THE SALIVARY GLANDS. 679 



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- 

 tion of the secretion center by nervous 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 inhibitory action of nerve impulses arising in the 

 cerebral centers. 



Electrical Changes in the Gland during Activity. It has been 

 shown that the salivary as well as other glands suffer certain changes 

 in electrical potential during activity which are comparable in a gen- 

 eral way to the "action currents" observed in muscles and nerves. f 



The Digestive Action of Saliva Ptyalin. The digestive action 

 proper of the saliva is limited to the starchy food. In human 

 beings and most mammals the saliva contains an active enzyme 

 belonging to the group of diastases and designated usually as ptyalin 

 or salivary diastase. It may be prepared in purified form from saliva 

 by precipitation with alcohol, but its chemical nature, like that of the 

 other enzymes, is still an unsolved problem. Saliva or preparations 

 of ptyalin act readily upon boiled starch, converting it into sugar 

 and dextrin. This action may be demonstrated very readily by 

 holding a little starch paste or starchy food, such as boiled potatoes, 

 in the mouth for a few moments. If the solution is then examined the 

 presence of sugar is readily shown by its reducing action on solutions 

 of copper sulphate (Fehling's solution). There is no doubt that the 

 action of ptyalin upon the starch is hydrolytic. Under the influence 



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

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

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



f See Biedermann, " Electro-physiology, " translation by Welby, London, 

 1896. 



