54 PHYSIOLOGICAL CHEMISTRY. 



amount of watery saliva, while the presentation of moist food, 

 which was eagerly desired by the animal, called forth a much 

 smaller secretion, slimy in character. These experiments show it 

 to be rather difficult to differentiate between the influence of physio- 

 logical and psychical stimuli. 



The amount of saliva secreted by an adult in 24 hours has been 

 variously placed, as the result of experiment and observation, be- 

 tween 1000 and 1500 c.c., the exact amount depending, among other 

 conditions, upon the character of the food. 



The saliva ordinarily has a weak, alkaline reaction to litmus, but 

 becomes acid, in some instances, 2-3 hours after a meal or during 

 fasting. The alkalinity is due principally to di-sodium hydrogen 

 phosphate (Na 2 HPO 4 ) and its average alkalinity may be said to 

 be equivalent to 0.08-0.1 per cent sodium carbonate. The saliva 

 is the most dilute of all the digestive secretions, having an average 

 specific gravity of 1.005 an d containing only 0.5 per cent of solid 

 matter. Among the solids are found albumin, globulin, mucin, urea, 

 the enzymes, salivary amylase (ptyalin) and maltase, phosphates 

 and other inorganic constituents. Potassium thiocyanate, KSCN, is 

 also generally present in the saliva. It has been claimed that this 

 substance is present in greatest amount in the saliva of habitual 

 smokers. The significance of thiocyanate in the saliva is not known; 

 it probably comes from the ingested thiocyanates and from the 

 breaking down of protein material. 



The so-called tartar formation on the teeth is composed almost 

 entirely of calcium phosphate with some calcium carbonate, mucin, 

 epithelial cells and organic debris derived from the food. The cal- 

 cium salts are held in solution as acid salts, and are probably pre- 

 cipitated by the ammonia of the breath. The various organic sub- 

 stances just mentioned are carried down in the precipitation of the 

 .calcium salts. 



The principal enzyme of the saliva is known as salivary amylase 

 or ptyalin. This is an amylolytic enzyme (see p. 3), so-called 

 because it possesses the property of transforming complex carbo- 

 hydrates such as starch and dextrin into simpler bodies. The 

 action of salivary amylase is one of hydrolysis and through this 

 action a series of simpler bodies are formed from the complex 

 starch. The first product of the action of the ptyalin of the saliva 

 upon starch paste is soluble starch (amidulin) and its forma- 

 tion is indicated by the disappearance of the opalescence of the 

 starch solution. This body resembles true starch in giving a 



