DIGESTION. 235 



viz., (1) the true salivary, or (2) the mucous type. In the former case, 

 it has been noticed, as has been already described (p. 228), that during 

 the rest which follows an active secretion the lumen of the alveoli be- 

 comes smaller', the gland cells larger, and very granular. During secre- 

 tion the alveoli and their cells become smaller, and the granular appear- 

 ance in the latter to a considerable extent disappears, and at the end of 

 secretion, the granules are confined to the inner part of the cell nearest 

 to the lumen, which is now quite distinct (Fig. 171). 



It is supposed from these appearances that the first stage in the act of 

 secretion consists in the protoplasm of the salivary cell taking up from 

 the 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 consisting of the actual discharge of 



c 



J\. r> \j 



FIG. 171. Alveoli of true salivary gland. A, at rest; B, in the first stage of secretion; C, after 

 prolonged secretion. (Langley.) 



these granules, with or without previous change. The granules are taken 

 to represent the chief substance of the salivary secretion, i.e., the ferment 

 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 discharge 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 mucous-secreting gland, the changes in the cells during secre- 

 tion have been already spoken of (p. 228). They consist in the gradual 

 secretion 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. 



