386 MOVEMENTS OF ALIMENTARY CANAL. [BOOK ir, 



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 ; meanwhile the more 

 independent movements of the higher parts of the canal con- 

 tinue to drive the contents onward ; and hence the faeces accu- 

 mulate 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 obstruc- 

 tion is frequently thrown into violent peristaltic movements. 



227. 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 faeces, which is in part at 

 least the result of increased peristaltic action, is not an unfre- 

 quent 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 sug- 

 gested, 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 oxy- 

 gen ; 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, provoke 

 the movements in question. 



The movements thus produced are to some extent the result 

 of the deficient supply of blood acting directly on the walls of 

 the canal, though in asphyxia at all events this effect may be in- 

 creased by the too venous blood stimulating the central nervous 

 system and thus sending augment or impulses down the vagus. 



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

 peristaltic action it will be sufficient here to say that while some 

 such as nicotine appear to act directly on the walls of the canal, 

 others such as strychnia produce their effect chiefly by acting^ 

 through the central nervous system. 



