SECRETION. 221 



Water, 99-1.203 



Solids : 



Mucin and epithelial cells, 2.202 



Ptyalin and albumin, 1.390 



Inorganic salts, 2.205 



5.797 



1.000.000 

 (Potassium sulphocyanide, 0.041.) 



Of the organic constituents of the saliva the proteid exists in small and varia- 

 ble quantities, and its exact nature is not determined. The mucin gives to the 

 saliva its ropy, mucilaginous character. This substance belongs to the group 

 of combined proteids, glyco-proteids (see section on Chemistry), consisting of a 

 proteid combined with a carbohydrate group. The physiological value of this 

 constituent seems to lie in its physical properties, as described in the section on 

 Digestion. The most interesting constituent of the mixed saliva is the pty- 

 alin. This body belongs to the group of enzymes or unorganized ferments, 

 whose general and specific properties are described in the section on Digestion. 

 It suffices here to say only that ptyalin belongs to the diastatic group of enzymes, 

 whose specific action consists in a conversion of the starches into sugar by a proc- 

 ess of hydrolysis. In some animals (dog) ptyalin seems to be normally absent 

 from the fresh saliva. An interesting fact with reference to the saliva is the 

 large quantity of gases, particularly 0O 2 , which may be obtained from it when 

 freshly secreted. In an analysis by Pfliiger of the saliva from the submaxil- 

 lary gland the following figures were obtained: C0 2 , 65 per cent., of which 

 42.5 per cent, was in the form of carbonates; N, 0.8 per cent. ; O, 0.6 per 

 cent. For the parotid secretion Kiilz reports: CG 2 , 66.7 per cent., (if which 

 62 per cent, was in combination as carbonate; N, 3.8 per cent. ; O, 1.46 per 

 cent. 



The secretions of the parotid and submaxillary glands can be obtained easily 

 by inserting a cannula into the openings of the duets in the mouth. The secre- 

 tion of the sublingual can only be obtained in sufficient quantities for analysis 

 from the lower animals. Examination of the separate secretions shows that the 

 main difference lies in the fact that the parotid saliva contains no mucin, while 

 that of the submaxillary and especially of the sublingual gland is rich in 

 mucin. The parotid saliva of man seems to be particularly rich in ptyalin as 

 compared with that of the submaxillary, while the secretion of the latter and 

 that of the sublingual gland give a stronger alkaline reaction than the parotid 

 saliva. 



The Secretory Nerves. — The existence of secretory nerves was discovered 

 by Ludwig in 1851. He found that stimulation of the chorda tympani nerve 

 caused a How of saliva from the submaxillary gland. He established also 

 several important facts with regard to the pressure and composition of the 

 secretion which will be referred to presently. It was afterward shown that, 

 the salivary glands receive a double nerve-supply, in pari by way of the 

 cervical sympathetic and in part through cerebral nerves, as briefly described 

 on p. 218. It was discovered also that not only arc secretory fibres carried 



