436 THE HUMAN BODY 



ally free from the special chemical substances that usually are 

 present in the secretion. 



The efferent nerves to glands belong, without exception, to the 

 sympathetic system. Glands are, therefore, under reflex control, 

 and not subject to the will. 



Hormone Control of Gland Activity. Some of the digestive 

 glands, notably the pancreas, appear to be wholly, or at least 

 chiefly, independent of nervous influences. Their control is vested 

 in hormones. The details of this method of control will be de- 

 scribed in connection with the glands themselves. It may be 

 noted here, however, that in general those glands whose secre- 

 tions are needed early in the digestive process are under reflex 

 control, and those whose secretions may not be required for some 

 time after are under hormone control. 



Control of the Salivary Secretion. The salivary glands are sub- 

 ject to reflex stimulation. We must inquire, therefore, what sen- 

 sory stimuli may excite the reflex. At least three sorts of stimuli 

 are effective to this end; mechanical, the presence of dry sub- 

 stances in the mouth, or merely the rubbing of the tongue against 

 the palate and jaws; chemical, the presence of sapid substances 

 upon the tongue; and psychic, the thought of savory food, as 

 when the mouth "waters." It is an interesting fact that the 

 character of the saliva varies somewhat with the nature of the 

 exciting stimulus; mechanical stimulation causes the production 

 of an abundant but very watery secretion; whereas the chemical 

 stimulus of food in the mouth calls forth a secretion rich in ptyalin. 

 By this mechanism the character of the secretion is adapted to 

 the need which excites it. The mucous lining of the mouth and 

 throat requires constant moistening. For this a watery saliva is 

 adequate, and such a saliva is poured out whenever the dryness 

 of the mouth becomes pronounced enough to act as a stimulus. 

 When food is taken, on the other hand, the proper functioning of 

 saliva requires that it be rich in ptyalin. Chemical stimulation, 

 therefore, excites a secretion containing this substance. 



The watering of the mouth at the thought of food is an example 

 of an emotional reflex through the sympathetic system such as 

 was discussed earlier (Chap. XII). Inhibition of the salivary 

 glands, leading to dryness of the mouth, may occur also under 

 the influence of certain emotions, as in stage fright. 



