SECRETION. 225 



Theories of the Action of Trophic and Secretory Fibres. — The way 

 in which the trophic fibres act has been briefly indicated. They may be sup- 

 posed to set up metabolic changes in the protoplasm of the cells, leading to the 

 formation of certain definite products, such as mucin or ptyalin. That sucli 

 changes do occur is abundantly shown by microscopic examination of the rest- 

 ing and the active gland, the details of which will be given presently. In 

 general these changes may be supposed to be katabolic in nature; that is, to 

 consist in a disassociation or breaking down of the complex living material 

 with the formation of the simpler and more stable organic constituents of the 

 secretion. There is evidence to show that these gland-cells during activity 

 form fresh material from the nourishment supplied by the blood; that 

 is, that anabolic or building-up processes occur along with the katabolic 

 changes. The latter are the more obvious and are the changes which are 

 usually associated with the action of the trophic nerve-fibres. It is possible, 

 also, that the anabolic or growth changes may be under the control of separate 

 fibres for which the name anabolic fibres would be appropriate. Satisfactory 

 proof of the existence of a separate set of anabolic fibres has not yet been 

 furnished. 



The method of action of the secretory fibres proper is difficult to under- 

 stand. At present the theories suggested are very speculative, and a detailed 

 account of them is scarcely appropriate in this place. Heidenhain's own view 

 may be mentioned, but it should be borne in mind that it is only an hy- 

 pothesis, the truth of which is far from being demonstrated. The theory starts 

 from the fact that no more water leaves the blood-capillaries than afterward 

 appears in the secretion ; that is, no matter how long the secretion continues, 

 the gland does not become ©edematous nor does the velocity of the lymph- 

 stream in the lymphatics of the gland increase. This being the case, we must 

 suppose that the stream of water is regulated by the secretion, that is, by the 

 activity of the gland-cells. If we suppose that some constituent 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 from the basement 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 filtra- 

 tion. The action of the secretory fibres proper consists in so altering the 

 structure of this limiting layer of the cells that it oilers less resistance to filtra- 

 tion ; consequently the water under tension in the cells escapes into the lumen, 

 and the osmotic pressure of its substance again starts up a stream of water 

 from capillaries to cells, which continues as long as the Qerve-stim illation is 

 effective. 



Vol. I.— 15 



