When complete development is attained, the cerebral hemispheres completely cover the other parts of the brain, besides which they themselves are covered over with a multiplicity of folds constituting the convolutions. If we take a cross-section of the hemispheres, we find that they consist of an outer layer of gray matter formed of nerve cells, and of a central mass of white matter, formed of fibres.
Fig. 75. Brain of a Human Embryo after the Fourth Week.
The study of the convolutions is quite important from the anthropological standpoint, because their number is not identical in the different branches of the human race, and also because they differ both in number and in arrangement from the convolutions in the brain of the anthropoid apes. But however interesting they may be, considered as differentiating characteristics, we cannot linger over a study of this kind, which has a purely theoretic importance, and for the present cannot be applied in any practical and direct way to our problems of pedagogic anthropology. It will be sufficient to note rapidly that at the present time the study of the convolutions has received a new impulse through the labours of certain distinguished investigators, among whom we must once more include Dr. Sergio Sergi. Instead of studying the surface convolutions, Dr. Sergi studies the internal folds which are disclosed by separating the lips of the cerebral fissures; and from these he draws deductions which to a large extent correct those made by previous scientists, in regard to the eventual ancestry of the different species, the marks of biological superiority or inferiority, the differences in the brain due to sex, etc.
The surface fissures which divide the cerebral hemispheres into convolutions are shown in the two accompanying figures (Figs. 76 and 77), the first of which shows the outer side of the hemispheres, and the second the inner side.
Of chief importance to us is the arrangement of convolutions and furrows on the outer surface of the hemispheres.
The points to be noted are the following: the two great fissures, Rolando's, running longitudinally, and Silvius's running transversely, which, together with the perpendicular fissure, divide the hemisphere into four lobes: the frontal lobe and the parietal lobe, situated respectively in front and behind Rolando's fissure; the temporal lobe, situated below Silvius's fissure, and lastly, the occipital lobe at the posterior apex of the hemisphere.
Fig. 76.—Cerebral hemisphere; external face.