As intimated, especially if the cells of the duct are still in situ, it is impossible to follow the fine nerve-fibres any further. By pressing out the cells from the ducts, one can obtain portion of the duct, with the capillaries belonging to them, but they are short pieces, and the nerve fibril is not easily discoverable.
But there remains one fact to be mentioned, which may throw light on the question, as to how the nerves end and which in my opinion might and does clear up the difficulty.
Examining the capillaries supplying the gland alveoli, one is able to find nuclei, not belonging to the capillaries themselves. This statement can be made for two reasons. In the first place, the majority of these nuclei have not the form of the nuclei of the capillary, and in the second place these nuclei are situated on the outside of the capillary wall. Whoever has examined the capillaries of the frog’s muscle knows that just such or the same nuclei belong to nerve-fibres, supplying the capillary wall. In the mammal of course these nuclei are smaller and not so conspicuous, nor can we procure pieces of capilliary of any length (the structures under examination here, not being in one plane.) Thus we cannot expect to have matters so plain here and easy of demonstration. After finding these nuclei, it was of course my desire to find if they were united by fibres. It must in this matter be kept in mind, that if there are such fibres present to which these nuclei belong, these may be expected to be in the most intimate relation with the capilliary wall, if one is to influence the other—and therefore not of ready and easy demonstration.
I have, however, succeeded in showing in a number of instances that such fibres exist. Figs. 10 and 11 show such a fibre uniting several nuclei. The shortness of these fragments makes a more perfect demonstration almost impossible.
If this observation is correct, we find on the capillaries the same (only finer) fibres provided with nuclei, which we have lost out of sight following the branching out of the nerves.
There is also some evidence of fine fibres in the membrana propria of the ducts, inasmuch as we find here nuclei along-side of the capillaries, these not being the nuclei of the membrane, but resembling altogether the nuclei of the nerve fibres.
Again, as stated above, I have not been able to find any such evidence of nerves in connection with the gland cells themselves. In case, however, such an arrangement or something similar existed, why should it have been impossible for me to find evidences of such as well here on the gland cells as on the capillaries; and should they not be more readily found in the gland than in the capillary, inasmuch as we have larger structures to deal with here in the gland.
The conclusion then regarding the nerve-endings to which my work has led me are these:
There were no special nerve-endings found in connection with the gland cells themselves. We find, however, upon the capillaries evidence of the same kind of fine nucleated fibrils, which we find to be present in the fine nerve twigs, as far as we can follow them in their course towards their final distribution. To explain the action of nerves on the process of secretion we would have to assume either that the nerves accompanying the capillaries also influence the gland cells, or that the glands are stimulated to activity by some changes in the condition of the capillary wall furnishing a greater amount of oxygen and nutritive fluids.
The results are partly negative, partly positive. The latter I wish to emphasize, believing it worthy of attention, as it may throw light on the physiology of the capillary wall, which seems to be altogether too much neglected. The negative I have offered, knowing well enough that such negative results must be accepted as final with much hesitation.