226 APPLIED PHYSIOLOGY 



The ingredients of mixed human saliva are water, 

 mucin, ptyalin, a trace of globulin, and certain salts, of 

 which the chief are sodium chloride, sodium phosphate 

 (Na 2 HP0 4 ), along with earthy carbonates and phos- 

 phates, and a trace of sulphocyanide of potassium. 

 Its alkalinity is due to sodium phosphate, but there 

 is not a trace of sodium carbonate present.* The 

 production of gastric flatulence can therefore hardly 

 be due, as has been suggested by some, to the 

 liberation of C0 2 from a highly alkaline saliva by 

 the gastric juice. The alkalinity is greatest before 

 breakfast. Salivary calculi result from the separation 

 out of the lime-salts in the gland ducts. The meaning 

 of the presence of sulphocyanide of potassium of all 

 salts ! in the saliva has given rise to much speculation, 

 and attempts have been made without much success 

 to show that variations in its amount are of diagnostic 

 value in disease. It has also been suggested that it 

 may play the part of an antiseptic, but this is apparently 

 not the case,f and we are still really quite in the dark as 

 to its significance. 



The ptyalin of the saliva plays a considerable part 

 in the conversion of the starch of the food into dextrin s, 

 but its powers in this direction are chiefly exerted in the 

 stomach, where it remains active for a much longer time 

 than used to be supposed. It is most active in a neutral 

 medium, any degree of acidity being specially inimical to 



* See Chittenden and Richards, Amer. Journ. of PhysioL, 1898, 

 i. 461. 



t See Nicolas and Dubief, Journ. de PJiys. et PatJiol. Gen. 

 1899, i. 979. 



