The office of digestion is to dissolve the food, so that it can be absorbed through the mucous membrane of the alimentary canal, and to effect certain chemical changes in its constituents, in order that these constituents may be utilised (assimilated) by the system. These changes are brought about chiefly by certain unorganized ferments (p. 68), which are respectively secreted by the digestive glands, and which are characteristic components of the digestive fluids. Digestion commences in the mouth, and does not end until the residue of food quits the body in the form of dung. The action of digestion is not uniform during the progress of the food through the alimentary canal; for it begins slowly, increases steadily, and having reached its effective limit, it gets slower and slower. Owing to the evolutionary influence of the different constituents of food, the alimentary canal of animals is more or less divided into tracts (such as the mouth, stomach, small intestine, caecum, and remainder of the large intestine), which are respectively concerned in the digestion of special food substances. Hence the form and particular functions of the alimentary canal are dependent on the natural food.