Up to the present time it was the observer himself who pourtrayed, by means of his pencil, the objects which passed through the focus of his microscope. And, accordingly, we all know how widely these nominal drawings—even those made by masters of their pro fession—usually diverge from the truth ; simply because they can never express more than those details which the artist has perceived and recognized, and a species of unconscious selection from the objects which are passing before his eyes. It is, then, in presence of these deside rata, as regards graphic representation, in drawings made by hand that we feel the necessity of applying the marvellous resources now offered us by photography to the reproduction of microscopic objects.
The sensitized plate henceforward plays its part in the world of scientific investigation, in the study of the phenomena that occur in the world of the infinitely little, as well as in the study of those that occur in the world of the infinitely great—registering histological facts as well as astronomical phenomena, and thus becoming the impersonal and automatic pourtrayer of the most minute details that have impressed themselves upon it. Thus, wonderful to relate, photography, very much superior to drawing, not only reveals the objects which the eye perceives, but brings to light in addition a whole series of latent details, which await but the intervention of a simple lens to be successively recog nized upon the prints when obtained.
These new means of investigation, which the scien tific methods of the nineteenth century have placed within the reach of our generation, will, therefore, explain the progress accomplished, and show us once more that, in the long process of evolution which extends through ages, man only arrives step by step at the fragments of truth which he snatches, and that even his most persevering efforts only serve to cause the unknown to recede a few paces backwards. It is strange to find that, as fast as any progress is accomplished and new discoveries registered, new problems incessantly start up ; and that just when we thought we had arrived at the utmost limits of the. known world, at the demon stration of elements, simple, fixed, definite, our perfected methods of study enable us to see new complexities and unexpected horizons.
Thus, for instance, by means of high powers, the histological elements of the nerve-cell, hitherto con sidered as the primordial and irreducible units of the system, become themselves divisible into secondary elements.
Photo-chemical histology, indeed, shows us that the protoplasm of the cell, formerly described as a homo geneous substance, is arranged in a fibrillary trellis-work ; that its nucleus presents an arrangement of radiated fibres ; and that what was thought to be the nucleolus is itself a complex element. The nerve-cell thus becomes in its turn a little nervous organ sui generis. (See Fig. 2.) The same analytic processes enable us, moreover, to demonstrate that the network, so dense and compact, which unites all the nerve-cells of the cerebral cortex, for instance, one with another, is so delicate that, when enlarged to 235 diameters, the fibres of which it is composed become visible, like single hairs in appearance and magnitude, etc.
What will be the end of these unforeseen details which present themselves in the train of each adap tation of a new method of study, to our researches into the nervous system ? No one knows as yet. It seems as though the secrets of nervous organization escape from our eyes as fast as we press further into the regions where they conceal themselves, and while anticipating the new methods of analysis which the future holds in reserve, we cannot help thinking that there is still much to do, and that now, more than ever, we should remenrber that true saying of Serres : " We have been dissecting the brain since Galen's time, yet there is not an anatomist who has not left his successors something t o do." The labours of which I am about to give a rifsinne, are, then, but one of the phases of this long discussion concerning the structure of the nervous centres which has been going on for centuries.
If they do not establish the truth absolutely and finally, they will at least have the merit of being the result of contemporary science, and a sort of synthesis of the methods of work at our disposal.
The method I have employed for studying the organization of the cerebrospinal centre in man, I have already explained in my first work.* It essentially consists in the preparation of a series of sections made methodically, millimetre by millimetre, vertically, hori zontally, and antero-posteriorly ; and—these sections being thus made according to the three dimensions of the solid mass which was to be studied—in reproducing them all photographically.