THE MECHANISMS OF DIGESTION 501 



balloon method (Fig. 162). If the gut is pinched above the balloon, a 

 marked relaxation occurs over the latter, and this relation extends for about 

 two feet down the intestine. If, on the other hand, the gut is pinched 

 a little below the situation of the balloon, a long-continued contraction 

 occurs over the latter. The conclusion that we may draw from this result 

 is that the stimulation of the gut causes contraction above the point of 

 the stimulus and relaxation below, this being known as "the law of the 

 intestine" (Bayliss and Starling). We have seen that it applies also in 

 the case of the cardiac and pyloric sphincters. 



THE PHYSIOLOGICAL NATURE OF THE RHYTHMIC AND PERISTALTIC MOVE- 

 MENTS 



Interesting information in this connection has been gained by obser- 

 vation of the behavior of the movements after the application of drugs 

 to the gut or after cutting the nerve supply. The rhythmic movements 

 are not affected by the application of nicotine or cocaine. Since these 

 drugs paralyze nervous structures it has been concluded that the rhythmic 

 movements are myogenic in origin. The question is not a settled one, 

 however, for it has been found by Magnus that, although strips of the 

 longitudinal muscle, isolated in oxygenated saline solution, will continue 

 to beat, they do not do so if the adherent Auerbach's plexus of nerves 

 is stripped off from them. The nature of the peristaltic contractions is 

 more definite; they must clearly depend upon a local nervous struc- 

 ture, since they are paralyzed by the application to the gut of cocaine or 

 nicotine. This local nervous system no doubt also resides in Auerbach's 

 plexus, which must therefore be considered as complex enough to be (see 

 page 830) endowed with the power of directing nervous impulses so as to 

 bring about relaxation of the gut in front of the stimulus and contrac- 

 tion over it. 



NERVOUS CONTROL OF MOVEMENTS 



The influence of the central nervous system on the intestinal movements 

 has been studied by the usual methods of cutting and stimulating the 

 extrinsic nerve supply. Through the splanchnic nerves tonic inhibitory 

 impulses are conveyed to the intestine (except the ileocolic sphincter), 

 for after these nerves are severed the movements become more distinct. 

 Indeed, in many animals after opening the abdomen no intestinal move- 

 ment can be observed until these nerves have been cut. Stimulation of the 

 peripheral end of the nerve also inhibits any movement which may mean- 

 while be in progress. The impulses through the vagus nerve are of an 

 opposite character. Section of these nerves has little effect, but stimula- 

 tion causes contraction. (Figs. 163 and 164.) 



