752 PHYSIOLOGY 



almost entirely to the inner margin of the cell. Activity is thus 

 associated certainly with a discharge of granules, and probably 

 with some increased building up of protoplasm. We may regard the act 

 of 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 



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FIG. 328. Sub-maxillary gland of rabbit. (SCHAFER after E. MTJLLER.) 



The cells, all serous, are in different functional states : a, a loaded cell ; 



&, a discharged cell ; c, a secretory canaliculus penetrating into a cell. 



from the sub-maxillary gland, the mucin is precipitated 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. 

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

 cursor 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. 328) 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 pre- 

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



