384 AN AMERICA X TEXT-BOOK OF PHYSIOLOGY. 



contractions which caused the animal evident uneasiness. We may suppose 

 that under normal conditions each contraction of* the antrum pylori of the 

 stomach, which ejects chyme into the duodenum, is followed by a peristalsis 

 that beginning at the duodenum passes slowly downward for a part or all of 

 the small intestine. According to most observers, the movement is blocked 

 at the ileo-ca?cal valve, and the peristaltic movements of the large intestine 

 form an independent group similar in all their general characters to those of 

 the small intestine, but weaker and slower. 



Mechanism of the Peristaltic Movement. — The means by which the peri- 

 staltic movement makes its orderly forward progression have not been deter- 

 mined beyond question. The simplest explanation 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 con- 

 duction of the contraction change. This theory, however, does not explain 

 satisfactorily the normal conduction of the wave of contraction always in one 

 direction, nor the fact that a reversed piece of intestine continues to send its 

 waves in what was fir it the normal direction. Moreover, Bayliss and 

 Starling state that although the peristaltic movements continue after section 

 of the extrinsic nerves — indeed, become more marked under these conditions 

 — the application of cocaine or nicotine prevents their occurrence. Since these 

 substances may be supposed to act on the intrinsic nerves, it is probable 

 that the co-ordination of the movement 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 pendular movement. The oscillations occur at 

 regular intervals, and are usually ascribed to rhythmic contractions of the 

 longitudinal muscles. Mall, 1 however, believes that the main feature of this 

 movement is a rhythmic contraction of the circular muscles, involving a part 

 or all of the intestines. He prefers to speak of the movements as rhythmic 

 instead of pendular contractions, and points out that owing to the arrangement 

 of the blood-vessels in the coats of the intestine the rhythmic contractions should 

 act as a pump to expel the blood from the submucous venous plexus into the 

 radicles of the superior mesenteric vein, and thus materially aid in keeping up 

 the circulation through the intestine and in maintaining a good pressure in the 

 portal vein, in much the same way as happens in the case of the spleen (see p. 

 332). Bayliss and Starling corroborate this view, except that they find that 

 both the circular and longitudinal layers of muscle are concerned in the 

 movement. The rhythmic contractions, according to these observers, are 

 entirely muscular in origin, since they persist after the application of 

 nicotine or cocaine. 



Extrinsic Nerves of the Intestines. — As in the case of the stomach, the 

 1 The Johns Hopkins Hospital Reports, vol. i. p. 37. 



