PHYSIOLOGY OF DIGESTION AND SECRETION. 



due to the hardening reagents used. A truer picture of what occurs 

 may be obtained from a study of sections of the fresh gland. Lang- 

 ley,* who first used this method, describes his results as follows: 

 When the animal is in a fasting condition the cells have a granular 

 appearance throughout their substance, the outlines of the different 

 cells being faintly marked by light lines (Fig. 290, A). When the 

 gland is made to secrete by giving the animal food, by injecting 

 pilocarpin, or by stimulating the sympathetic nerves, the granules 

 begin to disappear from the outer borders of the cells (Fig. 290, B), 



Fig. 290. Parotid gland of the rabbit in a fresh state, showing portions of the secret- 

 ing tubules: A, In a resting condition; B, after secretion caused by pilocarpin; C, after 

 stronger secretion, pilocarpin and stimulation of sympathetic; D, after long-continued 

 stimulation of sympathetic. (After Langley.) 



so that each cell now shows an outer, clear border and an inner 

 granular one. If the stimulation is continued the granules become 

 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. 290, 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 



* "Journal of Physiology," 2, 260, 1879. 



