756 



PHYSIOLOGY OF DIGESTION AND SECRETION. 



Fig. 291. — Mucous gland: submaxillary of dog; rest- 

 ing stage. 



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, 

 secon(i, 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. 291), 

 with flattened nuclei^ 

 placed well toward the 

 base of the cell. When 

 the gland is made to se- 

 crete the nuclei become 

 more spherical and lie 

 more toward the middle 

 of the cell, and the cells 

 themselves become dis- 

 tinctly smaller. After 

 prolonged secretion the 

 changes become more 



marked (Fig. 292) and, according to Heidenhain, some of the 

 mucous cells may break down completely. According to most 

 of the later observers, however, the mucous cells do not actually 

 disintegrate, but form again new material during the period of 



Fig. 292.— Mucous gland: submaxillary o< 

 dog after eight hours' stimulation of the chorda 

 tympani. 



