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 juice. 



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 cleanses the mouth, preventing an encrusta- 

 tion of the teeth and tongue with food par- 

 ticles, bacteria, scuffed-off epithelial cells, and 



Table 16-1— Human Saliva; 

 Average Composition 



Percent 

 Components by Weight 



Water 99.5 



Inorganic salts (mainly chlorides, 

 bicarbonates, and phosphates 

 of sodium, potassium, and cal- 

 cium) 0.2 



Inorganic gases: oxygen and carbon 



dioxide Traces 



Organic substances 



Enzymes: ptyalin and maltase. . . Traces 

 Other proteins: mucin, globulin, 



albumen 0.2 



Wastes: urea, etc Traces 



Reaction: slightly acid (pH 6.5-6.8) 



other detritus of the oral cavity. Thus if 

 salivation is inhibited for a long period, as 

 occurs in certain fevers, the mouth tends to 

 foul, unless it is washed at frequent inter- 

 vals. The continuous flow of saliva also 

 lubricates the tongue during speech; if the 

 saliva does not flow properly the tongue 

 is apt to "cling to the roof of the mouth." 



The Salivation Reflex. The main flow 

 of saliva occurs at mealtimes; and the activ- 

 ity of the salivary glands is controlled en- 

 tirely by the nervous system. Other digestive 

 glands (see p. 306) are also activated by 

 hormones, which exert a slower and more 

 sustained effect. But the flow of saliva must 

 occur very rapidly, while the food is in the 

 mouth. Accordingly, salivation is entirely a 

 reflex act, which is effected through the nerv- 

 ous system. 



