CHAEACTEKISTICS OF THE DIGESTIVE APPAKATUS. 219 



in the mouth, pharynx, and anus, and pale, unstriped, involuntary fibres 

 elsewhere. The contractions of these muscular fibres in the small and 

 large intestines serve to cause the onward progression of the food or the 

 so-called peristaltic movement of the intestines. The mucous membrane 

 of the alimentary canal is epithelial in nature in the mouth, pharynx, 

 and gullet, and in the first three pouches of the ruminant stomach, and 

 in the cardiac half of the stomach of the horse. It is free from glands, 

 and is simply protective in nature. In the entire stomach of carnivorous 

 animals, the fourth stomach of ruminants, and the pyloric half of the 

 stomach of solipedes, as well as through the entire extent of the in- 

 testines of all mammals, it is glandular, and furnishes a more or less 

 active digestive secretion. 



Sensory nerves are supplied to the two extremities of the digestive 

 tube, while the intermediary portions are supplied with nerves whose 

 stimulation seems to lead to secretion, and not, as a rule, to individual 

 sensations. 



The extent of mucous membrane varies natural!}' with the length, 

 diameter, and complexity of the alimentary canal. It is, therefore, less 

 in carnivora, greater in omnivora, and immense in herbivora. The extent 

 of surface, therefore, depends upon the complexity of the food. The 

 more concentrated the food, as in carnivora, the less surface is required 

 for producing secretion, and the less for its absorption. In animals 

 living on a vegetable diet, where the nutritive principles of the food are 

 mixed w r ith a larger amount of non-nutritious residue, a greater surface is 

 required for secretion, greater time is required for digestion, and a 

 greater surface must be supplied for the absorption of digestive matters ; 

 we find, therefore, that in herbivorous animals the intestinal tube is 

 always longer, more complicated, and supplied with a larger extent 

 of mucous membrane than in the carnivora. Even in the herbivora 

 we find a difference in the distribution of the mucous surfaces ; thus, 

 the horse and ox are both herbivorous animals : the former is a 

 monogastric animal, the latter a pol}*gastric, or ruminant. The former 

 digests little by its stomach, and much by its intestinal tube; the 

 latter readily digests more by its vast and complex stomach than by its 

 narrow and small intestinal tube. Both, however, from the fact that 

 they are herbivorous animals, have a great extent of mucous mem- 

 brane, which may be twice or three times as extensive as their ex- 

 ternal body surface. Thus, the cutaneous surface of the horse is about 

 five or six square meters, while its mucous gastro-intestinal surface 

 may be as much as twelve square meters, of which one-thirtieth is 

 represented by the stomach and the rest by the intestines. An ox, on 

 the other hand, of about the same size, has a mucous membrane of about 

 seventeen square meters, of which nine square meters represent the 



