There is here a double influence, that of stature and that of race. We might have introduced a third element—the weight of the body, but it represents too many different things, and may vary according to the degree of stoutness of the individual, the dietary regimen, etc. C. Voit found, when operating on two dogs of nearly equal bulk, that the weight of the brain of the well-fed dog represented 1.1 per cent. of the weight of its body, whilst the brain of the dog which had fasted for twenty-two days represented 1.7 per cent. of the weight of the body.[109] At all events, we cannot deny the influence of the bulk of the active parts of the body on the volume of the brain.[110] But then a new question arises. Is the increase of the volume of the brain made at the cost of the white substance formed solely of conducting-fibres, or of the grey substance formed principally of cells with their prolongations (neurons), that is to say, of the part which is exclusively affected by the psychic processes? This question still waits its solution. It is not the gross weight of the brain, but really the weight of the cortical layer which should be compared in the different races and subjects, in order to judge of the quantity of substance devoted to the psychic functions in each particular case.[111] Before the very delicate weighings of this kind are made, we have a round-about method of ascertaining the quantity of that substance by the superficial area which it occupies. The cerebral cortex, composed of the grey substance, forms on the surface of the brain sinuous folds called cerebral convolutions. Now, in brains of equal volume, the greater the surface of the cortex, the more numerous, sinuous, and complicated will be these folds. As the thickness of the grey layer is very much the same in all brains, it is evident that the complexity in the structure of the convolutions corresponds to the increase of the grey substance, and consequently of the psychic force. Now, the little that is known of the cerebral convolutions in different races, and of various subjects in the same race, appears to conform to this deduction. The brains of idiots, of the weak-minded, present very simple convolutions, almost comparable to those of the anthropoid apes, whose brain is like a simplified diagram of the human brain. On the other hand, distinguished personages, great scholars, orators, men of action, exhibit a complexity, sometimes truly remarkable, of certain convolutions. I say expressly certain convolutions, for all these folds, arranged according to a certain plan, common to all men, have not the same value from the physiological point of view. In the grey layer of certain of them are the centres of motor impulses, and of the general sensibility of the body (for example, those which are arranged around the fissure of Rolando, Fig. [25], 2, 2), and only regulate the voluntary movements of the limbs, the trunk and the head; others are connected with different forms of sensibility—visual (Fig. [25], 4), auditory (Fig. [25], 6), gustatory, olfactory, etc. But there are, between the different motor or sensorial regions (centres of projection) which take nearly a third of the grey substance of the brain, a great many more convolutions the grey substance of which is connected with no special function (white spots in Fig. [25]). What is their purpose? Basing his opinion on the tardy myelinisation[112] of the nerve-fibres which terminate in it, subsequent to the birth of the individual and to the myelinisation of the fibres of the sensory and motor centres, Flechsig[113] supposes that these convolutions were designed to enable the different cerebral centres to communicate with each other and to render us conscious of this communication; therefore he has named their grey substance “centres of association” (Fig. [25], 1, 3, 5). Without the convolutions, the other centres would remain isolated and condemned to a very restricted activity. Now, as the eminent anatomist Turner[114] has shown so clearly, it is found that the convolutions of the sensory and motor centres do not present any great differences in the brain of a child, a monkey, a Bushman, or of a European man of science, like Gauss; what differentiates these brains is the degree of complexity of the convolutions concerned with association. There, then, is the part of the brain which we want to utilise for the purpose of comparison, reduced by almost a third. But let us suppose that differences of volume and weight are found in these two-thirds of the grey substance. Have we more reason to think that we are approaching the solution of the problem?
FIG. 25.—Brain with indication of the three “centres of projection”
(2, general sensibility; 4, visual; 6, auditory) and the three “centres of association”
(1, frontal; 3, parietal; 5, occipito-temporal); 1, fissure of Rolando; 7, Island of Reil.
(After Flechsig.)
It is believed that certain cells of the grey substance only, the great and the little pyramidal-shaped cells, are associated with the psychical functions, and that each of these, forming with its axis-cylinder, dendrons and other branching prolongations what is called a neuron, is not in constant connection with, and does not occupy a fixed position once for all in regard to, other similar neurons, but may by means of its prolongations place itself alternately in contact with a great number of these.[115] Hence the complexity of the nervous currents resulting from these continual changes of contact. Thus the cerebral activity might not merely be measured by the quantity and the size of the cells of the grey substance, but also by the number and the variety of the habitual contacts which are probably established after an education, a training of the cells. As from the same number of keys of a piano the tyro can produce only a few dissimilar sounds, while an artist elicits varied melodies, so from cerebral cells practically equal in number a savage is only able to extract vague and rudimentary ideas, while a thinker brings out of them intellectual treasures. How far are we, then, from the true appreciation of cerebral work with our rude weighings of an organ in which, with one part that would assuredly help us to the solution of the problem, we weigh at least three other parts having nothing or almost nothing to do with it! And even if we succeeded in finding the number, the weight, and the volume of the neurons, how are we to estimate the innumerable combinations of which they are capable? The problem appears almost insoluble. However, in science we must never lose hope, and—who knows?—perhaps some day the solution of the question will be found, and it will then appear as simple as to-day it appears a matter of course to see through the body with radioscopical apparatus.
CHAPTER III.
2.—PHYSIOLOGICAL CHARACTERS.