140 MANUAL OF PHYSIOLOGY. 



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 

 induces 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 intercentral chan- 

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

 secreting centre. 



Irritation of the gastric mucous membrane stimulates the 

 salivary glands as may be seen with a gastric fistula, or by the 

 sudden 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 

 action. 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 activ- 

 ity of the salivary glands, may receive impulses from many 

 distant and diverse nervous sources, or may be influenced directly 

 by the quality of the blood flowing through the special nerve 

 centre itself. 



The channels traversed by the efferent impulses going to the 

 salivary glands have been demonstrated by experiment. In the 

 case of the sub-maxillary, the route is especially distinct and 

 instructive, so that from this gland we obtain most of our knowl- 

 edge concerning the direct influence of nerve impulses on the 



