During the period of rest, i. e., no secretion flowing from the duct, and the gland being pale, the gland cells in the acini undergo a change which may be compared with that observed in the cells of the serous salivary glands. The division of the row of cells lining the acinus, into a central granular and outer clear zone, has already been mentioned.

Immediately after very active secretion, the central granular zone is reduced to a minimum, owing to the paucity of granules; and the outer zone occupies the greater part of the cell, the entire substance of which stains readily and looks like ordinary protoplasm. After rest, however, the granules reappear, and after the lapse of a short quiescent period, the inner granular zone has again encroached on the outer, owing to the accumulation of granules which, rapidly increasing, fill the greater part of the cells, and cause them to bulge inward and occlude the lumen of the gland. As digestion proceeds, the cells undergo a slight change in form, so that each individual cell is more distinctly seen, and its angles are retracted, giving a notched appearance to the margin of the acinus. The blood supply during this period is much increased, red arterial blood flowing from the veinlets of the gland. At the same time the granules are diminished in number, escaping at the free central margin of the cells into the lumen, toward which they appear to crowd, leaving the outer zone once more clear and free from granules, while the lumen of the saccule and of the ducts is filled with secretion.

Structural Changes In The Cells During Secretion 73One Saccule of the Pancreas of the Rabbit in different states of activity.

Fig 70. One Saccule of the Pancreas of the Rabbit in different states of activity.

A. After a period of rest, in which case the outlines of the cells are indistinct, and the inner zone, i. e., the part of the cells (a) next the lumen (c), is broad and filled with fine granules.

B. After the gland has poured out its secretion, when the cell outlines (a) are clearer, the granular zone (a) is smaller, and the clear outer zone is wider. {Kilhne and Lea).

The examination of a single cell shows that during the period of rest with a comparatively poor supply of blood, it receives its normal nutrition, which is accompanied by an accumulation of granules in the protoplasm next the free side of the cell. During secretion these granules are pushed out of the cell, and seem in some way to form the secretion.

It will be seen immediately that one of the most important functions of the pancreatic juice is the formation of peptone from proteid, which operation is carried out by a special ferment called trypsin. It has been found that this ferment can only be obtained from the active pancreas, and that the wider the inner granular zone of the cells is, the richer in ferment is the glycerine extract made from the gland. But it has also been found that if a glycerine extract be at once made from an actively secreting, absolutely fresh gland, i.e, removed from the dead animal while still warm, the extract is found to be quite inert toward proteids, while an extract made from a portion of the same pancreas which has been kept some hours after death is very active; and a portion of the fresh pancreas pounded in a mortar with a little weak acid so as to develop the trypsin acts in an alkaline solution and forms peptone energetically.

We must therefore conclude that the special proteolytic ferment of the pancreas does not exist prior to the period at which the secretion is poured out from the gland cells.

Although a definite relation seems to exist between the amount of granules in the active cells and the degree of efficacy of the secretion, the ferment does not appear in full force for some time after the height of the gland activity has been established, and it is likely that the presence of an acid helps in the birth of the ferment.

It has therefore been assumed that the granules of the gland cells give rise, not to the proteolytic ferment, but to a ferment-producing substance which is called Zymogen.

So that if we trace the history of the pancreatic proteolytic ferment, we shall find that, so far as this trypsin is concerned, there can be no question as to whether it preexists in the blood and is removed thence by the gland or not, because by studying the process the final elaboration of the secretion is seen to take place after it has got into the ducts or into the intestinal cavity. Thus the blood gives nutriment to the protoplasm of the gland cells. The protoplasm of the cells, by its intrinsic chemical processes, manufactures peculiar granules. These granules give rise, among other things, to zymogen, which in the presence of an acid begets trypsin.