SALIVARY DIGESTION 55 



ticular interest (dry bread), caused the secretion of a large 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 dif- 

 ferentiate between the influence of physiological and psychical stimuli. 



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

 ously placed, as the result of experiment and observation, between 1000 

 and 1500 c.c., the exact amount depending, among other conditions, 

 upon the character of the food. 



The saliva of adults ordinarily has a weak, alkaline reaction to litmus, 

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

 fasting. The saliva of the newborn is generally neutral to litmus, 

 whereas that of infants, especially those breast-fed, is generally acid. 1 

 The alkalinity of saliva is due principally to di-sodium hydrogen phos- 

 phate (Na2HP04) and its average alkalinity may be said to be equiva- 

 lent 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 

 and containing only 0.5 per cent of solid matter. Among the solids are 

 found albumin, globulin, mucin, urea, the enzymes salivary amylase 

 (ptyalin), maltase, and pep tide-splitting enzymes, phosphates, and 

 other inorganic constituents. Potassium thiocyanate, KSCN, is also 

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

 stance 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 attempts to show some relationship between 

 tooth decay and the thiocyanate content of the saliva secreted into 

 the mouth cavity have met with failure. The most recent experiments 2 

 indicate a virtual absence of such relationship. 



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 calcium 

 salts are held in solution as acid salts, and are probably precipitated by 

 the ammonia of the breath. The various organic substances just men- 

 tioned are carried down in the precipitation of the calcium salts. 



The suggestion has been made that mucin is the salivary constituent 

 " which is particularly influential in the development of local conditions 

 favoring the onset of dental decay." 3 



The principal enzyme of the saliva is known as salivary amylase or 

 ptyalin. This is an amylolytic enzyme (see page 4) , so called because it 



1 Allaria: Monatsschr. fiir Kinder heilkunde, 10, 179, 191 1. 



2 Lothrop and Gies: Journal of the Allied (Dental] Societies, 6, 65, 1911. 



3 Id.: Ibid., 5, No. 4, 1910. 



