DIGESTION IN THE MOUTH 147 



uents of mucin, etc. When the cells are secreting, their blood 

 supply is greatly increased. Granules appear in the cells, and 

 these apparently are the mother substance of mucin. The pro- 

 toplasm of the cells is in large part converted into saliva, and 

 then renewed from repair materials selected from the blood. 

 Each set of glands is controlled by nerves from the brain direct, 

 and from the sympathetic system. The nerves from the brain 

 dilate the arterioles, those from the sympathetic system con- 

 strict. When the former are stimulated (the chorda tympani 

 of the sublingual and submaxillary, and the >auriculotemporalis 

 branch of the fifth nerve, with its fibers from the glossophar- 

 yngeal for the parotid) much saliva is secreted, and it is of a 

 thinner, more watery character than the secretion resulting 

 from stimulation of the sympathetic. It is as yet undecided 

 whether the difference in character of the saliva under these 

 two types of stimulation is due to different responses on the 

 part of the same cells, response of different cells, the dilation or 

 constriction of the blood vessels, or to other causes. A flow of 

 saliva may be produced by mechanical irritation of the mouth, 

 by reflex action upon stimulation of the end organs of taste or 

 smell, by purely psychic influences, such as the thought or sight 

 of food, or by a general reflex, as in nausea preceding vomiting. 

 Its flow may be inhibited by nervous stress or anxiety, a fact 

 well known to amateur public speakers or by those who have 

 experienced dry ness of the throat in sudden fright. Some drugs 

 stimulate, others retard the flow of saliva. The composition of 

 the secretion is influenced by the nature of the stimulus pro- 

 ducing it. 



If a dog is fed meat, the saliva secreted is thick and mucous 

 (submaxillary secretion) but if dried pulverized meat is fed, 

 the saliva is thin and watery (parotid secretion). Thus there 

 appears to be a certain amount of adaptation of the character 

 of the secretion to the type of stimulus, and to the function 

 which the secretion will be called upon to perform. The thick 

 mucous saliva serves as a lubricant for the meat, whereas the 

 thin watery secretion moistens the dry meat powder. 



