DIGESTION. 293 



chyme when it first enters the caecum, there can be no doubt that the 

 absorption of liquid is not by any means concluded. The peculiar odor, 

 moreover, which is acquired after a short time by the contents of the 

 large bowel, would seem to indicate a further chemical change in the 

 alimentary matters or in the digestive fluids, or both. The acid reac- 

 tion, which had disappeared in the small bowel, again becomes very 

 manifest in the caecum probably from acid fermentation-processes in 

 some of the materials of the food. 



There seems no reason to conclude that any special ' secondary diges- 

 tive' process occurs in the caecum or in any other part of the large in- 

 testine. Probably any constituent of the food which has escaped diges- 

 tion and absorption in the small bowel may be digested in the large 

 intestine; and the power of this part of the intestinal canal to digest 

 fatty, albuminous, or other matters, may be gathered from the good 

 effects of nutrient enemata, so frequently given when from any cause 

 there is difficulty in introducing food into the stomach. In ordinary 

 healthy digestion, however, the changes which ensue in the chyme after 

 its passage into the large intestine, are mainly the absorption of the 

 more liquid parts; the chief function of the large intestine being to act 

 .as a reservoir for the residues of digestion before their expulsion from the 

 body. 



MOVEMENTS OF THE INTESTINES. 



It remains only to consider the manner in which the food and the 

 .several secretions mingled with it are moved through the intestinal canal, 

 so as to be slowly subjected to the influence of fresh portions of intes- 

 tinal secretion, and as slowly exposed to the absorbent power of all the 

 villi and blood-vessels of the mucous membrane. The movement of the 

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

 contractions 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 

 longitudinal musjcular 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 action next ensues. These 

 movements take place slowly, and, in health, commonly give rise to no 

 sensation; but they are perceptible when they are accelerated under the 

 influence 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 



