276 DIGESTION. 



exposed to the absorbent power of all the villi and blood- 

 vessels of the mucous membrane. The movement of the intes- 

 tines is peristaltic or vermicular, and is effected by the alternate 

 contractions and dilatations of successive portions of the intes- 

 tinal coats. The contractions, which may commence at any 

 point of the intestine, extend in a wave-like manner along the 

 tube. In any given portion, the longitudinal muscular fibres 

 contract first, or more than the circular ; they draw a portion 

 of the intestine upwards, or, as it were, backwards, over the sub- 

 stance to be propelled, and then the circular fibres of the same 

 portion contracting in succession from above downwards, or, 

 as it were, from behind forwards, press on the substance into 

 the portion next below, in which at once the same succession 

 of actions next ensues. These movements take place slowly, 

 and, in health, are commonly unperceived by the mind ; but 

 they are perceptible when they are accelerated under the in- 

 fluence of any irritant. 



The movements of the intestines are sometimes retrograde ; 

 and there is no hindrance to the backward movement of the 

 contents of the small intestine. But almost complete security 

 is afforded against the passage of the contents of the large into 

 the small intestine by the ileo-csecal valve. Besides, the orifice 

 of communication between the ileum and caecum (at the bor- 

 ders of which orifice are the folds of mucous membrane which 

 form the valve) is encircled with muscular fibres, the contrac- 

 tion of which prevents the undue dilatation of the orifice. 



Proceeding from above downwards, the muscular fibres of 

 the large intestine become, on the whole, stronger in direct 

 proportion to the greater strength required for the onward 

 moving of the fseces, which are gradually becoming firmer. 

 The greatest strength is in the rectum, at the termination of 

 which the circular unstriped muscular fibres form a strong 

 band called the internal sphincter, while an external sphincter 

 muscle with striped fibres is placed rather lower down, and 

 more externally, and holds the orifice close by a constant 

 slight contraction under the influence of the spinal cord. 



The peculiar condition of the sphincter, in relation to the 

 nervous system, will be again referred to. The remaining 

 portion of the intestinal canal is under the direct influence of 

 the sympathetic or ganglionic system, and, indirectly, or more 

 distantly, is subject to the influence of the brain and spinal 

 cord, which influence appears to be, in some degree, transmitted 

 through the vagus nerve. Experimental irritation of the brain 

 or cord produces no evident or constant effect on the move- 

 ments of the intestines during life ; yet in consequence of cer- 

 tain conditions of the mind, the movements are accelerated or 



