Each hemisphere presents an outer surface, which is convex to correspond with the vault of the cranium; an inner surface, flattened, and in contact with the opposite hemisphere, the two forming the sides of the longitudinal fissure; and an under surface or base, of more irregular form, which rests, in front, in the anterior and middle fossae at the base of the skull, and behind, upon the tentorium.

  Convolutions. If the pia mater is removed with the forceps, the entire surface of each hemisphere will present a number of convoluted eminences, the convolutions, separated from each other by depressions (sulci) of various depths. The oiiter surface of each convolution, as well as the sides and bottom of the sulci between them, are composed of grey matter, which is here called the cortical substance. The interior of each convolution is composed of white matter, white fibres also blend witli the grey matter at the sides and bottom of the sulci. By this arrangement the convolutions are admirably adapted to increase the amount of grey matter without occupying much additional space, and also afford a greater extent of surface for the fibres to terminate in it. On closer examination, however, the grey matter of the cortical substance is found subdivided into four layers, two of which are composed of grey and two of white substance. The most external is an outer white stratum, not equally thick over all parts of the brain, being most marked on the convolutions in the longitudinal fissure and 'on the under part of the brain, especially on the middle lobe, near the descending horn of the lateral ventricle. Beneath the latter is a thick reddish grey lamina, and then another thin white stratum; lastly, a thin stratum of grey matter, which lies in close contact with the white fibres of the hemispheres: consequently white and grey laminae alternate with one another in the grey matter of the convolutions. In certain convolutions, however, the cortical substance consists of no less than six layers, three, grey and three white, an additional white stratum dividing the most superficial grey one into two; this is especially marked in those convolutions which are situated near the corpus callosum.

  A perfect resemblance between the convolutions does not exist in all brains, nor are they symmetrical on the two sides of the same brain. Occasionally the free borders or the sides of a deep convolution present a fissured or notched appearance.

  The sulci are generally an inch in depth; they also vary in different brains, and in different parts of the same brain; they are usually deepest on the outer convex surface of the hemispheres; the deepest is situated on the inner surface of the hemisphere, on a level with the corpus callosum, and corresponds to the projection in the posterior horn of the lateral ventricle, the hippocampus minor.

  The number and extent of the convolutions, as well as their depth, appear to bear a close relation to the intellectual power of the individual, as is shown in their increasing complexity of arrangement as we ascend from the lowest mammalia up to man. Thus they are absent in some of the lower orders of this class, and they increase in number and extent through the higher orders. In man they present the most complex arrangement. Again, in the child at birth before the intellectual faculties are exercised, the convolutions have a very simple arrangement, presenting few undulations; and the sulci between them are less deep than in the adult. In old age, when the mental faculties have diminished in activity, the convolutions become much less prominently marked.

  Those convolutions which are the largest and most constantly present, are the convolution of the corpus callosum, the convolution of the longitudinal fissure, the supra-orbital convolution, and the convolutions of the outer surface of the hemisphere.

  The Convolution of the Corpus Callosum (gyrus fornicatus) is always well marked. It lies parallel with the upper surface of the corpus callosum, commencing, in front, on the under surface of the brain in front of the anterior perforated space; it winds round the curved border of the corpus callosum, and passes along its upper surface as far as its posterior extremity, where it is connected with the convolutions of the posterior lobe; it then curves downwards and forwards, embracing the cerebral peduncle, passes into the middle lobe, forming the hippocampus major and terminates just behind the point from whence it arose.

  The Supra-orbitar Convolution on the under surface of the anterior lobe is well marked.

  The Convolution of the Longitudinal Fissure bounds the margin of the fissure on the upper surface of the hemisphere. It commences on the under surface of the brain, at the anterior perforated spot, passes forwards along the inner margin of the anterior lobe, being here divided by a deep sulcus, in which the olfactory nerve is received; it then curves over the anterior and upper surface of the hemisphere, along the margin of the longitudinal fissure, to its posterior extremity, where it curves forwards along the under surface of the hemisphere as far as the middle lobe.

  The convolutions on the outer convex surface of the hemisphere, the general direction of which is more or less oblique, are the largest and the most complicated convolutions of the brain, frequently becoming branched like the letter Y in their course upwards and backwards towards the longitudinal fissure: these convolutions attain their greatest development in man, and are especially characteristic of the human brain. They are seldom symmetrical on the two sides.