II. The Chemistry of the brain:
(a) The chemical composition of the substances constituting the brain, which may be more or less complicated. (Recent studies of the chemical evolution of living organisms have demonstrated that the atomic composition is far more complex in the higher organisms.)
(b) The intimate interchange of matter in the cerebral tissues, in connection with their nutrition.
(c) The chemical stimuli coming from the so-called glands of internal secretion (thyroid, etc.).
All these conditions concur in determining the quality of the cerebral tissues. In its ontogenetic evolution, for example, the brain does not merely increase in volume, and its development is not limited to attaining a definite morphology; but its intimate structure and its chemical composition as well must pass through various stages of transition before attaining their final state. We know, for example, that the myelination of the nerve fibres takes place upward from the spinal marrow toward the brain, and that the pyramidal tracts (voluntary motor tracts) are the last to myelinate, and hence the last to perform their functions in the child.
The consistence of the cerebral mass and its specific gravity also differ in childhood from that of the adult state. The evolution of the brain is therefore a very complex process; and this process may not be fully completed (for instance, it may be completed in volume, but not in form or chemical composition, etc.).
Consequently, just as in the case of volume, there may be various qualitative conditions, such as would produce organic inferiority.
But supposing that qualitatively the evolution has been accomplished normally, where there is greater cerebral volume, is there a correspondingly greater intellect?
At this point it is necessary to take into consideration another series of questions regarding the brain considered as a material organ, and having reference to the relation between the volume of the brain and that of the stature.
The brain must govern the nerves in all the active parts of the body, especially the striped muscles, which perform all voluntary movement. Consequently the cerebral volume must be in proportion, not only to the intellectuality, but also to the physical activity.