202 TEXT-BOOK OF PHYSIOLOGY 



second. In how far this movement represents the normal movement as it 

 takes place under physiologic conditions and as observed by Cannon, remains 

 for further investigations to decide. 



The Nerve Mechanism of the Small Intestine. The causes of these two 

 forms of intestinal activity, rhythmic segmentation and peristalsis, have been 

 the subject of much investigation. Because of the presence of a network or 

 plexus (Auerbach's and Meissner's) of nerve-cells and nerve-fibers in the 

 walls of the intestines and in close relation to the muscle cells, the so-called 

 myenteric plexus and because of the fact that the intestines will contract 

 for some time after removal from the body of the animal, it has been difficult 

 to decide whether the contractions are myogenic or neurogenic. 



As the rhythmic contractions continue though the peristaltic are abol- 

 ished by the introduction of nicotin into the blood, an agent which tempo- 

 rarily paralyses peripheral nerve-cells, it was concluded by Bayliss and 

 Starling that the rhythmic contractions are myogenic and that the peristaltic 

 contractions are reflex in character, the coordination being carried out by 

 the local nerve mechanisms and initiated by stimulation of the intestine. 

 This observation has been corroborated by Cannon who has shown that if 

 the myenteric plexus is divided by incisions extending around the intestine, 

 at intervals of 1.5 to 2 cm. for a distance of 45 cm., incisions reaching to the 

 submucous coat, the peristaltic movement is totally abolished though 

 rhythmic segmentation develops as usual and continues for long periods. 



As the segmentation activity continues after interruption of the myenteric 

 plexus, the inference is justifiable that it is purely myogenic and is the re- 

 sponse to a stimulus within the intestine such as distension by food, when 

 the intestinal wall possesses the requisite degree of tonicity. 



Though the orderly and coordinated contractions and relaxations of the 

 muscle coat, which constitutes a peristaltic movement, are mediated by the 

 myenteric plexus, and therefore termed a myenteric reflex, nevertheless the 

 contraction may be augmented or inhibited by the central nerve system 

 through the vagus and splanchnic nerves respectively. 



Stimulation of the vagus is followed by an augmentation of the contrac- 

 tion, though not infrequently there is a primary inhibition of short duration. 

 Stimulation of the splanchnic is followed by a relaxation or inhibition of the 

 contraction, though according to some observers there is at times an opposite 

 effect. 



The extent to which the contraction is initiated or augmented by stimula- 

 tion of the vagus depends upon the extent to which the contraction at the 

 moment is inhibited by the splanchnic. Thus after the splanchnics are 

 divided, stimulation of the vagus causes a much more pronounced contrac- 

 tion than would otherwise be the case; a fact that indicates that the 

 splanchnic nerve-center is in a state of tonus or tonic activity and therefore 

 exerting a constant inhibitor effect on the muscle-fibers. Stimulation of the 

 peripheral end of the divided splanchnic causes an arrest or inhibition of 

 a preexisting contraction. 



The nerve-centers regulating the contraction and relaxation of the 

 muscle walls of the intestine are doubtless excited to activity by nerve impulses 

 transmitted through afferent nerves, probably the vagus, from the mucous 

 surface of the small intestine. These centers are also influenced by nerve 

 impulses descending from the cerebrum, though the route they take is not 



