SUBSTANCE.
EVERY one knows the external appearance of the cortical 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 fold ings and refoldings, which attain their maximum of development in the human species, appa rently 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 lei E hemisphere.
Take, for instance, a sheet of tracing paper, apply 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 homo logous 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 cer tain 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 ob served 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.