82 PHYSIOLOGY CHAP. 



given. There are marked variations in the different species of 

 animals on which observations have been made. While the horse 

 secretes 14*2 grrns. saliva to every gramme of gland per hour, 

 during mastication, the calf only secretes 8 grms. (Tuczek). From 

 this point of view it seems probable that the salivary glands are 

 the most active. In man the secretion of saliva amounts to some 

 1500 grms. per diem. 



The organic components of mixed saliva are : 



(a) Mucin, which precipitates with acetic acid or alcohol, and 

 is derived from the mucinogen of the glandular epithelium. 



(b) Ptyalin, an enzyme discovered by Leuchs in 1831. It is 

 derived from the ptyalogen of the gland-cells, and is constant in 

 the saliva of man, horse, rabbit, and of herbivora in general, while 

 it is regularly absent in that of dogs and of carnivora in general 

 (Hoppe-Seyler). It is an amylolytic enzyme, the action of which 

 will be discussed in treating of buccal digestion. 



The existence of a pro-enzyme of ptyalin (ptyalinogeri) corre- 

 sponding to what exists for the gastric and pancreatic enzymes 

 (propepsin and protrypsiri), was demonstrated by Miss Latimer. 

 On washing the salivary glands repeatedly with water and chloro- 

 form, they are freed from the active ptyalin which they contain ; 

 on then treating the gland with a dilute solution of acetic acid an 

 extract capable of saccharifying starch is obtained. 



(c) A globulin that precipitates with heat, on addition of 

 mineral acids and also on passing a current of carbonic acid. 



(d) Sulphocyanide of potassium or sodium, which is frequent 

 but not constant in human saliva in minute quantities of 0'016- 

 0'084 per thousand (Oehl), according to others in an average 

 quantity of 010 per thousand (Jacubowitsch). It may possibly 

 be formed in the mouth by the action of special microbes. 

 According to Kriiger it increases in smokers. 



Grober's latest investigations (1901) show that the sulpho- 

 cyanide is not formed by decomposition of saliva ; its elimination 

 probably depends on the general protein metabolism, since it is 

 eliminated little or not at all by persons suffering from cachexia. 



(e) Traces of urea, and, in abnormal conditions, of leucine and 

 of lactic acid. 



The inorganic compounds consist of small quantities of chlorine 

 and phosphoric acid combined with potash, soda, lime, and 

 magnesia ; small quantities of sodium carbonate and abundant 

 quantities of sodium chloride. 



The amount both of water and of solids in the saliva may 

 fluctuate considerably with food or abstinence, or other changeable 

 factors. We must confine ourselves to citing the results of 

 Hammerbacher's analyses, which are of the mixed saliva (1000 

 parts) of a young healthy man, and which agree perfectly with 

 those of Frerichs : 



