Many individuals present peculiarities in respect to the size of their brain; but the organ is subject to other and more essential deviations from its natural bulk. I shall treat first of unnatural excess, and then of unnatural diminution of its volume.

1. Unnaturally Large Size Of The Brain

Many morbid conditions augment the volume of the brain, as hyperemia, hydrocephalus, and oedema, adventitious growths, and hypertrophy, or a combination of any of them with one another. I have here to treat of increase of volume by hypertrophy, which is the most important of these morbid conditions; the others have already been, or will be, mentioned in their place.

Hypertrophy Of The Brain

Its general characters are, unnatural size and weight of the organ. It varies in degree; and its importance depends partly upon this variety, but mainly on the condition of the skull. The most serious conditions under which it occurs, are when the hypertrophy is far advanced, and the sutures are closed, as the skull then resists the increase of the volume of the brain.

The best plan will be to begin by depicting such a case.

When the skull-cap is removed, the brain, closely covered by the dura mater, swells up palpably (turgescirt); on slitting open the dura mater, the swelling is still more distinct, and it costs some trouble to fit the skull-cap on again. All the membranes of the brain are remarkably thin; the dura mater especially is delicate, pale, reddish, and transparent. The inner membranes lie close upon both the dura mater and the surface of the brain. Their lack of the fluid which usually moistens the arachnoid, and occupies the tissue of the pia mater, is quite conspicuous; they are dry, and their vessels are bloodless and flattened.

Before further dissection and comparison the cerebral hemispheres appear large. Their convolutions are compressed and flattened, and the sulci between them are scarcely discernible.

The usual horizontal section through the hemispheres, a little above the level of the corpus callosum, displays a centrum ovale of unusual size.

The ventricles are remarkably small.

When the whole brain is removed from the cranium, the size of the cerebrum again arrests attention, especially when compared with the cerebellum and other parts at the base of the brain, and with the nerves.

It is quite clear throughout the examination that it is the white substance that is increased in volume, the white substance of the cerebral hemispheres. The cineritious matter is generally of a pale grayish-red color, the medullary is always dazzling white, and remarkably pale and anaemic; a circumstance both of interest and importance, because it distinguishes the increase in the volume of the brain occasioned by hypertrophy, from that which is produced by congestion.

The consistence of the hypertrophied white substance is quite peculiar; it is elastic, and has the somewhat firm resisting feeling of rising dough.

Having thus sketched the most essential and most striking of the appearances in hypertrophy of the brain, I proceed to detail some results which attend it in advanced degrees of the disease, and at particular periods of life.

a. When the hypertrophy is very far advanced, and the sutures are closed, the pressure sets up some absorption at the inner table of the skull, and it becomes rough: the absorption may even go so far as to make the wall of the cranium distinctly thinner than natural. Inferiorly the cerebellum and the structures at the base of the brain are flattened and spread out, evidently by pressure from above. The absorption of the inner table generally begins and goes farthest at the vault of the skull, though it appears indeed, to be most advanced at the base, as the bone, which was there originally thin, is in some parts perforated with holes, produced by the absorption; the orbital and cribriform plates, and the roof of the sphenoidal sinuses, are thus perforated.