SECTION VIII 

 FUNCTIONS OF THE LARGE INTESTINE 



GREAT differences are found in the structure of the large intestine of different 

 animals, differences which depend, not on the zoological position of the 

 animal, but entirely on the nature of its food. In the carnivora the large 

 intestine is short and narrow and possesses little or no caecum. In herbivora 

 the large intestine is well developed with sacculated walls, and the caecum, 

 i.e. that part of the large gut distal to the opening of the ileum into the 

 colon, is .very large. Man occupies a somewhat intermediate position be- 

 tween these two classes. The differences between the total length of the 

 alimentary canal in various animals are largely determined by the varying 

 development of the large intestine. The relation of these differences to the 

 diet is seen if we compare the length of the intestine with the length of the 

 animal. Thus in the cat the intestine is three times the length of the animal, 

 in the dog from four to six times, in man from seven to. eight times, in the 

 pig fourteen times, and in the sheep twenty-seven times. The great develop- 

 ment of the large intestine in vegetable feeders is due to the fact that in this 

 class of food all the nutritious material is enclosed in cells surrounded by 

 cellulose walls. In order that the food-stuffs e.g. proteins, starch, &c. 

 may be dissolved by the digestive juices and absorbed by the wall of the 

 gut, these cellulose walls must be disintegrated. In none of the higher verte- 

 brates do we find any cellulose digesting ferment, cytase, produced in the 

 alimentary canal. The cellulose has therefore to be dissolved either by the 

 agency of bacteria or by means of cellulose-dissolving ferments which may be 

 present in the vegetable cells themselves. Thus in ruminants the masses of 

 grass and hay are first received into the paunch, where they are kept warm 

 and moist with saliva. In the paunch opportunity is thus given for the 

 development of huge numbers of micro-organisms which can dissolve cellu- 

 lose. From time to time portions of the sodden mass are returned to 

 the mouth, chewed and then swallowed again to be subjected to the action 

 of the proper digestive juices. In the horse and rabbit the chief part of the 

 digestion of the cellulose occurs in the caecum. Even after abstinence from 

 food for some time the caecum is still found to contain food material. In the 

 caecum, under the action of bacteria, 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, 



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