Everyone knows that this is one of those complicated, much discussed questions that spread outside of the purely scientific circles and become one of the stock themes of debate among classes incompetent to judge; consequently it has been colored by popular prejudice, rather than by the light of science. It is well that persons of education should acquire accurate ideas upon the subject.
If the volume of the brain should be in proportion to the intellectual development, argues the general public, what sort of a head must Dante Alighieri have had? He would have had to be the most monstrous macrocephalic ever seen upon earth. And on the basis of this superficial observation, they wish to deny any quantitative relation whatever between brain and intelligence. And yet it is this same general public that keeps insisting: Woman has less intelligence than man, because she has a smaller brain.
A single glance up and down the zoological scale suffices to show that throughout the whole animal series a greater development of brain is accompanied by a correspondingly greater development of psychic activity; and that there is a conspicuous difference between the human brain and that of the higher animals (anthropoid apes), corresponding to the difference between the level of man's psychic development and that of the higher mammals; and this justifies the assertion that, as a general rule, there is a quantitative relation between the brain and the intellect.
This suggests the thought that the perfect development of this delicate instrument, the brain, demands a variety of harmonious material conditions, among others the volume, in order to render possible the conditions of psychic perfection.
From this premise, we may pass on to a more particularised study of the material conditions essential to the superior type of brain. The volume is the quantitative index; but the quality may be considered from various points of view, which may be grouped as follows:
I. The General Morphology of the Brain in reference to:
(a) The harmonious, relative volumetric proportions between the lobes of the brain (namely, the proportion between the frontal, parietal, temporal and occipital lobes). It was formerly believed that a superior brain ought to show a prevalence of the frontal lobes, since a lofty forehead is a sign of intellect; but it was afterward established that there is no direct relation between the development of the forehead and the development of the frontal lobes; a higher forehead results from a greater volume of the entire cranial contents; the superior brain, on the contrary, is that in which no one lobe prevails over another, but all of them preserve a reciprocal and perfect harmony of dimensions.
(b) The form, number and disposition of the cerebral convolutions, and of the folds of the internal passage (Sergio Sergi).
(c) The form, number and disposition of the cells in the cortical strata of the brain, and the proportion between the gray matter and the white, that is to say, between the cells and fibres; in short, the histological structure of the brain.