268 THE TISSUES AND MECHANISMS OF DIGESTION. 



The Changes in a Gland constituting the Act of Secretion. 



203. We have now to consider what are the changes in the glandular 

 cells and their surroundings which cause this flow of fluid possessing specific 

 characters into the lumen of an alveolus, and so into a duct. It will be con- 

 venient to begin with the pancreas. 



The thin extended pancreas of a rabbit may, by means of special precau- 

 tions, be spread out on the stage of a microscope and examined with even 

 high powers, while the animal is not only alive but under such conditions 

 that the gland remains in a nearly normal state, capable of secreting vigor- 

 ously. It is possible under these circumstances to observe even minutely the 

 appearances presented by the gland when at rest and loaded, and to watch 

 the changes which take place during secretion. 



When the animal has not been digesting for some little time, and the gland 

 is therefore " loaded," the outlines of the individual cells are very indistinct, 

 the lumen of the alveolus is invisible or very inconspicuous, and each cell is 

 crowded with small refractive spherical granules, forming an irregular gran- 

 ular mass which hides the nucleus and leaves only a very narrow clear outer 

 zone next to the basement membrane, or it may be hardly any such zone at 

 all. (Fig. 82, A.) 



The blood-supply, moreover, is scanty, the small arteries being constricted 

 and the capillaries imperfectly filled with corpuscles. 



If. however, the same pancreas be examined while it is in a state of activity, 

 either from the presence of food in the stomach or from the injection of some 

 stimulating drug, such as pilocarpine, a very different state of things is seen. 

 The individual cells (Fig. 82, ^) have become smaller and much more dis- 



FIG. 82. 



B 



A Portion of the Pancreas of the Rabbit. (Kuhne and Sheridan Lea.) A at rest, B in a state 

 of activity, o the inner granular zone, in which A is larger, and more closely studded with fine 

 granules, than in B, in which the granules are fewer and coarser, b the outer transparent zone, 

 small in A, larger in B, and in the latter marked with faint striae, c the lumen, very obvious in 

 B, but indistinct in A. d an indentation at the junction of two cells, seen in B, but not occurring 

 in A. 



tinct in outline and the contour of the alveolus, which previously was even, 

 is now wavy, the basement membrane being indented at the junction of the 

 cells, also the lumen of the alveolus is now wider and more conspicuous. 

 In each cell the granules have become much fewer in number and, as it were, 

 have retreated to the inner margin, so that the inner granular zone is much 

 narrower and the outer transparent zone much broader than before ; the 

 latter, too, is frequently marked at its inner part by delicate striae running 

 into the inner zone. At the same time the bloodvessels are largely dilated 

 and the stream of blood through the capillaries is full and rapid. 



With care the change from the one state of things to the other may be 



