166 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



a more irritable condition, so that the few secretory fibres proper in the sym- 

 pathetic branches are now effective in producing a flow of water. 



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 such 

 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 oedematous 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, then, while the gland is in the resting state, water 

 will be absorbed 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 the osmotic tension comes to an equilibrium, and the diffu- 

 sion stream from blood to cells is at 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 fibres- 

 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 up a stream of water from capillaries to cells, which continues as 

 long as the nerve-stimulation is effective. 



