424 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



caeca are consequent^ rudimentary or absent. In vegetable-eating birds, 

 on the other hand, the caecum acquires a high degree of importance. 

 The caecal apparatus in these animals is double, and consists of two long 

 tubes, symmetrically located to the right and left, and connected with 

 the alimentary canal at the point of junction of the small and large 

 intestines. Their mucous membrane is arranged in folds, so as to 

 increase their internal surfaces. Their internal surface is supplied with 

 glands, which secrete a fluid, and with villi, which facilitate absorption. 

 While it thus would appear that in these animals, as we find it the case 

 in mammals, the development of the caecum is in proportion to the com- 

 plexity of the food, there are, nevertheless, certain exceptions to this 

 rule. In the nocturnal birds of prey the caecum is highly developed. 

 This is, perhaps, to be explained from the fact that it acts as a compen- 

 sation to the extreme shortness of the intestinal canal in this species. 

 On the other hand, in the gallinaceous birds the caecum is voluminous, 

 and yet in the pigeon it is entirely absent. This latter fact is, perhaps, 

 to be explained by the statement that in the pigeon the starchy mat- 

 ters are completely digested before the caecum is reached, while such 

 is not the case in the gallinaceae. Moreover, in the pigeon the crop is 

 double, so, perhaps, acting as a substitute for the caecum. In mammals, 

 also, the caecum varies in importance according to the character of the 

 substances with which they are nourished. In carnivora, such as the 

 dog and cat, whose foods are readily digestible and assimilable, 

 the caecum is absent or rudimentary. Its structure is analogous to the 

 large intestine ; that is to say, this organ forms part of the excretory 

 portion of the alimentary canal. The herbivora, on the other hand, find 

 their food in substances which are poor in nutritious principles, and in 

 which the nutritive matters are inclosed in resisting cellulose envelopes. 

 As a consequence, we find the digestive apparatus in these animals 

 reaching a high degree of perfection. Their apparatus of mastication is 

 complete; their salivary secretion abundant. Their intestine is extremely 

 long and of considerable volume, so as to multiply the secretory and 

 absorbent surfaces and prolong the action of the digestive juices. 



We find in the ruminants a complex and voluminous stomach, where 

 the food is delayed for a considerable time before being subjected to the 

 action of the solvent juices : but in the horse and rabbit the stomach is 

 simple and small, and we find in the highly developed ca?cum a sub- 

 stitute for the voluminous gastric pouches of the ruminant. In these 

 animals the caecum takes on the form of an immense pocket, whose 

 length may exceed that of the body, and whose capacity may be two 

 or three times greater than that of the stomach, while its volume is 

 so great as to cause it to occupy the greater part of the abdominal 

 cavity. In structure, also, the caecum no longer resembles that of the 



