The form of the brain generally corresponds to that of the skull, but anomalies occur, principally in the cerebrum. In the first place, the brain is subject to variations, in respect to its length, its breadth, and its height: in the next place, some of the larger portions of it, the anterior or posterior lobes, for instance, may exceed, or come short of, their normal development: the two halves of the brain may be unsymmetrical, in consequence either of inequality of size generally, or of a difference in a particular diameter, or in consequence of a change of relative position in the horizontal or in the vertical direction, etc.; and lastly, its form varies in respect to the stronger or fainter marking out of the separate lobes, and to the number, depth, and symmetry of the convolutions of the cerebrum, and laminae of the cerebellum.

Alterations of shape, similar to these congenital deformities, occur also, as results of disease of the brain: the principal instances are those in which the symmetry of the two halves is deranged, by an increase or diminution in the volume of the whole, or of part of one side, as well as by flattening, and loss of the convolutions, etc.

The most striking deformity of the brain is that in which the cerebrum is single; it occurs in conjunction with cyclopia, and with partial or total absence of the face. (Ateloprosopia; Aprosopia.) The brain forms a single sac, open behind, but completed by the arachnoid, and filled with serum: its posterior lobes are so defective, that the cerebellum, corpora quadrigemina, and even the optic thalami, appear uncovered; whilst many other parts, the fornix, corpus callosum, septum, small commissures, etc, are also wanting. The cerebrum is sometimes thus single in cases of hemicephalus, and this combination associates the latter with cyclopia. (Otto.) Another anomaly, observed in the interior of the brain, is allied to that which has just been mentioned: it is that in which the optic thalami and corpora striata of the two sides are fused together in the middle: a double or a large commissura mollis is a modification of the same anomaly.

From the opposite condition, deficiency of the commissures, a division of the brain results. The fornix is very rarely wanting, while, on the other hand, deficiency of the soft commissure of the optic thalami is not unfrequent.

The other smaller organs of the brain are rarely misshapen from any fault in their original formation.