All mammiferous animals, excepting those mentioned in the preceding paragraphs, have convolutions which exhibit more or less of complication. This complication has evidently no connection with the general organization of the animal, inasmuch as we find animals, in the same family with those which possess numerous convolutions, exhibiting a very slight developement of them. The monkeys, the dolphin, the elephant, exhibit the most nume rous convolutions of any of the Mammalia inferior to man, in vvhose brain the convoluted surface reaches its highest peint.
Each fold on the surace of the bmin is ordinarily called a convolution, whatever be its position, size, or direction. It consists of a fold of gre. matter, enclosing a process of white or fibrous matter. On each side of 'Cis a sulcus or groove, in which we find the same elements, a fold of grey or vesicular matter— concave externally, convex internally — the fibrous matter adhering to its convex surface. As the convolution exhibits no essential diffe rence of structure from the sulcus, it is plain that the fortner portion of the brain's surface cannot differ in physiological office from the latter. We describe particular convolutions, not because they are to be regarded as endowed with special functions distinct from the less prominent portions of the cerebral surface, the sulci, which are continuous and identical in structure with them, but because they afford good indications of a particular arrangement of the surface of the hemispheres by which one brain may he coveniently com pared with another, whether they belong to the same or to different groups of animals.
The folded arrangement of the surface of the hemispheres, dependent as it is upon the grey matter, is evidently destined to bring the central and deep-seated parts of the hemispheres into union with a large extent of vesicular surface.
That the disposition of the convolutions, like that of all other parts of animal bodies, follows a particular law, is well illustrated by comparing the brains of different groups of animals, in their gradation from the more simple to the more complex.
M. Leuret very justly makes a distinction between those convolutions which are constant, and to be found throughout the whole series of convoluted bmins, occupying the same position, and differing only in size and extent of connec tions, and those which are not constant, even in the brains of the same group -of animals, but are dependent on the extent of the primary ones, and the connections which they form with others near them. According to this idea
we may classify the convolutions as primary and secondary.
The primary convolutions are all formed after one type. Of this, as M. Leuret suggests, the brain of the fox may be taken as the basis. The fissure of Sylvius is well marked in this brain ; it is bounded by a prominent convolu tion, which encloses it above, below, and behind—thus forming a curve, the concavity of which is directed forwards and downwards. Above and behind this we find a second con volution fonning a similar curve and parallel to the first. It exhibits a slight undulation, and is marked by a short fissure--signs of ad vancing complication. Still further back and upwards there is a third convolution, parallel and curved similarly to the second; this bifur cates at one point. Above all, near the summit of the hemisphere, a fourth is found disposed in the same curved manner, but ex hibiting some sinuosities or undulations at its anterior portion. A fifth convolution exists on the inferior surface of the anterior lobe and rests upon the roof of the orbit. Leuret de signates it the supru-orbitar convolution. The sixth convolution is of great extent; the prin cipal portion of it is found on the inner surface of each hemisphere above the corpus callosum; in front it bends downwards and backwards to the fissure of Sylvius, and behind it extends to the middle lobe and forms the hippocampus major. This convolution exists in a high state of developement in the human brain, and has attracted very generally the attention of ana tomists. Foville describes it by the name convobaion d'ourlet.
Such is the most simple arrangement of the convolutions. The complication of this takes place by undulations being formed in the con volutions themselves, by a subdivision of them at certain situations, by the junction of neigh bouring ones through smaller folds crossing the sulci between them, and in the highest classes by the addition of totally new convolutions.