494 MOVEMENTS OF ALIMENTARY CANAL. [BOOK n. 



inhibits the circular coat. It has however been urged by other 

 observers that stimulation of the sacral nerves causes contractions 

 of both circular and longitudinal coats, and that stimulation of the 

 sympathetic fibres, while sometimes giving rise to a brief con- 

 traction, has for its more marked effect an inhibition of all move- 

 ments of the part of the canal in question. According to this 

 latter view the actions of the sacral and of the sympathetic fibres- 

 on the colon and rectum are analogous to the actions of the vagus 

 and splanchnic nerves on the rest of the intestine and the 

 stomach. 



Leaving this matter at present undecided, we may here call 

 attention to the fact that in man at least the movements of the 

 end of the canal, of the rectum and sigmoid flexure, like the 

 movements of the oesophagus at the beginning of the canal, are 

 more directly dependent on the central nervous system than are 

 the movements of the middle portion of the canal. While the 

 movements of the stomach and small intestine readily go on with- 

 out the intervention of the central nervous system, those of the 

 rectum and end of the colon are not so spontaneous and seem to 

 have greater need of some initiation furnished by impulses from 

 the spinal cord. 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 

 defaecation. The presence of faeces in the sigmoid flexure no 

 longer stirs up the reflex mechanism for their discharge; mean- 

 while the more independent movements of the higher parts of the 

 canal continue to drive the contents onward ; and hence the faeces 

 accumulate in the colon and sigmoid flexure awaiting the delayed 

 action of the imperfect reflex mechanism. With regard to the 

 exact manner in which the presence of food provokes peristaltic 

 movements 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 pro- 

 ceeds and the bulk of the contents diminishes, yet in the intestine 

 distension of the bowel up to certain limits most distinctly in- 

 creases 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. 



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 



