322 A MANUAL OF PHYSIOLOGY 



of salts, is increased by the addition of very small amounts of 

 acids and of the neutral salts of strong monobasic acids. The 

 action is decreased by larger amounts of acid (0-0007 to 0-0012 

 per cent, of hydrochloric acid) and by neutral salts of weak acids. 

 An acidity equal to that of a O'l per cent, solution of hydrochloric 

 acid stops salivary digestion completely, although the ferment is 

 still for a time able to act when the acidity is sufficiently reduced. 

 Strong acids or alkalies permanently destroy it. These facts in- 

 dicate that in the mouth, where the reaction is weakly alkaline, 

 the conditions are comparatively favourable to the action of the 

 ptyalin. They are still more favourable in the stomach for some 

 time after the beginning of a meal, while the reaction is yet 

 weakly acid. It has been observed that (in cats) salivary digestion 

 may go on for an hour or more in the cardiac end of the stomach, 

 since free hydrochloric acid does not appear here before that time. 

 Since the contents of the cardiac end are not freely intermixed 

 with those of the pyloric end, a greater proportion of sugar is 

 found in the former, and the difference is more marked with solid 

 than with liquid food (Cannon and Day). But during the greater 

 part of gastric digestion the degree of acidity is such that the 

 ptyalin must be hindered. Although the food stays but a short 

 time in the mouth, there is no doubt that, in man at least, some 

 of the starch is there changed into sugar (p. 424). But this 

 is not the case in all animals. Something depends on the 

 amylolytic activity of the saliva, and something upon the form 

 in which the starchy food is taken, whether it is cooked or raw, 

 enclosed in vegetable fibres or exposed to free admixture with 

 the secretions of the mouth. 



The fact already mentioned that hydrolytic changes of the 

 same nature as those produced by enzymes can be brought about 

 in other ways holds good for ptyalin. If starch is heated for a time 

 with dilute hydrochloric or sulphuric acid, it is changed first into 

 dextrin, and then into a form of reducing sugar, which, however, 

 is not maltose, but dextrose. If maltose is treated with acid in 

 the same way, it is also changed into dextrose. When glycogen 

 (p. i) is boiled with dilute oxalic acid at a pressure of three 

 atmospheres, isomaltose and dextrose are formed (Cremer). 

 Facts will be cited later on which show that the action of the 

 other digestive ferments, as already mentioned, can also be 

 imitated by purely artificial means. Indeed, we may say that 

 the ferments accomplish at a comparatively low temperature 

 what can be done in the laboratory at a higher temperature, 

 and by the aid of what may be called more violent methods. 



Gastric Juice. The Abbe Spallanzani, although not, perhaps, 

 the first to recognise, was the first to study, systematically, the 

 chemical powers of the gastric juice, but it was by the careful 



