770 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 experiments in the medulla oblongata, in 

 the formatio reticularis lateral to the facial nucleus. The experi- 

 ments f consisted in dividing the chorda and the nervus tympan- 

 icus and, after a suitable interval, examining sections of the medulla 

 for the appearance of degenerative changes (chromatolysis, p. 128). 

 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 stimula- 

 tion 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 vomit- 

 ing, may be accompanied by a flow of saliva, the effect in this case 

 being due probably to stimulation of the secretion center by ner- 

 vous impulses descending from the higher nerve centers. Lastly, 

 the medullary center may be inhibited as well as stimulated. The 

 well-known effect of fear, embarrassment, or anxiety in producing 

 a parched throat may be explained in this way as due to the inhibit- 

 ory action of nerve impulses arising in the cerebral centers. 



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

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

 1904, and "Archives Internationales de physiologic," 1, 119, 1904.' 



t Yagita and Hayama, "Neurologisches Centralblatt," 738, 1909. 



