Evidently, a greater mass of body demands a greater nervous system to give it motive power.
The biological law is of a general nature: if the brain of a rat weighs 40 centigrams, that of an ox weighs 734 grams, and that of an elephant 4,896 grams.
"The absolute volume of the brain increases with the total volume of the body."
But this correspondence is not proportional. There are two facts that alter the proportions. One of these is that the mass of the body increases faster than the brain, throughout the biological series of species, so that the smaller the body the greater the proportional quantity of brain. Just the opposite from what was found to hold true for the absolute weight.
It may be affirmed as a biological law that "the relative volume of the brain increases as the size of the body diminishes." For instance, the tiny brain of a rat is a 43d part of the total volume of its body; the brain of an ox, on the contrary, is a 750th part. Consequently we may say that the little rat has relatively a far larger brain than the huge ox.
And the same thing holds true among men; those of small build have a proportionately larger brain than those of large build.
A second fact which alters the absolute proportion between the volume of brain and the volume of body has reference to the "functional capacity" of the active parts. The muscles which are capable of the best activity and the greatest agility are the ones more abundantly stimulated through their nerves than those which are capable only of slow and sluggish action. The same may be said of the organs of sensation; the more highly the sensibility is developed, the larger are the corresponding nerves, and consequently the greater is the corresponding quantity of cerebral cells. Accordingly the animal which is nimblest in its movements, and most capable of sensations has in proportion to this greater functional activity a greater cerebral volume. In this same way we may explain the enormous difference in relative brain volume between the extremely active, sensitive and intelligent little beast which we call the rat, and the sluggish and stupid animal which we call the ox. Consequently this functional activity has a correspondingly greater volume of brain, without a correspondingly greater volume of the various highly sensitized organs. In such a case it may be stated as a general law that "the relative volume of the brain is in direct proportion to the intelligence (or, more broadly, to the functional activity), while the absolute volume is in direct relation to the total mass of the body."
Man has a cerebral volume of 1,500 cubic centimetres, a volume equal to a fortieth part of the whole body. Consequently he has a brain twice the actual size of that of the ox, while considered in its relation to bodily bulk, he has more brain than the smallest rat (man = 1/40; rat = 1/43). A volume so far exceeding the proportions found in animals, is beyond doubt directly related to human intelligence.
Relation between Cerebral and Intellectual Development in Man.—This ends our examination of the generic question of the relation between cerebral volume and intellect.
Granting these biological principles, and wishing to apply them to normal man, let us go back to our first question: "Do persons of greater intelligence have a greater cerebral volume, and consequently a larger head?"