342 DIGESTION 



as dextrin; in dialyser digestions the residue of dextrin may be 

 little more than 4 per cent. This goes far to explain the complete 

 digestion of starch which takes place in the alimentary canal, a 

 digestion so exhaustive that, although dextrins may be found in 

 the stomach after a starchy meal, they do not occur in the intestine, 

 or only in minute traces. Here the amylolytic ferment of the 

 pancreatic juice, which is essentially the same in its action as the 

 amylase of saliva, only more powerful, must effect a very complete 

 conversion of the starch molecules accessible to its attack. It is 

 not inconsistent with this, that unchanged starch granules may 

 sometimes be excreted in the faeces, especially when imbedded in 

 raw vegetable structures. For it may be easily shown that un- 

 boiled starch is digested by amylase with far greater difficulty than 

 boiled starch, an illustration of the important part played by 

 cooking in the preparation of the food for digestion. 



It is a notable fact that amylases, also called diastases, are not 

 confined to the animal body, but are widely distributed in plants. 



The polysaccharide starch forms the great reserve of carbohydrate 

 material in plant nutrition, and is mobilized for the use of the 

 vegetable cells by being hydrolysed to simple sugars under the in- 

 fluence of these enzymes, just as the polysaccharide glycogen, the 

 great carbohydrate reserve of animal nutrition (p. 525), is mobilized 

 in the form of dextrose under the influence of the diastase of the 

 liver. A diastase, which is present in all sprouting seeds, and 

 may be readily extracted by water from malt, forms dextrin and 

 maltose from starch. The optimum temperature of malt diastase, 

 however, is about 55 C., while that of ptyalin is about 40 C. 



While a neutral or weakly alkaline reaction is not unfavourable 

 to salivary digestion, it goes on best in a slightly acid medium. 

 It has been shown that the activity of ptyalin on starch, both 

 having been previously dialysed to get rid as far as possible 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 t 0*0012 per cent, of hydrochloric 

 acid) and by neutral salts of weak acids. An acidity equal to that 

 of a o-i 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 indicate 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 hydro- 

 chloric acid does not appear here before that time. Since the con- 



