I set myself, then, to make a series of successive horizontal sections of the brain, previously hardened in a chromic acid solution, from apex to base, at intervals of about one millimetre, and as perfect as possible ; each being in its turn reproduced by photography.
I made similar sections of the brain in a vertical and antero-posterior direction, and at regular intervals from behind forwards.
These operations having been thus regularly con ducted, this method enabled me to have representations of the reality as exact as possible ; to keep the natural relations of the most delicate portions of the nervous centres each by each, according to their normal connec tions, and, in fact, without deranging anything. Thus by comparing the sections, horizontal or vertical, one with another, I could follow a given order of nerve fibres in its progress, see its point of origin, and its point of termination ; study the natural increase in com plexity of the different kinds of nerve fibrils, millimetre by millimetre, changing nothing, lacerating nothing, and leaving everything pretty much in its normal position.
By means of these new photographic methods of reproduction, which are all the more precise because impersonal, I had only, then, to register the details the sun himself had printed, to place the prints in juxta position, to compare them one with another, and thus to make a single synthesis of the multiple elements of analysis I had thus obtained by the automatic co-opera tion of the light.
The general view of cerebral topography having thus been fixed by these processes, the regions of more delicate texture, the special points which it was necessary to study in their minute elements, were further suffi ciently magnified and reproduced, with successively in creasing powers. I could thus render visible to the naked eye, and exhibit on a plan, details of structure which, up to that time, had only been seen in isolation under the tube of the microscope. By this means the mind of the observer, penetrating successively from the known to the unknown, from well-defined regions to those which are not so as yet, can easily make itself familiar with the details of the minute structure of the final nerve elements.
The cerebrospinal system in man and the vertebrates consists of three departments, independent one of another, and yet very intimately connected. These are : I. The cerebrum proper.
2. The cerebellum and the apparatuses of cerebellar innervation annexed thereto.
3. The medulla spinalis and its encephalic expansions. In this study we shall occupy ourselves with the cerebrum proper only.
The cerebrum consists of two lobes or hemispheres united to one another by a series of white transverse fibres, which form an anastomosis between the homo logous regions of each lobe, so as to constitute a twin system, of which all the molecules are consonant one with another.
Each cerebral lobe, taken alone, presents for considera tion in its turn : I. Masses of grey matter.
2. Agglomerations of white fibres.
The masses of grey matter, which are composed of many myriads of cells, and are the essentially active regions of the system, are arranged at the periphery in the form of a thin, undulating, continuous layer, which constitutes the cerebral cortex ; and in the central regions in the form of two grey ganglions, coupled together, which are simply the grey substance of the optic thalami and corpora striata (opto-striate ganglions).
The white substance, essentially composed of nerve tubules in juxtaposition, occupies the spaces comprised between the cortical periphery and the central ganglions.
The fibres of which it consists, and which merely represent lines of union between such and such regions of the cortical periphery and such and such regions of the central ganglions, run, like a series of electric wires stretched between two stations, in two principal direc tions.
1. Some directly unite the different points of the cortical periphery with the central ganglions, and are lost in their mass.
These are like the spokes of a wheel which unite its circumference to the central nave, which serves as their point of support. We may therefore describe them under the name of converging fibres.
2. The others, on the contrary, have a transverse direc tion. They proceed from one hemisphere to the other, thus uniting the homologous regions of the brain, right and left.
It may therefore be said that they serve as an anas tamosis and commissure between these homologous regions, and that they are thus the agents whick pro duce unity of action between the two cerebral hemi spheres. This order of fibres, by reason of its origin and connections, may legitimately be designated by the name of commissural fibres.
These data being admitted, it may be said that the anatomic formula by means of which we may define the structure of the cerebrum, of man as of the other verte brates, is this : "The cerebrum is the sum total of the cerebral convolutions, united one with another, with those on the same side and with those on the other, and simultaneously with the central opto-striate ganglions." We shall now pass in review the different agglome rations of the grey matter, and at the same time give a sketch of the principal details of the organization of the white matter.