116 DIGESTION. 



upon it in such a way as to liquefy and dissolve it. These fluids 

 are secreted by the mucous membrane of the alimentary canal, and 

 by certain glandular organs situated in its neighborhood. Since the 

 food always consists, as we have already seen, of a mixture of vari- 

 ous substances, having different physical and chemical properties, 

 the several digestive fluids are also different from each other ; each 

 one of them exerting a peculiar action, which is more or less con- 

 fined to particular species of food. As the food passes through the 

 intestine from above downward, those parts of it which become 

 liquefied are successively removed by absorption, and taken up by 

 the vessels ; while the remaining portions, consisting of the indi- 

 gestible matter, together with the refuse of the intestinal secretions, 

 gradually acquire a firmer consistency owing to the absorption of 

 the fluids, and are finally discharged from the intestine under the 

 form of feces. 



In different species of animals, however, the difference in their 

 habits, in the constitution of their tissues, and in the character of 

 their food, is accompanied with a corresponding variation in the 

 anatomy of the digestive apparatus, and the character of the secreted 

 fluids. As a general rule, the digestive apparatus of herbivorous 

 animals is more complex than that of the carnivora ; since, in vege- 

 table substances, the nutritious matters are often present in a very 

 solid and unmanageable form, as, for example, in raw starch and 

 the cereal grains, and are nearly always entangled among vegetable 

 cells and fibres of an indigestible character. In those instances 

 where the food consists mostly of herbage, as grass, leaves, &c., the 

 digestible matters bear only a small proportion to the entire quan- 

 tity ; and a large mass of food must therefore be taken, in order 

 that the requisite amount of nutritious material may be extracted 

 from it. In such cases, accordingly, the alimentarjr canal is large 

 and long ; and is divided into many compartments, in which 

 different processes of disintegration, transformation, and solution 

 are carried on. 



In the common fowl, for instance (Fig. 16), the food, which con- 

 sists mostly of grains, and frequently of insects with hard, coria- . 

 ceous integument, first passes down the oesophagus (a) into a 

 diverticulum or pouch (b) termed the crop. Here it remains for 

 a time mingled with a watery secretion in which the grains are 

 macerated and softened. The food is then carried farther down 

 until it reaches a second dilatation (c), the proventriculus, or 

 secreting stomach. The mucous membrane here is thick and 



