674 PHYSIOLOGY OF DIGESTION AND SECRETION. 



stituent of these cells has an attraction for water, or, to use the 

 modern expression, exerts a high osmotic pressure, then, while the 

 gland is in the resting state, water will diffuse first from the base- 

 ment membrane, this in turn supplies its loss from the surrounding 

 lymph, and the lymph obtains the same amount of water from the 

 blood. As the amount of water in the cell increases a point is reached 

 at which an equilibrium is established, and the osmotic stream from 

 blood to cells comes to a standstill. The water in the cells does not 

 escape into the lumen of the tubule or of the secretion capillaries, 

 because the periphery of the cell is modified to form a layer offering 

 considerable resistance to filtration. The action of the secretory 

 fibers proper consists in so altering the structure of this limiting layer 

 of the cells that it offers less resistance to filtration; consequently 

 the water under tension in the cells escapes into the lumen, and the 

 osmotic pressure of its substance again starts a stream of water 

 from capillaries to cells, which continues as long as the nerve-stimu- 

 lation is effective. 



Histological Changes during Activity. The cells of both the 

 albuminous and mucous glands undergo distinct histological changes 

 in consequence of prolonged activity, and these changes may be 

 recognized both in preparations from the fresh gland and in preserved 

 specimens. In the parotid gland Heidenhain studied the changes in 

 stained sections after hardening in alcohol. In the resting gland 

 the cells are compactly filled with granules that stain readily and are 

 imbedded in a clear ground substance that does not stain. The 

 nucleus is small and more or less irregular in outline. After stimula- 

 tion of the tympanic nerve the cells show but little alteration, but 

 stimulation of the sympathetic produces a marked change. The 

 cells become smaller, the nuclei more rounded, and the granules 

 more closely packed. This last appearance seems, however, to be 

 due to the hardening reagents used. A truer picture of what occurs 

 may be obtained from a study of sections of the fresh gland. Lang- 

 ley,* who first used this method, describes his results as follows: 

 When the animal is in a fasting condition the cells have a granular 

 appearance throughout their substance, the outlines of the different 

 cells being faintly marked by light lines (Fig. 257, A). When the 

 gland is made to secrete by giving the animal food, by injecting 

 pilocarpin, or by stimulating the sympathetic nerves, the granules 

 begin to disappear from the outer borders of the cells (Fig. 257, B), 

 so that each cell now shows an outer, clear border and an inner, 

 granular one. If the stimulation is continued the granules become 

 fewer in number and are collected near the lumen and the margins of 

 the cells, the clear zone increases in extent, and the cells become 

 smaller (Fig. 257, C, D). Evidently the granular material is used 

 * "Journal of Physiology," 2, 260, 1879. 



