468 MOVEMENTS OF ALIMENTARY CANAL. [BOOK 11. 



nervous mechanisms, by means of which the more complicated 

 movements of the stomach, including the closing and opening of 

 the sphincters, are carried out, is at present very imperfect. We 

 cannot add to what we have incidentally said in speaking of 

 vomiting. 



The movements of the rectum, including the sigmoid flexure, 

 appear to be much more closely dependent on the central nervous 

 system than are those of the rest of the alimentary canal. As 

 we have said the movements of both large and small intestine 

 are rather assisted and augmented than primarily called forth by 

 impulses descending from the central nervous system along the 

 vagus fibres. As the large intestine however passes into the 

 rectum government by the vagus is replaced by government 

 through the lumbar cord and the nerves just previously mentioned ; 

 and this government appears to be not so much mere augmen- 

 tation as the actual carrying out of the movements through reflex 

 action. Hence this is the part of intestinal movement which fails 

 in diseases of the central nervous system; the failure leading to 

 obstinate constipation if not to actual difficulty of defecation. The 

 presence of faeces in the sigmoid flexure no longer stirs up the 

 reflex mechanism for their discharge ; meanwhile the more in- 

 dependent movements of the higher parts of the canal continue 

 to drive the contents onward ; and hence the fseces accumulate 

 in the sigmoid flexure and colon awaiting the delayed action 

 of the imperfect reflex mechanism. With regard to the exact 

 manner in which the presence of food acts as a stimulus it may 

 be worth while to remark, that, though in the stomach as we 

 have seen mere fulness is not the efficient cause of the movements, 

 since these become more active as digestion proceeds and the bulk 

 of the contents diminishes, yet in the intestine distension of the 

 bowel up to certain limits most distinctly increases the vigour of 

 the movements just as distension of the cardiac cavities within 

 certain limits improves the cardiac stroke. This is well seen in 

 obstruction of the bowels in which cases the bowel distended 

 above the obstruction is frequently thrown into violent peristaltic 

 movements. This effect is in part at least due to the distension 

 extending the muscular fibres and so in a direct manner promoting 

 their contraction (see 81), but may be in part due to augmentor 

 impulses excited in a reflex manner. Probably in an intestine 

 isolated from the central nervous system, food provokes peristaltic 

 movements much more by causing distension and so stretching 

 the muscular coats than by acting as a stimulus to the mucous 

 membrane either through chemical action or in any similar way. 



277. Next to the presence of food in the interior of the 

 alimentary canal, a deficient oxygenation of the blood supplied 

 to the walls of the canal or the sudden cutting off of the supply 

 of blood, may be regarded as the most powerful provocatives 

 of peristaltic action. When the aorta is clamped or when the 



