DIGESTION IN THE LARGE INTESTINE. 425 



larger intestine, but more nearly approaches that of the small intestine. 

 In it are found numerous folds of mucous membrane, villi, glands and 

 follicles, and large lymphatics. 



Comparative anatomy thus shows that in all animals whose diet is 

 composed of substances difficult to digest and rich in cellulose, the 

 caecum is highly developed, if some other organ is not specialized for 

 this purpose, as in ruminants. "While its volume is proportionate to the 

 volume of food which these animals require for their nourishment, while 

 it is absent in the sable and those of the bear tribe which nourish them- 

 selves on fruits or substances easily digested, and while it is slightly 

 developed in the carnivora and herbivora which feed on tender plants, it 

 is highly developed and acquires a value closely allied to that of the 

 ' stomach in animals whose ordinary food is composed of bulky vegetable 

 substances difficult to digest. Such a state' of affairs is seen in most of 

 the rodents. It reaches its maximum development in the solipedes, the 

 rhinoceros, and some herbivorous marsupials. Its development, again, is 

 not only dependent upon the degree of the digestibility of the food, but 

 it is inverse with the size of the stomach (hence its great size in soli- 

 pedes) and with the presence of special organs to facilitate gastric diges- 

 tion (hence its small size in ruminants). In cases, therefore, where we 

 have a voluminous stomach provided with an extensive mucous mem- 

 brane and followed by a long small intestine, we may be sure the caecum 

 will be poorly developed. These facts prove that the cscum is a com- 

 pensatory organ to the stomach, and that the development of the two is 

 in inverse ratio. 



Apart from the oesophageal dilatations of the ruminant, we have 

 certain other animals in which annexes to the alimentary tube serve to 

 assist in the digestion of almost indigestible food. These annexes may 

 replace the oesophageal pouches and caecum. Thus, we have in birds the 

 crop, analogous to the ruminant's pouches, highly developed in pigeons, 

 chickens, and geese, with the addition of a muscular, crushing stomach. 

 In these birds, with the exception of the pigeon, the caecum is also 

 present, and all together are needed for the digestion of foods. Here 

 the apparatus of mastication is absent. The crop, gizzard, and caeca all 

 fulfill the same end. In the solipedes, again, the caecum therefore fulfills 

 the same general functions as the pouches of the ruminant's stomach, 

 which again are analogous to the crop of the gallinaceous birds. 



Most of the earlier physiologists regard the caecum as a second 

 stomach from some obscure analogy of form, and from the fact that its 

 contents were said to be almost invariably acid. This theory as to the 

 analogy of the functions of the caecum and stomach held until it was 

 discovered that the glands of the caecum, instead of secreting an acid 

 fluid, poured out an alkaline secretion like that coming from the glands 



