In the imperfectly developed brains of the infant or young child, the convolutions are quite symmetrical. They are so likewise in idiots, or persons of very inferior intellect, and, as has been already stated, in sorne Negro brains.
The following convolutions of the human brain are constant in their position, although they differ much in different brains in size and developement.
1. The internal convolution, or that of the corpus callosnm, called by Foville convolution d'ourlet (process° cristato, Rolando). The principal portion of this convolution is above and parallel to the corpus callosum : in front it curves down parallel to the anterior reflection of the corpus callosum, as far as the locus per foratus, connecting itself with some of the ante rior convolutions. Behind it passes in a similar manner round the posterior reflexion, connecting itself with some of the posterior convolutions, and in the middle lobe forming the hippocam pus major, the anterior extremity of which is situate immediately behind the fissure of Sylvi us and locus perforatus. Its horizontal portion appears to be connected with some nearly ver tical ones, which seem indeed to branch off from it. (Fig. 395, O.) This is the most constant and regular con volution of the brain. It exhibits with its fel low of the opposite side very exact symmetry. Its inferior or concave border is smooth and uninterrupted, and founs the superior boun dary of a sulcus, which intervenes between it and the surface of the corpus callosum. It forms, to use Foville's expression, a hem or selvage to the cortical layer of the cerebral hemisphere. The fibrous matter which is in closed- by the cortical layer of this convolu tion consists of longitudinal fibres following the same general direction, a large number of them no doubt bending inwards into the cor tical layer. These fibres are evidently com missural in their office, and will be referred to by-and-bye as constituting the superior longi tudinal cominissure.
The free margin of this convolution varies in its characters in different brains, according to the degree of tortuosity it exhibits, and the number of small fissures which are niet with in it. The small folds which connect it with other convolutions on the inner surface of the hemi sphere vary in number, and are generally found most numerous at its posterior part. Some of
these fotds are not distinctly visible unless the sulcus above it has been freely opened, as they are situated quite on its floor.
2. 7'he convolution of the Sylvian fissure.— This convolution forms the immediate boun dary of this great fissure. NN'e have seen its early developement in the simple brain of the fox, and we may observe it gradually rising in complexity through all the intermediate stages rip to the most highly developed brains. In the elephant it is remarkably tortuous, and is connected anteriorly as well as posteriorly with convolutions which pass to the anterior and su perior and to the posterior part of the brain.° In man it is also very tortuous, and the nume rous folds which pass from it forwards or back wards, forrning primary or secondary convolu tions, render it difficult to isolate it sufficiently for the anatomist to follow it throughout its entire course. Its inner border is likewise interrupted by the connections which it forms with the con volutions of the floor of the Sylvian fissure.
3. Within the fissure of Sylvius we find that remarkable group of convolutions called by Reit itzsda, the island. It consists of a series of small folds radiating from a common centre and connected with the convolution last described by still smaller folds, which cannot be seen unless when the fissure has been very freely laid open. The centre from which the convolutions radiate is the apex of a cone, the base of which adheres to the floor of the fissure.
4. On the inferior surface of the anterior lobe there is a pair of longitudinal convolutions which enclose between them the fissure of the olfactory process. The external of these con volutions is continuous with the convolution of the Sylvian fissure.
The numerous secondary convolutions which are found over the surfaces of the brain render it difficult to distinguish the primary ones. These latter are indicated hy the antero-poste rior course which they take--the former being more or less vertical. The largest and most tortuous convolutions are found on that part of the external surface which corresponds to the parietal bone. Next to them, in point of size, are the convolutions of the anterior lobe, but the smallest of all are those of the posterior lobes.