If one were to ask the physiologist: What do you expect us to find in our anatomical investigations on the nerve-endings in the submaxillary gland, he would undoubtedly answer: “You will find an arrangement analogous, if not similar, to that in the striped muscle; the nerve-fibre will be seen to approach the alveolus, penetrate its covering, and the protoplasm of the nerve and the gland-cell will be found to come in contact, if not to coalesce.” And that is what has been found by Pflüger.
It has been my endeavor for some time to find something similar to the descriptions of Pflüger, but so far in vain, although many an alveolus has been carefully examined, flattened out and slowly crushed, so that each nucleus could be diagnosed, but never have I found anything which would remind me of the drawings by Pflüger in Stricker’s ‘Histological Handbook.’
In this matter I reasoned thus: If there are any such structures as specialized nerve-endings present in the glandular elements, we might look for an analogy either to the striped muscle fibres or to the capillaries, in both of which nerve-endings are known to be present. In the former case we have the so-called end-plate—the nerve fibre ends in a multi-nucleated mass—but as the muscle fibre, standing for a number of cells, is so different a structure from the gland-cells, we might, with greater probability, expect some such arrangement as we see on the capillaries. Here we see fine fibrils, provided with nuclei, exceeding the fibres two to four times in thickness, entwining the capillary, thus coming in contact with each of the cells composing the capillary wall. In the (submaxillary) gland there are similar structures to be influenced; at least in the capillary as well as in the gland, we have cells lying side by side to be influenced by nerves—hence we might expect similar structures.
Although the books in general have very little to say about the nerve supply of the capillaries, yet it is a fact, and can well be demonstrated, e. g., in the frog’s muscle, that such is the arrangement. As a rule, the fibril cannot so readily be detected, but the nuclei belonging to it are not difficult to recognize. A still more convenient and instructive locality is the base of the frog’s mouth, where everything is one plane, and where the nerve fibres can be followed to their ultimate ramifications.
Let us now see what evidence of nerve-endings we find in or on the alveoli of the gland, examining one, or more accurately, a part of one (the round eminence). We shall see what fig. 1 presents. We see there two sets of nuclei; one kind oval, longish, stained lighter than the rest, of a pink hue (fig. 1, m), and these a further examination will demonstrate to be the nuclei belonging to the membrana propria, which have also been described before. Besides these nuclei, fewer in number, there is another kind, making up the bulk of the nuclei, more rounded in form (fig. 1, g), and of a deep-red color, which, in their turn, will prove to be the nuclei of the gland-cells; and these two are the only elements or structures that I have been able to demonstrate in a glandular alveolus.
At one time I thought I had found in or on the alveoli the evidence of a network of fine nerves. Quite frequently one can observe longish, even spindle-shaped, often deeply stained, nuclei, which resemble very much those belonging to the fine nerve fibres. (Fig. 2, n, such nuclei are figured.) But so far, whenever I examined such a spindle-shaped nucleus more carefully, I found that it proved to be one of the nuclei belonging to the membrana propria, already described, and the form, as well as its dark color, was due to the fact that it had been examined edgewise. If, by manipulation, such a nucleus was turned over and compressed to present a flat surface, it would have an oval outline and be of a light-red color. Fig. 3 (a, b, c), shows the same nucleus as it appeared when seen edgewise, when seen from the flat and when seen in a position midway between these. Thus is also to be explained the demilune: If the section would strike a nucleus so that it would be seen only edgewise—especially if the membrana propria has a chance to roll up slightly—we should then have a deeply stained body, somewhat half-moon-shaped, the demilunes of the books.
So far, nothing but the nuclei have been mentioned, and the observation might very appropriately be made, that there might exist some exceedingly fine and delicate nerve fibrils without nuclei. Such a system of structures would, of course, be almost impossible to demonstrate with any degree of certainty. I have not, of course, sufficient reason to deny such an arrangement, and would not consider it improbable if we had not, in the capillaries of the frog’s muscle (and other structures) evidence certainly not in favor of such an assumption. The fine nerve fibrils supplying the capillaries are richly supplied with nuclei, although the cells which build up the walls of the capillary are by far more insignificant elements than the secreting cells of the gland. Certain it is that as yet there is no evidence of the existence of such fine non-nucleated fibrils, arranged either in the form of a network or a bundle of fine branches.
The examination of the alveoli not leading to any results as to nerve-endings—no structures being discovered there that might be called nervous—I attacked the problem from the other side. I took up the nerve trunks accompanying the ducts, and tried how far I could follow their divisions and branches, and can say that I have spent a great deal of time on this point. In speakings of the bloodvessels, the fact was mentioned that, where arteries and veins of any size run along-side of the duct, these are accompanied by nerve branches, i. e., bundles of nerve fibres, inclosed in a sheath, and these—at least in the cat, the calf and the ox—are made up (excepting, possibly, the very large trunks) of fine nerve fibres. Thicker or medullary nerves I have not been able to discover, even in branches of considerable size, consisting of, say six or eight fibres, and giving off smaller twigs containing one to three fibres. Fig. 7 shows such a nerve-trunk with side branches; neither in the main stem nor in the offshoots are there any medullary nerves discoverable.
If this observation is correct, we see that what has to be traced further, is not the more easily recognized medullary fibre, but exceedingly fine fibrils carrying in certain intervals nuclei, the only distinguishing feature. It will be admitted also (first) that it must be a matter of considerable difficulty to follow such indifferent looking structures, as delicate fibres. And (secondly), one cannot expect to find medullary nerves approach the alveoli, penetrate the membrane, come in connection with the secretory cells, etc., etc, as has been done by Pflüger. If the question should be asked how far I have been able to follow these nerves, I should say: Small twigs consisting of one to three fine fibrils, enclosed in a sheath branch off from a trunk, not much larger, and apply themselves to the membrana propria of the duct; their sheaths in all probability coalescing with that membrane. Here they have been seen to apply themselves to some bloodvessel and are now lost out of sight, being hid by all the many structures mentioned above, the nuclei of membrane, capillary, duct cells, and the fibre cannot so readily be distinguished from the membrana propria of the duct upon which it rests. But I do not wish to be understood that because the nerves have been lost out of sight, along-side of vessels, that they have their “ending” there, inasmuch as the network of bloodvessels is such a close one that the nerves cannot but help coming in contact with them. Fig. 8 shows such a fine nerve-trunk. By the aid of aniline blue the fine fibrils have been made quite plain. Such fine fibrils can by the same means be demonstrated in much larger trunks, as shown in fig. 7, and it can be shown that even such large trunks consist of nothing but these fine fibrils.
I have also seen, exceptionally however, fine nerve twigs run into the alveoli directly, but the nerves here were of the same nature as the ones described going to the duct. Fig. 9 shows a fine fibril passing on to an alveolus and running probably on to a vessel; the nerve fibre resting upon gland cells, also stained, could not with absolute certainty be traced to the vessel. There is a small break shown in the drawing.