DIGESTION. 165 



Starch -= Soluble Starch = I Erythrodextrin - 



< 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: 



3 (C 6 H 10 5 ) + H 2 - C 12 H 22 O n + C 6 H 10 5 

 Starch + Water = Maltrose + Dextrin. 



The amylolytic or starch-changing action of saliva depends on 

 A the presence of an unorganized ferment or enzyme known as w/w. 

 \This enzyme is present in the secretion of each of the^HW^ 

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

 are reasons for believing that it partakes of the nature of a proteid. 

 It is a product in all probability of the katabolic activity of the secre- 

 tory cells. According to Latimer and Warren, ptyalin is a deriva- 

 tive of the zymogen, t 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 moder- 

 ately alkaline. Its activity is arrested either 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 question 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 1 06 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 Secretion. The secretion of the 

 saliva is a complex act and involves the cooperation of gland-cells, 

 blood-vessels, and nerves. During the intervals of mastication the 

 glands are practically at rest as far as the discharge of saliva is con- 

 cerned. The cells, however, are actively engaged in absorbing from 

 the surrounding lymph-spaces materials derived from the blood 

 from which they construct their characteristic contents. The blood- 



