THE MUSCULAR MECHANISMS OF DIGESTION. 305 



mented than primarily culled 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 augmentation 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 dif- 

 ficulty of defecation. The presence of feces in the sigmoid flexure no 

 longer stirs up the reflex mechanism for their discharge ; meanwhile the 

 more independent movements of the higher parts of the canal continue to 

 drive the contents onward, and hence the feces 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, although 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 distention of the 

 bowel up to certain limits most distinctly increases the vigor of the move- 

 ments just as distention of the cardiac cavities within certain limits im- 

 proves the cardiac stroke. This is well seen in obstruction of the bowels, 

 in which case the bowel distended above the obstruction is frequently 

 thrown into violent peristaltic movements. This effect is in part at least 

 due to the distention extending the muscular fibres, and so in a direct 

 manner promoting their contraction (see 77), but may be in part due 

 to augmentor impulses excited in a reflex manner. Probably in an in- 

 testine isolated from the central nervous system food provokes peristaltic 

 movements much more by causing distention 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. 



238. 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 respiration is seriously interfered with, peristaltic 

 movements become very pronounced. Thus, in death by asphyxia or suf- 

 focation, an involuntary discharge of feces, which is in part at least the 

 result of increased peristaltic action, is not an unfrequent result; and 

 the marked peristaltic movements which are so frequently seen in an animal 

 when the abdomen is laid open immediately after death appear to be due to 

 the cessation of the circulation and the consequent failure in the supply of 

 blood to the walls of the alimentary canal, and not, as has been suggested, 

 to the contact with air of the peritoneal surface. Since it is blood which 

 brings oxygen to the tissues, failure in the supply of blood is tantamount 

 to failure in the supply of oxygen ; but the blood current brings other 

 things besides oxygen and also takes things away ; and the failure of this 

 action also, probably, as well as failure in the supply of oxygen, provokes 

 the movements in question. 



The movements thus produced are to some extent the result of the defi- 

 cient supply of blood acting directly on the walls of the canal, though in 

 asphyxia at all events this effect may be increased by the too venous blood 

 stimulating the central nervous system, and thus sending augmentor impulses 

 down the vagus. 



With regard to the mode of action of the drugs which promote peri- 

 staltic action, it will be sufficient here to say that while some, such as 



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