. Principles of modern biology. Biology. of the mouth near the roots of the lower canine teeth (Fig. 16-16). Composition and Functions of the Sa- liva. The composition of human saliva, based on an analysis of the mixed juices of all the glands, is given in Table 16-1. This shows that saliva, like all other digestive juices, contains a large proportion of water, which serves as a solvent for all other com- ponents of the Fig. 16-16. Drawing of the salivary glands super- imposed upon a photograph. The sublingual gland, lower front; the submaxillary gland, lower back; the parotid gland, a
. Principles of modern biology. Biology. of the mouth near the roots of the lower canine teeth (Fig. 16-16). Composition and Functions of the Sa- liva. The composition of human saliva, based on an analysis of the mixed juices of all the glands, is given in Table 16-1. This shows that saliva, like all other digestive juices, contains a large proportion of water, which serves as a solvent for all other com- ponents of the Fig. 16-16. Drawing of the salivary glands super- imposed upon a photograph. The sublingual gland, lower front; the submaxillary gland, lower back; the parotid gland, above. (From The Digestion of Foods. Encyclopedia Britannica Films, Inc.) The functions of saliva are partly chemical and partly mechanical. Among the chemical reagents in saliva, the enzyme ptyalin is a very active amylase. Thus if a suspension of boiled starch is incubated at body tempera- ture with a few drops of saliva, a hydrolysis of the starch to maltose is completed within about 20 minutes (see Fig. 5-5). The opaque starch suspension gradually becomes first translucent and then transparent, which in- dicates that the large starch molecules are decomposing into smaller and smaller frag- ments. Finally all the starch is converted into maltose. Moreover, some of the maltose is converted to glucose, since human saliva con- tains small amounts of the enzyme maltase. As to its mechanical functions, saliva facili- tates swallowing by softening and lubricat- ing the food, and by binding the separate food particles into a plastic mass, the bolus, which can be swallowed as a whole. Thus when the flow of saliva is inhibited—as by fright—a cracker or other dry food can scarcely be swallowed. Saliva also augments The Digestive System - 301 taste. The taste buds are sensitive only to dissolved substances; and by dissolving the dry components of the food, the saliva brings out their taste. A small flow of saliva continues even be- tween meals. This flow is important because it clea
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