334 FOOD AND DIGESTION 



lymph certain materials from which it manufactures the elements of its own 

 secretion, and which are stored up in the form of granules in the cell during 

 rest; the second stage consists of the actual discharge of these granules, with 

 or without previous change. The granules are zymogen granules, and repre- 

 sent the chief substance of the salivary secretion, ptyalin. In the case of the 

 submaxillary gland of the dog, at any rate, the sympathetic nerve fibers ap- 

 pear 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 discharge from the cells of the materials of secretion, together with a 



FIG. 251. Alveoli of True Salivary Gland. A, At rest; B, in the first stage of secretion; 

 C, after prolonged secretion. (Langley.) 



considerable amount of fluid. The latter is an actual secretion by the 

 protoplasm, as it ceases to occur when atropine has been subcutaneously 

 injected. 



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

 have been already spoken of. They consist in the gradual production 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 muci- 

 gen 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 

 are the agents by which the special constituents of the saliva are formed. The 

 material which they have incorporated within themselves, which is doubtless 

 a product of the metabolism of the protoplasm of the cells, is given up again 

 almost at once in the form of a fluid, secretion, which escapes from the ducts 

 of the gland. The cells themselves undergo diminution in the mass of their 

 protoplasm, which is again renewed in the intervals of the active exercise of 

 the functions. The source whence the cells obtain the materials for the con- 

 struction of secretion is the blood plasma, which is filtered off from the circu- 

 lating blood into the interstices of the glands, as in all living tissues. 



