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.

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 conducted, 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 connections, and, in fact, without deranging anything. Thus by comparing the sections, horizontal or vertical, one with another, I could follow a given order of nervefibres in its progress, see its point of origin, and its point of termination; study the natural increase in complexity of the different kinds of nerve fibrils, millimetre by millimetre, changing nothing, lacerating nothing, and leaving everything pretty much in its normal position.*

*The plan of this work does not permit me to insist upon the innumerable difficulties I have surmounted, in arriving at the clear result already recorded in my photographic iconography. (Luys, “Iconographie des centres nerveux,'' J. B. Baillière, Paris, 1872.)

In the first place I had to invent cutting instruments sufficiently delicate to make complete sections of the brain, of the thickness of about one millimetre. But these pieces, when sufficiently hardened to undergo the action of the cutting instrument, had acquired, on coming out of the bath of chromic acid,

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 juxtaposition, 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-operation 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 sufficiently magnified and reproduced, with successively increasing 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 cerebro-spinal 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.

that peculiar uniform greenish colouring which renders them completely unfit for photogenic action. It was therefore necessary to discover a perfectly novel series of processes, in order to purify these sections from the chromic acid, and, without altering them, to impart to them photogenic properties. (See Journal d'Anatomie de Robin, Paris, 1872, for the whole series of the processes employed to bleach the sections tinted with chromic acid.)

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 homologous 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 consideration in its turn :

1. 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 nervetubules 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 directions.

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 direction. 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 anastamosis and commissure between these homologous regions, and that they are thus the agents which produce unity of action between the two cerebral hemispheres. 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 vertebrates, 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 agglomerations of the grey matter, and at the same time give a sketch of the principal details of the organization of the white matter.

CHAPTER II.

CORTEX OF THE BRAIN-THE GREY CORTICAL

SUBSTANCE.

EVERY one knows the external appearance of the cortical substance of the brain. It is sufficient to recall that of the brains of sheep, as served at table, to see at a glance that the grey cortical substance presents the appearance of a grey undulating layer, folded a great number of times upon itself, and thus forming a series of multiple sinuosities of which the sole object is the obtaining of increased surface.

These foldings and refoldings, which attain their maximum of development in the human species, apparently obey some fixed laws as regards their distribution.* Some, in fact, have permanent characters which render them easily discoverable in all human brains; others, and these form the greater number, present all possible varieties of external configuration, not only in different individuals, but even in the same individual, according as we inspect homologous regions in the right or left hemisphere.

Take, for instance, a sheet of tracing paper, apply

* See the interesting description of the topography of the cerebral convolutions given by Prof. Charcot in his lectures to the Faculty.-Progrès Médical, 1875, p. 283, 353, &c.