Pedagogical Anthropology - Maria Montessori (best free novels TXT) 📗
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Another cause for lack of correspondence between the cerebral and the cranial volume may be the abnormal production of cerebro-spinal fluid within the brain: hydrocephalic cranium.
The Development of the Brain.—In the earliest period of embryonal life, the brain consists of a single vesicle, the continuation of which forms the spinal marrow: later on, this vesicle divides into three superimposed vesicles which represent respectively the embryonal beginnings of the anterior, middle and posterior brain; continuing their development, the anterior and posterior brains each divide in turn into two other vesicles, so that there result in all five primitive vesicles of the brain, superimposed one upon another (see Fig. 75); the anterior vesicle which is destined to grow enormously, dividing into two parts, right and left, with a longitudinal division, will constitute the cerebral hemispheres; the second vesicle will constitute the optic thalami; the third vesicle, the corpora quadrigemina; the fourth vesicle, the cerebellum, and the fifth vesicle, the medulla oblongata.
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.
In the third frontal convolution are situated Broca's centres, which are believed to be the seat of articulate speech; while along Rolando's fissure, in the ascendant convolutions, is the locality designated by physiologists as the motor centres.
The occipital lobe is the location of the zone of sight; and the temporal lobe, that of hearing.
It is important for us to observe the volume of the brain, and therefore that of the head, in relation to the rest of the body; it is enormous in the embryo; and even at birth and during childhood the head is quite voluminous as compared with the body, as appears from the diagram in Fig. 16, in which a new-born child and an adult man are reduced to the same scale, each retaining his relative bodily proportions. In Fig. 22 a new-born child is shown in two positions: from the front and from behind; the head is very large and the cranial nodules are plainly visible. Figs. 80 and 81 represent the same child at the age of six months and a year and a half; in the first picture the head is still very large as compared with the body, and the forehead protrudes (infantile forehead); in the second, the proportion between head and body has already altered.
A knowledge of the laws governing the growth of the brain is of particular importance in relation to pedagogic anthropology.
Fig. 77.—Cerebral hemisphere, internal face.
Within the last few years anthropologists have established certain principles that are well worthy of notice:
The child's head is normal when its volume and cephalic index come within the limits of normality (even if the shape appears abnormal: Simon, Binet, etc.). When the volume of the head is too small it frequently indicates psychic deficiency; when it is too large, even up to the age of twenty years, it indicates a predisposition to precocious mortality (see below).Very frequently when the size of the head is larger than normal and is not due to pathological causes (rickets, hydrocephaly, etc.), it is associated with an excessive development of the brain, and also with an intellectual precocity. A high percentage of this type die before reaching the age of twenty years; and this fact confirms the popular belief that children who are too intelligent or too good cannot live long.
This indication alone ought to be sufficient to prove the pedagogic importance of the cerebral volume.
The researches made by various authors in regard to the growth of the brain are not rigorously in accord as to the limits of volume: but they do agree as to the rhythm of growth.
Welcker gives the following figures:
WEIGHT OF THE BRAIN IN GRAMS
(According to Welcker)
Accordingly, the weight of the brain is doubled before the end of the first year; according to Massini it is very nearly doubled at the end of the first six months:
MASSINI'S FIGURES AS TO THE WEIGHT OF THE BRAIN
Age Total weight Increase At birth 352 68 279 First month 420 211 From first to third month 631 From third to sixth month 675 44 63 From sixth month to 1 year 694 19Fig. 78.—Spheroidal cranium lateral norm (Sergi's collection).
Fig. 79.—Spheroids typicus (from Sergi's collection).
Fig. 80.—A child six months old.
Fig. 81.—The same child a year and a half old.
It follows from these figures that by the end of the sixth month the weight of the brain is already very nearly doubled; but the maximum growth takes place between the ages of one month and three, after which it shows a notable diminution of rate.
But while the weight of the whole body is increased threefold by the end of the first year, that of the brain is very far from being tripled, since the rate of growth is still further diminished during the second six months; in fact even according to Welcker the weight at the end of the first year has little more than doubled.
Accordingly the rhythm of cerebral growth is not identical with that of the increase in weight of the body taken as a whole.
According to Massini, the relation between the cerebral weight and the weight of the body, at the various successive ages, is as follows:
RELATION BETWEEN WEIGHT OF BRAIN AND TOTAL WEIGHT
(According to Massini)
In other words, the body grows more rapidly than the brain, and consequently, than the head: a fact which results in the different proportions already noted between head and body.
The rhythm of brain growth considered by itself has been set forth in a most noteworthy and accurate fashion by Boyd, based on the study of about two thousand cases; from the figures given by Boyd, I have calculated the amount of increase from period to period, as well as from year to year, the whole result being set forth in the following table:
RHYTHM OF GROWTH OF BRAIN
(Males: According to Boyd)
In the above table, the first column of figures gives the mean average weight of the brain, obtained by direct observation of individual subjects; while from all the others the rhythm of cerebral growth and involution throughout the successive periods of life may be computed.
We see that the maximum growth takes place in the first years of life, the intensity is greater in the first year than in the second, and greater in the first three months than in those that follow. Already at the end of the first year the brain has surpassed one-half of the maximum weight which the individual is destined to attain in adult life (last column: proportions computed on scale of 100). A notable rate of increase continues up to the age of four, after which it moderates, but receives a new impulse at about the fourteenth year (period of puberty); hence it appears that at this important epoch of life the brain not only shares the general rapid growth of the body, but that by the end of the fourteenth year the brain has already practically completed its development; in fact, assuming that 100 represents its complete development, the weight of the brain is already 95.3; and at thirty it will be only 99.3.
By studying the above table we can obtain a clear analysis of these phenomena.
For women, Boyd gives the following figures:
THE GROWTH OF THE BRAIN IN WOMEN
(Figures Given by Boyd)
The rhythm of growth of the female brain is analogous to that of the male, except for the more precocious attainment of the maximum weight, which corresponds to the more precocious evolution of the female organism.
It should be noted that in the tables above cited the maximum is actually
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