SALIVA 309 



plasm is extremely active in manufacturing the granules, whereas the chorda 

 tympani is concerned in the production of the second act, the actual discharge 

 from the cells of the materials of secretion, together with a considerable 

 amount of fluid, the latter being an actual secretion by the protoplasm, as 

 it ceases to occur when atropine has been subcutaneously injected. 



In the mucus-secreting gland, the changes in the cells during secretion 

 have been already spoken of. They consist in the gradual production by the 

 protoplasm of the cell of a substance called mucigen, which is converted into 

 mucin, and discharged on secretion into the canal of the alveoli. The mucigen 

 is, for the most part, collected into the inner part of the cells during rest, 

 pressing the nucleus and the small portion of the protoplasm which remains, 

 against the limiting membrane of the alveoli. 



The process of secretion in the salivary glands is identical with that of 

 glands in general. The cells which line the ultimate branches of the ducts 

 are the agents by which the special constituents of the saliva are formed. The 

 material which they have incorporated within themselves, which is doubtless 

 a product of the metabolism of the protoplasm of the cells, is given up again 

 almost at once in the form of a fluid, secretion, which escapes from the ducts 

 of the gland. The cells themselves undergo diminution in the mass of their 

 protoplasm, which is again renewed in the intervals of the active exercise .of 

 the functions. The source whence the cells obtain the materials for the con- 

 struction of secretion is the blood-plasma, which is filtered off from the circu- 

 lating blood into the interstices of the glands, as in all living tissues. 



Saliva. Saliva, as it commonly flows from the mouth, is the mixed 

 secretion of the salivary glands proper and of the glands of the buccal mucous 

 membrane and tongue. When obtained from parotid ducts, and free from 

 mucus, saliva is a transparent watery fluid, the specific gravity of which varies 

 from 1004 to 1008, and in which, when examined with the microscope, are 

 found floating a number of minute particles, derived from the secreting ducts 

 and vesicles of the glands. In the impure or mixed saliva are found,' besides 

 these particles, numerous epithelial scales separated from the surface of the 

 mucous membrane of the mouth and tongue, and the so-called salivary cor- 

 puscles, discharged probably from the mucous glands of the mouth and the 

 tonsils, which subside when the saliva is collected in a deep vessel and left at 

 rest. They form a white opaque sediment leaving the supernatant fluid trans- 

 parent and colorless, or with a pale bluish-gray tint. Saliva also contains 

 various kinds of micro-organisms (bacteria). The saliva, when first secreted, 

 appears to be always alkaline in reaction; the alkalinity is about equal to .08 

 per cent of sodium carbonate, and is due to the presence of disodium 

 phosphate, Na 2 HPO 4 . 



The mucin is the largest representative of the organic nitrogenous class 

 of bodies in the saliva. It may be thrown down by addition of acetic acid. 

 It gives the three chief proteid reactions, and may easily be split up by the 



