DIGESTION IN LARGE INTESTINE. 273 



There seems no reason, however, to conclude that any special, 

 " secondary," digestive process occurs in the caecum or in any 

 other part of the large intestine. Probably any constituent 

 of the food which has escaped digestion 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, albu- 

 minous, 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 stom- 

 ach. 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, and the 

 addition of the special excretory products which give it the 

 characteristic odor. At the same time, as before said, it is 

 probable that a certain quantity of nutrient matter always 

 escapes digestion in the small intestine, and that this happens 

 more especially when food has been taken in excess, or when 

 it is of such a kind as to be difficult of digestion. Under 

 these circumstances there is no doubt that such changes as 

 were proceeding in it at the lower part of the ileum may 

 go on unchecked in the large bowel, the process being as- 

 sisted by the secretion of the numerous tubular glands therein 

 present. 



By these means the contents of the large intestine, as they 

 proceed towards the rectum, become more and more solid, and 

 losing their more liquid and nutrient parts, gradually acquire 

 the odor and consistence characteristic of faeces. After a 

 sojourn of uncertain duration in the rectum, they are finally 

 expelled by the contraction of its muscular coat, aided, under 

 ordinary circumstances, by the contraction of the abdominal 

 muscles. 



For a description of the mechanism by which the act of 

 defecation is accomplished, see p. 183. 



The average quantity of solid fecal matter evacuated by 

 the human adult in twenty-four hours is about five ounces ; 

 an uncertain proportion of which consists simply of the undi- 

 gested or chemically modified residue of the food and the re- 

 mainder of certain matters which are excreted in the intesti- 

 nal canal. 



