700 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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.* 



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 

 of the enzyme the starch molecules take up water and undergo 

 cleavage into simpler molecules. The steps in the process and the 

 final products have been investigated by a very large number of 

 workers, but much yet remains in doubt. The following points 

 seem to be determined: The end-result of the reaction is the 

 formation of maltose, a disaccharid, having the general formula 

 12 H 22 O U , and some form of dextrin, a non-crystallizable poly- 

 saccharid. When the digestion is effected in a vessel some dextrose 

 (C 6 H 12 O 6 ) may be found among the products, but this is explained on 

 the assumption that there is present in the saliva some maltase, an 

 enzyme capable of splitting maltose into dextrose. So far as the 

 ptyalin itself is concerned, its specific action is to convert starch to 

 maltose and dextrin. It seems very certain, however, that a number 

 of intermediate products are formed consisting of a variety of dex- 

 trins, so that the hydrolysis probably takes place in successive 

 stages. There is little agreement as to the exact nature of the in- 

 termediate dextrins. The following facts, however, may be easily 

 demonstrated in a salivary digestion carried on in a vessel and ex- 

 amined from time to time. The starch at first gives its deep-blue 



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

 1896. 



