DIGESTION IN THE MOUTH 



671 



secretion as determined by the alteration of the granules and their discharge, 

 together with water and salts, to form the specific secretion of the gland. 

 During rest the granules are re-formed by precipitation in or modification 

 of the protoplasm surrounding the nucleus. We have evidence that although 

 the granules form the secretion, they represent, not the secretion itself, but 

 a precursor of some at any rate of its constituents. Thus if acetic acid be 

 added to the saliva obtained from the sub-maxillary gland, the mucin is pre- 

 cipitated as threads and films. If the granules in the secreting cells also 

 consist of mucin we should expect acetic acid to have a coagulating effect 

 upon them. We find, on the contrary, that on allowing acetic acid to flow 

 over a section of the fresh gland the granules at once swell up and burst. 



FIG. 328. Mucous cells from a fresh 

 subma xillary gland of a dog. (LANG- 



LEY.) 



a, mucous cell examined fresh from 

 a resting gland ; a', the same cell 

 treated with weak alcohol ; 6 and &', 

 cells from a discharged gland before 

 and after treatment with weak alcohol. 



FIG. 329. Acini of a serous salivary 



gland. (LANGLEY.) 



A. resting condition ; B, discharged 

 condition. 



We must regard these granules therefore, not as mucin, but as a precursor 

 of mucin, mucigen. The effect of ordinary hardening reagents, such as dilute 

 alcohol up to 70 per cent, or Miiller's fluid, is to cause these granules to swell 

 up so that the cells become filled with a mass of mucin giving the typical 

 hyaline appearance of ordinary sections of these glands. In the case of the 

 serous gland the granules (Fig. 330) are apparently protein in nature. Where 

 ptyalin is a constituent of the saliva we are probably justified in assuming 

 that it is contained either pre-formed or more probably as a precursor in 

 the granules. In the glands of the stomach we have evidence that the 

 granules are not pepsin the characteristic ferment of gastric juice but a 

 precursor of this substance, namely, pepsinogen. It is very customary 

 therefore to speak of the granules in a secreting gland as zymogen granules, 

 i.e. the precursors of zymins or ferments. It is probable that we ought to 

 regard these granules not merely as material precursors of the constituents of 

 the secretion, but as little machines or cell laboratories in which proceed a 

 whole series of chemical and osmotic changes which determine the production 

 of the fully formed secretion directly from the protoplasm and indirectly 

 from the ordinary constituents of the surrounding lymph. 



