CH. XXXII. ] COMPOSITION AND ACTION OF SALIVA 505 



parotid saliva is poorest in total solids (0'3 to 0*5 per cent.), and 

 contains no mucin. Mixed saliva contains in man an average of 

 about 0'5 per cent, of solids : it is alkaline in reaction, due to the salts 

 in it ; and has a specific gravity of 1002 to 1006. 



The solid constituents dissolved in saliva may be classified thus : 



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

 _ J b. Ptyalin : an amylolytic enzyme. 



1 c. Protein : of the nature of a globulin. 



V_ d. Potassium sulphocyanide. 



( e. Sodium chloride : the most abundant salt. 

 Inorganic . -| /. 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 substances 

 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 enzymes which are called 

 amylolytic (starch-splitting) or diastatic (resembling diastase, the 

 similar enzyme 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 equa- 

 tion : 



4nH 2 



[Water.] 



10(C 6 H 10 5 )n 2 



[Starch.] 



. 4nC 12 H 22 O 



[Maltose.] [Achroo-dextrin.] [Erythro-dextrin.] 



1222u (C 6 H 10 5 )_ + (C 6 H IO O 6 V 



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 swallowed saliva in the 

 stomach continues for a certain time. 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 proteins of the food, but when free acid appears ptyalin is de- 

 stroyed, and so it cannot resume work when the acid is neutralised 

 in the duodenum. Another amylolytic enzyme contained in pan- 



