64 NUTRITIONAL PHYSIOLOGY 



acting upon the endings of nerves excitable by its chemical 

 ingredients and by its temperature more than by its mere 

 contact. We have to do with something more than 

 the typical reflex, however, because it is a familiar fact 

 that the appeal to consciousness has much influence upon 

 the flow of saliva. The "watering of the mouth" at the 

 approach of acceptable food is a hard phenomenon to 

 classify. It is a reflex, but it is one which would not 

 occur in an unconscious subject. For such actions the 

 term psychoreflex is often used. 



The saliva is a bland fluid which one would hardly sup- 

 pose to be endowed with active powers of digestion. In 

 some animals it does not have any apparent chemical ac- 

 tion. Still, it has valuable properties which we shall do 

 well to recognize. Whether it is a digestive juice or not, 

 its physical effect is useful in mastication, since it softens 

 the food, makes it cohere into the pellets which are pre- 

 pared for swallowing, and later lubricates their transit to 

 the stomach. Moreover, it has a defensive use, protecting 

 the mouth from injury when food or fluid is taken too hot 

 or when some corrosive liquid calls for dilution. As it issues 

 fresh from the glands it is slightly alkaline in reaction. 

 If it stagnates for a long time in the by-places of the mouth, 

 as happens during sleep, and if it contains at the same time 

 traces of carbohydrate food in solution, bacterial fermenta- 

 tion may make it acid and the effect upon the teeth may 

 be injurious. The value of an alkaline mouth-wash, like 

 milk of magnesia, used at bedtime is evident. 



Human saliva contains various salts. Attention need 

 be called only to its lime compounds, which are always de- 

 posited more or less upon the back surfaces of the teeth, 

 a process which reminds one of the formation of stalactites 

 and stalagmites in caverns. The hard crust that results 

 is the tartar. It is not very unlike the original substance 

 of the teeth in its chemical composition, and its occurrence 

 might seem to indicate a mode of making good the wearing 

 away of the teeth. Unfortunately we cannot regard it in 

 this favorable light, for the lime salts are always contami- 



