350 DIGESTION. 



secretion, and as slowly exposed to the absorbent power 

 of all the villi and blood-vessels of the mucous mem- 

 brane. The movement of the intestines is peristaltic 

 or vermicular, and is effected by the alternate con- 

 tractions and dilatations of successive portions of the 

 intestinal 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 longi- 

 tudinal muscular fibres contract first, or more than the 

 circular ; they draw a portion of the intestine upwards, or, 

 as it were, backwards, over the substance 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 retro- 

 grade ; and there is no hindrance to the backward move- 

 ment 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- 

 caecal valve. Besides, the orifice of communication 

 between the ileum and csecum (at the borders of which 

 orifice are the folds of mucous membrane which form the 

 valve) is encircled with muscular fibres, the contraction 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 



