THE CHEMISTRY OF DIGESTION 419 



change, like all digestive reactions, is a hydrolysis. It does not 

 occur in a single stage; that is, the starch molecule is not split 

 directly into maltose, but first into a dextrin which is hydrolyzed 

 into a simpler dextrin, and this in turn into maltose. In effecting 

 the change the ptyalin is not altered ; a very small amount of it can 

 convert a vast amount of starch, and does not seem to have its 

 activity impaired in the process. 



In order that the ptyalin may act upon starch certain conditions 

 are essential. Water must be present, and the liquid must be 

 neutral or feebly alkaline i j-cidfLjetardf or if stronger^ entirely 

 stop the process: The change takes place most quickly at about 

 the temperature of the Human Body, and is greatly checked by 

 cold. BoilingJJie_saliva,destroys its ptv_alin^and renders it quite 

 incapable of converting starch. Cooked starch is changed more 

 rapidly and completely than raw. 



It will be noted that salivary digestion is only a stage in the 

 preparation of starch for the use of the Body, since starch, in 

 common with the other carbohydrates taken as food, is finally 

 converted to single sugar before it is absorbed. 



The Gastric Juice. The food having entered the stomach is 

 subjected to the action of the gastric juice, which is a thin, color- 

 less or pale yellow liquid, of a strongly acid reaction. It contains 

 as specific elements free hydrochloric acid (about 0.2 per cent), and 

 an enzym called pepsin which, in acid liquids, has the power of 

 converting the ordinary proteins which we eat by hydrolysis, into 

 closely allied bodies, proteases and peptones. 



In neutral or alka^e, jgedia the pepsin is Jnacjbive; and cold 

 checks its activity. Boiling destroysTb7 In addition to pepsin, 

 gastric juice contains another enzym (rennin) which coagulates 

 the casein of milk, as illustrated by the use of " rennet," prepared 

 from the mucous membrane of the calf's digestive stomach, in 

 cheese-making. The acid of the natural gastric juice would, it is 

 true, precipitate the casein, but such precipitate is quite different 

 from the true tyrein, and neutralized gastric juice still possesses 

 this power; moreover, boiled gastric juice loses the milk-clotting 

 property, and a very little normal juice can coagulate a great 

 quantity of milk. The curdled condition of the milk regurgitated 

 by infants is, therefore, not any sign of a disordered state of the 

 Stomach, as nurses commonly suppose. It is proper for milk to 



