PHYSIOLOGICAL CHEMISTKY 373 



of maltase (see p. 403). There are several varieties of dextrin formed 

 during the hydrolysis, one of these gives the iodine reaction described 

 above, and is called erythro-dextrin ; another, called achroo-dextrin, 

 gives no reaction with iodine. The latter exists as an intermediate 

 stage between erythro-dextrin and maltose. 



The very first effect of ptyalin on starch is to convert it into so-called 

 soluble starch (sometimes called amylodextrin). This gives a clear 

 solution with water and a blue reaction with iodine. During each step 

 in the hydrolytic break down, a certain amount of maltose is set free. 

 This is small in amount at first, but becomes progressively more with 

 each successive dextrin formed. 



EXPERIMENT VII. Place some of 0-5 per cent, solution of starch in the 

 mouth, and after about two minutes transfer it to a test tube. Ascer- 

 tain if reduction of cupric salts occurs. Repeat this experiment with 

 some unboiled starch, and note the difference in the two cases (see 

 Carbohydrates, p. 290). 



The ptyalin will only act in neutral or very faintly alkaline reaction, 

 but not in the presence of free acid (e.g. 0*003 % HC1 can practically 

 stop its action). Stronger alkalinity destroys it. 



EXPERIMENT VIII. If Experiment VI. be repeated with the addition 

 of a few drops of 0*2 per cent, hydrochloric acid, so that the fluid just 

 reacts acid to litmus, it will be noticed that no dextrin is produced. (If 

 the acid mixture be heated for a considerable time a trace of reducing 

 sugar may appear because of the hydrolysing action of the acid.) 



From the result of Experiment VIII. we may conclude that it would 

 be impossible for the action of the ptyalin to proceed in the stomach 

 after the gastric contents had become distinctly acid. If the stomach be 

 empty at the beginning of the meal, however, the action of ptyalin 

 may proceed in this viscus for some considerable time, since the first 

 portion of acid which is secreted becomes bound to protein, so that it 

 does not exercise its inhibiting influence on the ptyalin which has been 

 swallowed. 



A certain amount of the hydrochloric acid secreted by the stomach 

 will also combine with the alkalies of saliva to form chlorides. These 

 chlorides have a marked accelerating influence on the action of saliva. 

 Although, therefore, ptyalin has little chance in the mouth to carry its 

 action on starch far, it can, nevertheless, continue acting for some con- 

 siderable time in the fundus of the stomach. The extent of this action 

 no doubt varies in different cases, being probably more prolonged when 

 the food is taken without much liquid. 



One of the chief functions of the saliva is undoubtedly a mechanical 

 one, acting as a solvent for certain foods, and assisting in the mastica- 



