among men of genius, has unquestionably done more than any other man living to enlarge our knowledge of the minute structure of the brain, for we owe to him, besides invaluable researches, the invention of an entirely novel method of study, by which a few of the cells of the brain are marked out with the utmost distinctness by a deep deposit of color, while most of the cells and tissues of the brain are left translucent and lightly tinged. The finest ramifications of these cells can be followed in such preparations under the microscope, yet they have never been proved to unite with the ramifications of other cells. Another method is that which consists in treatment by chloride of gold, as long employed in histology for tracing the finest thread of nervous substance, yet with this also it has hitherto been impossible to demonstrate any actual continuity of cell with cell. There are, however, certain authorities who still uphold the older view. Thus Adam Sedgwick, guided by certain general theoretical considerations as to the laws of cell connection, expects to find the continuity hypothesis re-established. Recently Prof. Dogiel, of the Siberian University at Tomsk, has published an article in Russian, in which he apparently seeks to verify the same hypothesis by actual observation, but unfortunately his results are not yet fully accessible to me. The settling of the problem is beset with the greatest difficulties.
The physiological consequences of the theory of non-continuity reach very far. Thus, if the sensory fibers simply branch within the brain, then there must occur a leap from those fibers to the cells which are to send out the reflex response to the sensation. So in other cases there must be a leap from one cell to another. Perhaps the leap or transfer is comparable to an electric induction. But it is obviously useless to ramble into sheer speculation.
The Fourth Discovery.—The zones of His were vaguely recognized by Löwe, but to His belongs the honor of having first clearly recognized them and established their morphological importance. There are four zones of His—two on each side; they run the entire length of the brain and spinal cord, except that in the partially aborted end of the latter the zones are imperfectly developed. Each zone is a thickening of the wall of the medullary tube. We distinguish the dorsal and ventral zones. The dorsal zone was termed by His the Flügelplatte (wing plate) and the ventral zone the Grundplatte (basilar plate), but the new names proposed appear to me preferable. At an early stage of development the two zones are very clearly marked off from one another; but after a more advanced stage is reached, although they preserve their characteristic differences, their delimitation is far less conspicuous. They persist throughout life, and can be identified in the adult. Thus, for example, in the cerebral region proper, or,