152 TEXT-BOOK OF PHYSIOLOGY 



on the presence of an unorganized ferment or enzyme known as ptyalin or 

 amylase. This enzyme is present in the secretion of each of the salivary 

 glands. The chemic character of ptyalin is unknown, though there are 

 reasons for believing that it partakes of the nature of a protein. It is a prod- 

 uct in all probability of the katabolic activity of the secretor cells. According 

 to Latimer and Warren, ptyalin is a derivative of the zymogen, ptyalogen. 

 This latter compound has been shown to be present in the glands of the 

 dog, cat, and sheep. Ptyalin effects the transformation of starch merely 

 by its presence, and undergoes no perceptible consumption in the process. 

 The activity of this enzyme is very great, and unless interfered with by an 

 excess of sugar and dextrin, it acts practically indefinitely. 



The activity of ptyalin is modified by various external conditions, among 

 which may be mentioned the chemic reaction of the medium in which it is 

 placed. It is most active when the medium is moderately alkaline. Its 

 activity is arrested by strong alkalies or acids, though the presence of a 

 small percentage of an acid does not appear to have any effect in either 

 hastening or retarding the process. This fact has a bearing upon the ques- 

 tion as to whether the action of the saliva is interfered with in the stomach 

 by the presence of the gastric juice. At present it is a disputed matter, but 

 the weight of authority is in favor of the view that the transforming action 

 may continue for almost half an hour during the early stages of gastric 

 digestion. The temperature also influences the rapidity with which the 

 transformation of the starch is effected. At a temperature of 95 to io6F. 

 the ptyalin acts most energetically, while its activity is entirely arrested by 

 reducing the temperature to the freezing-point or raising it to the boiling 

 point. 



The Mode of Secretion of Saliva. The secretion of saliva is a com- 

 plex act and involves the cooperation of gland-cells, blood-vessels, efferent 

 and afferent nerves contained in different cranial nerves, and a central 

 mechanism by which they are excited to and coordinated in activity. The 

 central mechanism is located in the medulla oblongata in the gray matter 

 beneath the floor of the fourth ventricle and subject to cerebral and surface 

 stimulation. 



During the intervals' of mouth digestion the glands are practically at rest 

 as far as the discharge of saliva is concerned. The cells, however, are 

 actively engaged in absorbing from the surrounding lymph-spaces materials 

 derived from the blood, out of which they construct their characteristic con- 

 stituents. The blood-vessels possess that degree of dilatation necessary for 

 nutritive purposes. 



With the development of the sensation of hunger, the sight and the odor of 

 agreeable foods develop visual and olfactory sensations, which give rise to 

 psychic states more or less agreeable. With their development the entire 

 mechanism is excited to activity and a more or less abundant discharge of 

 saliva into the mouth frequently takes place. This is, therefore, known as a 

 psychic secretion. The fluid thus secreted has given rise to the expression 

 "watering of the mouth." 



With the introduction of food into the mouth and the stimulation of 

 afferent nerves (nerves of general sensibility and nerves of taste) and the 

 onset of mastication, the blood-vessels suddenly dilate, the blood-supply is 

 increased, and the gland-cells begin to discharge water, inorganic salts, and 



