it to a fresh vertical section of the brain, mark with a brushful of water-colour the contour of the cortical substance of one hemisphere, and fold the paper over; you will thus see very clearly that the outline of the convolutions of one side does not adapt itself to those of the other. I have made such tracings repeatedly, and have never yet found a human brain completely symmetrical, completely balanced in its peripheral regions, and with the left regions of the cortical substance exactly corresponding to the homologous regions of the opposite side.

There is another peculiarity, which it is important to notice, in the external examination of the cortical substance.

In the adult, in vertical or horizontal sections of the brain, it is evident that the line of the summits of the convolutions is continuous, that their culminating points are all on the same level; there is some uniformity in the distribution of the activity of nutrition over the whole mass.

As old age advances different appearances begin to show themselves, and in studying the different effects of senescence in all the organs, it is curious to observe its characteristics in the human brain.

We observe, then, that the grey substance becomes diminished in thickness; that its colour changes to yellowish white in consequence of the passing of the nerve-cells into the granulo-fatty state; and that besides, the convolutions settle down in isolated groups, like mountains, undermined at their bases, which insensibly subside. Thus, in many old men in their dotage, we may note that the line joining the summits of certain

groups of convolutions becomes interrupted; that a certain number of them are retracted and have sunk below the level of the surrounding convolutions; and that thus, from the effect of time, there exists a slow and progressive absorption of the nervous substance.

In individuals who fall prematurely into dotage from alterations of the cerebral substance, under the action of mental diseases, we find the same atrophy of the cortical layer. Thus I have very frequently observed atrophy of the convolutions in young subjects attacked by paralytic dementia, persons affected by hallucinations, and patients who have suffered from melancholic delirium.

The thickness of the cortical substance in the adult is on the average about two to three millimetres. Generally it is more abundant in the anterior than the posterior regions. Its mass varies according to age, and especially according to race, Gratiolet remarking that in races of low stature the mass of the cortical substance is but small.*

Its colour presents some varieties. It is uniformly greyish, and as it were gelatinous, in the new-born infant; in the child during its first years it is of a rosy grey; in the old man it acquires somewhat of a yellowish-white colour, its vascularity being less distinct than in the adult. In the negro this substance is of a darker colour than in the white man.

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 # " Gratiolet, Bulletin de la Société d'Anthropologie," 1859, p. 38.

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 substance, compress it between two pieces of glass, and examine it by holding it up to the light, as Baillarger 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, amorphous 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 photography 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 returned from distant lands, to bring back various photographic 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

"Mémoires de l'Académie de Médecine de Paris," 1840.

FIG. 1.-Half-diagrammatic figure of the cerebral cortex, magnified about 280 diameters, giving a view of the entire arrangement of the different zones of cells, and their relations to one another, and to the surrounding neuroglia. The region A corresponds to the sub-meningeal network of the neuroglia. The region в to the sub-meningeal zones of small cells (region of the sensorium commune); the region c is intermediate between the sub-meningeal and the deeper zones of cells which are indicated at D. At E we note the dipping of the fasciculi of white substance into the plexus of F represents a capillary at the moment when it plunges into the tissues of the cortex. cortical cells.

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 constitution 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 enter 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 characteristic arrangement. It is indeed very curious to observe that, while they are all, as we have seen, pyramidal