760 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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. In some animals 

 (horse) it is stated that the ptyalin as secreted from the gland is in 

 an inactive or zymogen form. On coming into contact with the 

 mucous membrane of the mouth it is converted to active ptyahn 

 by an organic kinase, orokinase,^ said to be produced in the 

 small buccal glands found in the mucous membrane of the 

 cheek and lips. 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 

 C12H22O11, and some form of dextrin, a non-crystallizable poly- 

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

 (CgHijOg) 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- 



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

 1896. 



t See Palmer, et. al., "American Journal of Physiology," 43, 457, 1917. 



