of the superior meseriteric 

 vein, and thus materially aid in keeping up the circulation through 

 the intestine and in maintaining a good pressure in the portal vein, 



*" Proceedings of the Royal Society," London, 41, 1887. 

 t" Johns Hopkins Hospital Reports," 1, 37, 1896. 



