SECRETION OF SALIVA. 131 



nerves, which carry the impulses towards the centre, and efferent 

 nerves which carry them from the centre. 



If we review the ordinary circumstances giving rise to a flow 

 of saliva, there will be no difficulty in determining the nerves 

 which act as the afferent channels in the simple reflex act. 



Stimulation of the mucous membrane of the tongue and mouth, 

 whether chemically, as with irritating condiments, or mechani- 

 cally, as by the motions of mastication, is generally transmitted 

 to the centre by the sensory branches of the fifth cranial nerve, 

 which supply the mouth, and by the branches of the glosso- 

 pharyngeal. 



The stimulus of the sense of taste is sent by the nerves of that 

 sense, mainly the glosso-pharyngeal, to the taste centre in the 

 cortex cerebri, and from thence to the secreting centre by means 

 of intercentral fibres. 



The stimulating of the olfactory region with certain odors in- 

 duces salivation through a channel of a similar kind passing 

 along the olfactory nerve to the brain, and thence to the special 

 salivary centre. Even in the absence of taste or smell, mental 

 emotion may be excited by seeing or thinking of food, and may 

 cause activity of the salivary glands, here the inter-central chan- 

 nel is the only one occupied in bearing the impulse to the special 

 secreting centre. 



Irritation of the gastric mucous membrane stimulates the sali- 

 vary glands as may be seen with a gastric fistula, or by the sud- 

 den flow of saliva which commonly precedes vomiting. In this 

 case the impulses are carried by the gastric branches of the vagus. 



The stimulation of the central end of the cut sciatic is said to 

 cause an increase in the flow of saliva, so that it would appear 

 that even an ordinary sensory nerve can excite the centre to ac- 

 tion. Lastly, many drugs, when introduced into the blood, cause 

 a flow of saliva, among these are pilocarpin, physostigma, and 

 curara, while atropia and some others, on the other hand, check 

 the action of the glands. 



From this we learn that the nerve centre, controlling the ac- 

 tivity of the salivary glands, may receive impulses from many 

 distant and diverse nervous sources, or may be influenced directly 



