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AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



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

 (Fig. 55, C, D). Evidently the granular material is used up 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 rest- 

 ing gland contain the ptyalin, or rather a preliminary material from which the 

 ptyalin is constructed 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 formation 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 an- 

 abolism is set up, leading to the formation of new protoplasmic 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 suppose that the clear substance during the 

 resting periods undergoes metabolic changes, whether of a katabolic or anabolic 

 character cannot 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, according 

 to Heidenhain, the cells are large and clear (Fig. 56), with flattened nuclei 



Fk;. 56.— Mucous gland : submaxillary of dog ; rest- Fio. 57.— Mucous jdaud: submaxillary of dog 

 ing stage. after eight hours' stimulation of the chorda tym- 



pani. 



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. 57) 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 



