696 



PHYSIOLOGY OF DIGESTION AND SECRETION. 



fewer in number and are collected near the lumen and the margins 

 of the cells, the clear zone increases in extent, and the cells become 

 smaller (Fig. 266, C, D). Evidently the granular material is used 

 in some way to make the organic material of the secretion. Since the 

 ptyalin is a conspicuous organic constituent of the secretion, it is 

 assumed that the granules in the resting gland contain the ptyalin, 

 or rather the preliminary material from which the ptyalin is con- 

 structed during the act of secretion. On this latter assumption the 

 granules are frequently spoken of as zymogen granules. During the 

 act of secretion two distinct processes seem to be going on in the cell, 

 leaving out of consideration, for the moment, the secretion of the 

 water and the salts. In the first place, the zymogen granules undergo 

 a change such that they are forced or dissolved out of the cell, and, 

 second, a constructive metabolism or anabolism is set up, leading to 



the formation of new pro- 

 toplasmic material from 

 the substances contained 

 in the blood and lymph. 

 The new material thus 

 formed is the clear, non- 

 granular substance, 

 which appears first 

 toward the basal sides of 

 the cells. We may sup- 

 pose that the clear sub- 

 stance during the resting 

 periods undergoes meta- 

 bolic changes, whether of 

 a catabolic or anabolic 

 character can not be 



safely asserted, leading to the formation of new granules, and the 

 cells are again ready to form a secretion of normal composition. 

 It should be borne in mind that in these experiments the glands 

 were stimulated beyond normal limits. Under ordinary conditions 

 the cells are probably never depleted of their granular material to 

 the extent represented in the figures. 



In the cells of the mucous glands changes equally marked may 

 be observed after prolonged activity. In stained sections of the 

 resting gland the cells are large and clear (Fig. 267), with flattened 

 nuclei placed well toward the base of the cell. When the gland is 

 made to secrete the nuclei become more spherical and lie more 

 toward the middle of the cell, and the cells themselves become 

 distinctly smaller. After prolonged secretion the changes become 

 more marked (Fig. 268) and, according to Heidenhain, some of the 

 mucous cells may break down completely. According to most of 



Fig. 267. Mucous gland: submaxillary of dog; rest- 

 ing stage. 



