Purpose of Digestion.--We have learned that starch and protein food of plants are formed in the leaves. A plant, however, is unable to make use of the food in this condition. Before it can be transported from one part of the plant body to another, it is changed into a soluble form. In this state it can be passed from cell to cell by the process of osmosis. Much the same condition exists in animals. In order that food may be of use to man, it must be changed into a state that will allow of its passage in a soluble form through the walls of the alimentary canal, or food tube. This is done by the enzymes which cause digestion. It will be the purpose of this chapter to discover where and how digestion takes place in our own body. Alimentary Canal.--In all vertebrate animals, including man, food is taken in the mouth and passed through a _food tube_ in which it is digested. This tube is composed of different portions, named, respectively, as we pass from the _mouth_ downward, the _gullet_, _stomach_, _small_ and _large intestine_, and _rectum_. [Illustration: The digestive tract of the frog and man. _Gul_, gullet; _S_, stomach; _L_, liver; _G_, gall bladder; _P_, pancreas; _Sp_, spleen; _SI_, small intestine; _LI_, large intestine; _V_, appendix; _A_, anus.] Comparison of Food Tube of a Frog and Man.--If we compare the food tube of a dissected frog with the food tube of man (as shown by a manikin or chart), we find part for part they are much the same. But we notice that the intestines of man, both small and large, are relatively longer than in the frog. We also notice in man the body cavity or space in which the internal organs rest is divided in two parts by a wall of muscle, the _diaphragm_, which separates the heart and lungs from the other internal organs. In the frog no muscular diaphragm exists. In the frog we can see plainly the silvery transparent _mesentery_ or double fold of the lining of the body cavity in which the organs of digestion are suspended. Numerous blood vessels can be found especially in the walls of the food tube. Glands.--In addition to the alimentary canal proper, we find a number of _digestive glands_, varying in size and position, connected with the canal. [Illustration: Diagram of a gland. _i_, the common tube which carries off the secretions formed in the cells lining the cavity _c_; _a_, arteries carrying blood to the glands; _v_, veins taking blood away from the glands.]