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.