THE SALIVARY GLANDS. 



675 



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 



D 



Fig. 257. 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 oy pilocarpin; C, after 

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

 stimulation of sympathetic. (After Langley.) 



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 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. 



