750 PHYSIOLOGY OF DIGESTION AND SECRETION. 



Composition of the Secretion. — The saliva as it is found in the- 

 mouth is a colorless or opalescent, turbid, and viscid liquid with 

 a specific gravity of about 1.003 and a neutral or slightly acid 

 reaction (pH = 6.6 to 7.1). It may contain numerous flat cells- 

 derived from the epithelium of the mouth, and the peculiar 

 spherical cells known as salivary corpuscles, which seem to be 

 altered leucocytes. The important constituents of the secretion 

 are mucin, a diastatic enzyme known as ptyahn, maltase, traces- 

 of protein and of potassium sulphocyanid, and inorganic salts, 

 such as potassium and sodium chlorid, potassium sulphate, 

 sodium carbonate, and calcium carbonate and phosphate. The 

 carbonates are particularly abundant in the saliva, and the 

 secretion, in addition, contains much carbon dioxid in solution. 

 Thus, Pfliiger found that 65 volumes per cent, of CO2 might be 

 obtained from the saliva, of which 42.5 per cent, was in the form 

 of carbonates. The amount of CO2 in solution and combined is 

 an indication of the active chemical changes occurring in the; 

 gland. 



Of the organic constituents of the saliva the protein exists in- 

 small and variable 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 proteins, glyco- 

 proteins (see Appendix), consisting of a protein combined with at 

 carbohydrate group. The most interesting constituent of the mixed, 

 saliva is the ptyalin or salivary diastase. This body belongs to the- 

 group of enzymes or unorganized ferments, whose general properties 

 have been described. In some animals (dog) ptyalin seems to be 

 normally absent from the fresh saliva. 



The secretions of the parotid and the submaxillary glands can ha 

 obtained separately by inserting a cannula into the openings of the 

 ducts in the mouth, or, according to the method of Pawlow, by trans- 

 ferring the end of the duct so that it opens upon the skin instead of 

 in the mouth, making thus a salivary fistula. The secretion 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. 



The Secretory Nerves. — The existence of secretory nerves to the 

 salivary glands was discovered by Ludwig in 1851. The discovery is 

 particularly interesting in that it marks the beginning of our knowl- 

 edge of this kind of nerve fiber. Ludwig found that stimulation of 

 the chorda tympani nerve causes a flow of saliva from the submaxil- 

 lary gland. He established also several importar'' ^'acts with regard. 



