NUTRITION 151 



of digestion is basically similar in every case: it is a process of 

 hydrolysis. This is a chemical reaction in which a molecule of 

 the substance to be digested combines with a molecule of water to 

 form a new compound. Then this splits into two or more simpler 

 molecules and, by repeated hydrolyses, exceedingly complex food 

 substances become relatively simple ones. Hydrolyses are brought 

 about through the activities of special catalytic agents, the fer- 

 ments, or enzymes — a special enzyme for each kind of chemical 

 reaction being supplied by the alimentary canal. Moreover, the 

 diverse enzymes, carrying out chemical simplification of various 

 foodstuffs, produce just a few relatively simple substances: amino 

 acids from proteins, fatty acids and glycerol from fats, and 

 simple sugars from carbohydrates. (Fig. 113.) 



The wall of the alimentary canal consists of three chief cellular 

 layers: a lining epithelium, a connective tissue layer, and a muscu- 

 lar layer. The epithelium which lines the alimentary canal and 

 its derivatives is the digestive tract proper in the sense that it is 

 of basic functional importance in secreting the digestive fluids 

 and in absorbing the products of digestion. The other layers per- 

 form accessory functions such as support, conduction of blood 

 vessels, and movements of the canal. (Fig. 34.) 



A. Buccal Cavity, Pharynx, and Esophagus 



The entrance to the alimentary canal is the mouth, a transverse 

 aperture in the head, which leads into the mouth-chamber, or 

 buccal cavity, supported by the jaws. The buccal cavity gradu- 

 ally merges into the throat, or pharynx, which in the Vertebrates 

 acts as a passage both for the food and the respiratory gases. The 

 respiratory current of water in aquatic forms soon passes to the 

 exterior by a series of perforations, the gill slits, through the 

 pharynx and body wall; while the respiratory current of air in 

 higher forms enters the lungs. On the other hand, in all Verte- 

 brates the preparation of the food for its passage through the 

 alimentary canal starts in the buccal cavity. (Figs. 110, 117.) 



The human buccal cavity is lined with a membrane, continu- 

 ous at the lips with the outer skin, which is provided with uni- 

 cellular glands that secrete mucus. This and saliva, secreted 

 by three pairs of large salivary glands, lubricate the food so 

 that it may be more readily moved about by the tongue for 

 mastication by the teeth and passed on toward the esophagus. 



