132 



DIGESTION. 



Fig. 26. 



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

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

 canal 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 indigestible matter, to- 

 gether 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, the differ- 

 ence 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 herb- 

 ivorous animals is more complex than that of 

 the carnivora ; since, in vegetable substances, 

 the nutritious matters are often present in a 

 comparatively solid and unmanageable form, as, 

 for example, in raw starch and the cereal grains, 

 and are nearly always entangled among vege- 

 table cells and fibres of an indigestible character. 

 In those instances 'where the nutriment consists 

 mostly of grass, leaves, twigs, and roots, the 

 digestible matters bear only a small proportion 

 to the entire quantity ; and a large mass of food 

 must therefore be taken, in order that the re- 

 quisite amount of nutritious material may be 

 extracted from it. In such cases, accordingly, 

 the alimentary canal is large and long ; and is 

 divided into many compartments, in which dif- 

 ferent processes of disintegration, transforma- 

 tion, and solution are carried on. 



In the common fowl, for instance (Fig. 26), 

 the food, consisting mostly of grains, or of in- 

 sects with hard, coriaceous integument, first 

 passes down the oesophagus (a) into a diverti- 

 culum or pouch (6) termed the crop. Here it 

 remains for a time mingled with a watery secre- 

 tion in which the grains are macerated and 

 softened. The food is then carried farther down 

 until it reaches a second dilatation (c)^ the pro- 

 ventriculus, or secreting stomach. The mucous membrane here is thick 

 and glandular, and is provided with numerous secreting follicles. From 

 them an acid fluid is poured out, by which the food is subjected to 

 further changes. It next passes into the gizzard (d), or triturating 



ALIMENTARY CANAL 

 OF FOWL. a. (Esophagus. 

 b. Crop. c. Proventriculus, 

 or secreting stomach, d. Giz- 

 zard, or triturating stomach. 

 e. Intestine. /. Two long 

 caecal tubes which open into 

 the intestine a short distance 

 ahove its termination. 



