478 SALIVA. [CH. xxx. 



The solid constituents dissolved in saliva may be classified thus : 



!a. Mucin : this may be precipitated by acetic acid. 

 b. Ptyalin : an amylolytic ferment. 

 c. Proteid : of the nature of a globulin. 

 d. Potassium sulphocyanide. 

 | e. Sodium chloride : the most abundant salt. 



Inorganic . <f. Other salts: sodium carbonate, calcium phosphate and 

 ( carbonate ; magnesium phosphate ; potassium chloride. 



The action of saliva is twofold, physical and chemical. 



The physical use of saliva consists in moistening the mucous 

 membrane of the mouth, assisting the solution of soluble sub- 

 stances in the food, and in virtue of its mucin, lubricating the 

 bolus of food to facilitate swallowing. 



The chemical action of saliva is due to its active principle, 

 ptyalin. This substance belongs to the class of unorganised 

 ferments, and to that special class of unorganised ferments which 

 are called amylolytic (starch splitting) or diastatic (resembling 

 diastase, the similar ferment in germinating barley and other 

 grains). 



The starch is first split into dextrin and maltose ; the dextrin 

 is subsequently converted into maltose also : this occurs more 

 quickly with erythro-dextrin, which gives a red colour with 

 iodine, than with the other variety of dextrin called achroo-dextrin, 

 which gives no colour with iodine. Brown and Morris give the 

 following equation : 



io(C 6 H 10 5 ) n 



[starch] [water] 



= 4 nC 12 H, 2 O n + (C 6 H 10 5 ) n + (C 6 H 10 5 ) n 



[maltose] [achroo-dextrin] [erythro-dextrin] 



Ptyalin acts in a similar way, but more slowly on glycogen : it 

 has no action on cellulose ; hence it is inoperative on uncooked 

 starch grains, for in them the cellulose layers are intact. 



Ptyalin acts best at about the temperature of the body (35 

 40 C.). It acts best in a neutral medium; a small amount of 

 alkali makes but little difference ; a very small amount of acid 

 stops its activity. The conversion of starch into sugar by saliva 

 in the stomach continues for a short time (15 to 30 minutes). It 

 then ceases owing to the hydrochloric acid secreted by the glands 

 of the stomach. The acid which is first poured out neutralises 

 the saliva, and combines with the proteids of the food, but when 

 free acid appears ptyalin is destroyed, and so it cannot resume 

 work when the acid is neutralised in the duodenum. Another 

 amylolytic ferment contained in pancreatic juice (to be considered 

 later), however, digests starch in the intestine. 



