690 PHYSIOLOGY OF DIGESTION AND SECRETION. 



intrinsic nerve ganglion cells of the glands (preganglionic fibers), and 

 from these latter cells axons (postganglionic fibers) are distributed 

 to the secreting cells, passing to these cells along the ducts. The 

 nerve fibers terminate in a plexus upon the membrana propria of the 

 alveoli, and from this plexus fine fibrils pass inward to end on and 

 between the secreting cells. It would seem from these observations 

 that the nerve fibrils do not penetrate or fuse with the gland cells, 

 as was formerly supposed, but form a terminal network in contact 

 with the cells, following thus the general schema for the connection 

 between nerve fibers and peripheral tissues. 



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

 mouth is a colorless or opalescent, turbid, and mucilaginous liquid of 

 weakly alkaline reaction and a specific gravity of about 1.003. 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 ptyalin, 

 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 CO 2 might be 

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

 carbonates. The amount of CO 2 in solution and combined is an 

 indication of the active chemical changes in the gland. 



Of the organic constituents of the saliva the protein exists iri 

 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 a 

 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 be 

 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 



