DIGESTION. 149 



/ Achroodextrin. 



Starch-Soluble Starch- " Maltose. 



This change consists in the assumption by the starch of a molecule of water, 

 and for this reason the process is termed hydrolysis. The nature of the 

 chemic change is shown in the following formula: 



6105 3 - I , 2an 6 



Starch + Water - Maltose + Dextrin. 



The amylolytic 1 , amyloclastic, or starch-changing action of saliva depends 

 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 106 F. 

 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 Nerve Mechanism of the Secretion of Saliva. The secretion of 

 saliva is a complex 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. 



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- 



1 The term amylolytic has been objected to on the ground that it does not correctly express 

 the fact, but is analogous with electrolytic and means a transformation by means of starch. 

 Armstrong has suggested the use of the term amyloclastic as well as proteoclastic and lipoclastic 

 for the terms now generally employed. 



