FOOD AND DIGESTION. 345 



ments of its own secretion, and which are stored up in the form of 

 granules in the cell during rest, the second stage consisting of the actual 

 discharge of these granules, with or without previous change. The 

 granules are zymogen granules, and represent the chief substance of the 

 salivary secretion, i.e., ptyalin. In the case of the submaxillary gland of 

 the dog, at any rate, the sympathetic nerve-fibres appear to have to do 

 with the first stage of the process, and when stimulated the protoplasm is 

 extremely active in manufacturing the granules, whereas the chorda 

 tympani is concerned in the production of the second act, the actual dis- 

 charge of the materials of secretion, together with a considerable amount 

 of fluid, the latter being an actual secretion by the protoplasm, as it 

 ceases to occur when atropin has been subcutaneously injected. 



In the mucus-secreting gland, the changes in the cells during secre- 

 tion have been already spoken of. They consist in the gradual secre- 



Fig. 839. Alveoli of true salivary gland. A, at rest; B, in the first stage of secretion ; C, after pro- 

 longed secretion. (Langley.) 



tion by the protoplasm of the cell of a substance called mucigen, which 

 is converted into mucin, and discharged on secretion into the canal of 

 the alveoli. The mucigen is, for the most part, collected into the inner 

 part of the cells during rest, pressing the nucleus and the small portion 

 of the protoplasm which remains, against the limiting membrane of the 

 alveoli. 



The process of secretion in the salivary glands is identical with that 

 of glands in general; the cells which line the ultimate branches of the 

 ducts being the agents by which the special constituents of the saliva 

 are formed. The materials which they have incorporated with them- 

 selves are almost at once given up again, in the form of a fluid (secre- 

 tion), which escapes from the ducts of the gland; and the cells, them- 

 selves, undergo disintegration again to be renewed, in the intervals of 

 the active exercise of the functions. The source whence the cells obtain 

 the materials of their secretion is the blood, or, to speak more accu- 

 rately, the plasma, which is filtered off from the circulating blood into 

 the interstices of the glands as of all living textures. 



