THE SALIVARY GLANDS. 699 



and various other chemical or mechanical stimuli applied to the 

 tongue or mucous membrane of the mouth produce a flow of saliva. 

 The normal flow during mastication must be effected by a reflex of 

 this kind, the sensory impulse being carried to a center and thence 

 transmitted through the efferent nerves to the glands. It is found 

 that section of the chorda prevents the reflex, in spite of the fact that 

 the sympathetic fibers are still intact. No satisfactory explanation 

 of the normal functions of the secretory fibers in the sympathetic 

 has yet been given. Various authors have suggested that possibly 

 the three large salivary glands respond normally to different stimuli. 

 This view has been supported by Pawlow, who reports that in 

 the dog at least the parotid and the submaxillary may react quite 

 differently. When fistulas were made of the ducts of these glands it 

 was found that the submaxillary responded readily to a great num- 

 ber of stimuli, such as the sight of food, chewing of meats, acids, etc. 

 The parotid, on the contrary, seemed to react only when dry food, 

 dry powdered meat/or bread was placed in the mouth. Dryness in 

 this case appeared to be the efficient stimulus. 



Pawlow lays great stress upon the adaptability of the secretion of saliva 

 to the character of the material chewed. Dry, solid food stimulates a large 

 flow of saliva, such as is necessary in order to chew it properly and to form it 

 into a bolus for swallowing. Foods containing much water, on the contrary, 

 excite but little flow of saliva. If one places a handful of clean stones in 

 the mouth of a dog he will move them around with his tongue for a while 

 and then drop them from his mouth ; but little or no saliva is secreted. 

 If the same material is given in the form of fine sand a rich flow of saliva 

 is produced, and the necessity for the reflex is evident in this case, since 

 otherwise the material could not be conveniently removed from the mouth. 

 Such adaptations must be regarded from the physiological point of view 

 as special reflexes depending upon some difference in the nervous mechanism 

 set into play.* 



Since the flow of saliva is normally a definite reflex, we should 

 expect a distinct salivary secretion center. This center has been 

 located by physiological means in the medulla oblongata; its exact 

 position is not clearly defined, but possibly it is represented by the 

 nuclei of origin of the secretory fibers which leave the medulla by 

 way of the facial and glossopharyngeal nerves. Owing to the wide 

 connections of nerve cells in the central nervous system, we should 

 expect this center to be affected by stimuli from various sources. 

 As a matter of fact, it is known that the center and through it the 

 glands may be called into activity by stimulation of the sensory 

 fibers of the sciatic, splanchnic, and particularly the vagus nerves. 

 So, too, various psychical acts, such as the thought of savory food and 

 the feeling of nausea preceding vomiting, may be accompanied by a 

 flow of saliva, the effect in this case being due probably to stimula- 



*See Pawlow, "The Work of the Digestive Glands," translation by 

 Thompson, London, 1902; also " Ergebnisse der Physiologic," vol iii., part i, 

 1904, and "Archives internationales\de physiologie," 1, 119, 1904. 



