In the adult in whom development is regularly accomplished, the cortical substance presents itself very clearly to the naked eye, in the form of stratified zones, differing slightly in colour. We observe, in fact, that there exists a superficial sub-meningeal zone of a greyish colour, and transparent ; and a deeper zone, underlying the preceding, of a more distinctly reddish colour.
When we take a thin section of this cortical sub stance, compress it between two pieces of glass, and examine it by holding it up to the light, as Bail larger first pointed out,* we see that it divides into secondary zones of unequal transparency, and that these zones cleave with a regular and fixed striation. We shall see that these appearances are merely the result of the minute structure of the cortical substance.
Such are the characters which the cerebral cortex presents to the naked eye, and which every one may observe in fresh brains.
Let us now penetrate, by means of magnifying glasses, into the interior of this soft substance, amorph ous in appearance, of which the homogeneous aspect is far from revealing to us its marvellous details.
Let us push our researches still further by means of thin sections rendered transparent and methodically coloured ; let us employ gradually increasing powers to pass from a known to an unknown region ; and avail ourselves of the magnifying processes that photo graphy places at our disposal. We shall then be able to penetrate into these almost unknown regions of the world of the infinitely little, and, like travellers re turned from distant lands, to bring back various pho tographic images—indisputably faithful reproductions of the details which have struck us in the course of our voyage of discovery.
We now find in the cortical substance a fixed anatomical element—an ultimate morphological unit, This is the nerve-cell, with its various attributes and definite configuration, its nerve-fibres, connective-tissue, and capillaries ; and we must now examine the consti tution of this cortical nerve-cell, its forms, its connections, and its relations.
Imagine a number of small pyramidal bodies, disposed in series, parallel to one another, united to one another by means of an intermediary network, and moreover regularly stratified, and thus forming layers successively piled up, like the strata of the terrestrial cortex. Such
is the general aspect that a thin complete section of the cortical substance presents.
If we add that the cerebral nerve-fibres entet into intimate connection with this network of cells, and are insensibly lost in the surrounding tissue, we shall then have a complete expression of the organization of the cerebral cortex.
Now if we observe each of the nerve-cells singly, we discover that they all have a pyramidal form ; that they are of unequal volume ; that the smaller occupy the superficial or sub-meningeal, the larger the deeper regions ; that these latter are on an average double the size of their fellows, and that the transition from small to large cells is accomplished by insensible gradations, the cells of the intermediate zones in general presenting mixed characteristics.
The cells have in addition one extremely remarkable peculiarity, which gives to the histological preparations of this region a special physiognomy, viz., their character istic arrangement. It is indeed very curious to observe that, while they are all, as we have seen, pyramidal in form, the summit of each is, so to speak, attracted towards the superficial regions, like a series of needles magnetized so as to point towards the pole; so that their bases are all parallel and are directed towards the point from which the nerve-fibres arrive.
They give off from their substance a species of very delicate, rootlet-like, hirsute fringe, which gradually spreads out and forms on all sides a surrounding network ; and as each cell presents a similar arrange ment, the result is that a close union between them is produced, so that they form throughout the cerebral cortex a. continuous true plexus, all the molecules of which are by some means arranged so as to vibrate in unison.
By their prolongations, which form the base of the pyramid, they enter more or less directly into relation with the afferent nerve-fibres ; while their apices send out a filamentous prolongation, which proceeds either to be lost in the surrounding network, or to enter into contact with certain zones of cells situated above.