SECTION VII 

 . 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 between 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 development 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 vertebrates 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 develop- 

 ment of huge numbers of micro-organisms which can dissolve cellulose. 

 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, 



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