814 PHYSIOLOGY 



the cellulose is dissolved and the cells are opened up so as to allow 

 their contents to escape. The products of digestion of cellulose 

 include a number of organic acids, chiefly of the lower fatty acid 

 series, as well as methane, carbon dioxide, and hydrogen. In the 

 paunch the acids accumulate so that fermentation occurs in an acid 

 medium, *whereas in the caecum the acids are neutralised by the 

 secretion of alkalies and the reaction remains practically neutral. 

 The products of digestion of cellulose, as well as the contents of the 

 vegetable cells set free by the solution of the cell walls, are gradually 

 absorbed by the walls of the large gut. In carnivora the large 

 intestine has very unimportant functions to discharge in digestion 

 and absorption. The proteins of meat are practically entirely 

 absorbed by the time that the food has arrived at the ileocolic valve, 

 and the same applies to fat. In man the importance of the large 

 intestine will vary with the nature of his food. Under the con- 

 ditions of civilised life the food material is almost- entirely absoibed 

 by the time that it reaches the lower end of the ileum. If, however, 

 a large quantity of vegetable food be taken, such as fruit or green 

 vegetables, or cereals roughly prepared and coarsely ground, a con- 

 siderable amount of material may reach the large intestine un- 

 absorbed. A certain proportion of this may undergo absorption 

 in the large intestine, while the rest will pass out with the faeces, 

 increasing their bulk. 



It is hardly possible to speak of a secretion by the mucous mem- 

 brane of the large intestine. In herbivora alkaline carbonates are 

 secreted to neutralise the acids produced in the bacterial fermenta- 

 tion of the food, but the processes of absorption and secretion 

 keeping pace, there is no accumulation of the products of secretion 

 in the intestine. A section of the mucous membrane shows a number 

 of simple tubular glands. The greater number of the cells lining 

 these glands are typical ' goblet ' cells and contain plugs of mucin. 

 The secretion of mucus not only aids the passage of the faeces along 

 the gut, but probably impedes the propagation of the bacteria 

 which are present in countless numbers in the faeces. This may 

 account for the fact that although bacteria are so numerous in the 

 faeces, it is difficult to cultivate any large numbers, most of them 

 being dead. 



As an absorbing organ the large intestine of man is of little import- 

 ance. From observations on fistulae in man it has been calculated 

 that about 500 c.c. of water pass the ileocolic valve in the twenty- 

 four hours. Of these about 400 c.c. undergo absorption in the large 

 intestine. The absorption of any of the food substances by this 

 part^of 'the'gut is'much slower'than that which takes place on intro- 

 duction by the mouth. Feeding by nutrient enemata is thus always 



