CHAP, i.] DIGESTION. 275 



During activity (Fig. 54 B\ the cells become smaller, their outlines 

 more distinct, and the granules disappear especially from the outer 

 portions of each cell. After prolonged activity, as in Fig. 54 C, the 

 cells are still smaller with their outlines still more distinct, and the 

 granules have disappeared almost entirely, a few only being left at 

 the extreme inner margin of each cell abutting upon the conspicuous 

 almost gaping lumen of the alveolus. And upon special examina- 

 tion it is found that the nuclei are large and round. In fact we 

 might almost take the parotid as thus studied, to be more truly 

 typical of secretory changes than even the pancreas. For, as we 

 have already stated, the demarcation of an inner and outer zone is not 

 a necessary feature of the cell at rest. What is essential is that the 

 protoplasm manufactures granules, which for a while, that is during 

 rest, are deposited in the cell, and during activity these granules 

 are used up, their disappearance being earliest and most marked at 

 the outer portions of each cell, and progressing inwards towards the 

 lumen, the whole cell becoming smaller and as it were shrunken. 



It would hardly be profitable to enter more fully into the 

 discussion of this matter, and especially of the differences, to which 

 we have just called attention, as occurring in different glands; 

 enough has been seen to justify us in the conclusion, which 

 further study will be found to strengthen, that the act of secretion 

 is not a mere filtration from the blood but a complicated business, 

 which we may picture to ourselves somewhat as follows. 



The protoplasm of the secreting cell lives upon its 'internal 

 medium/ the lymph filling the lymph spaces by which the 

 alveolus is surrounded ; this lymph being constantly renewed from 

 the blood-stream. We have no reason to think that the main 

 nutritive constituents of the lymph in the interstices of a gland 

 are different from those in the interstices of a muscle ; but are led 

 to believe that the same substances are built up in the one 

 case into muscular and in the other into glandular protoplasm by 

 the specific activity of the already existing protoplasm which is 

 different in the one case and the other. The cell substance which 

 has thus built itself up out of the lymph materials sooner or later 

 breaks down again: the constructive metabolism is inevitably 

 followed by a destructive metabolism. In this downward path are 

 probably many steps, two of which become conspicuous: the 

 formation of some intermediate product or 'mesostate,' as we may 

 call it, such as zymogen or mucigen, and the conversion of the 

 zymogen into an actual ferment or of the mucigen into mucin, that 

 is of the mesostate into the final product, which is discharged as a 

 constituent of the secretion. 



In what we may consider the common or typical case where 

 periods of rest alternate with periods of secretory activity, the 

 downward metabolism stops short at the formation of zymogen, 

 which becomes deposited, commonly in the form of granules in the 

 meshes of the protoplasm, the constructive metabolism or growth 



18-2 



