the ultimate element of organisms, nerve-cells are undoubtedly of the greatest importance for all the higher manifestations of life. Embryology teaches that not only in the spinal cord, but in the nervous system generally, the cellular formation precedes the fibrous, which latter is said to have its origin in the former. It is taught too that 'spontaneous motion on inward impulse, or the power of itself to change its form, is the inherited quality of all cells,' which, moreover, increase in number by division. It is even said that, ' in certain circumstances, after a cell has been artificially divided, the separate parts display the same phenomena of life as the entire cell, viz. breathing, motion, nutrition, and increase in number.'1

I have said enough to show the importance of nerve-cells, and we will now glance at the development-history of the brain, for further insight into their relation to that organ.

Researches into the embryonic development of the brain have shown that it begins as a bladder-like swelling on the upper, completed end of the cylindrical spinal tube. At first uniform, it soon receives two indentations crossways, and thus falls into three particular, but connected parts, which are the groundwork of that number of typical divisions of the brain,

1 Address of Professor Preyer of Jena, 'On the Investigation of Life,' at the 50 years' Jubilee of the annual assembling of German Naturalists (Naturforscher), &c., 1872.

Development-history of the Brain

29

called the front brain (prosencephalon), the middle brain (mesencephalon), and the hind brain (ephencephalon). Of the three named bladders, the middle one alone maintains a simple character, the front and the hinder becoming greatly extended by their anterior ends, bag-like, rising upwards and turning backwards; and thus to the original bladder-like formation a supplementary one is added, which, as in course of time it grows into a particular part of the brain, must be regarded as a further typical division of the first formation. Both supplementary bladders are at first small and inconsiderable, but they soon sprout over the original bladders to which they belong to such an extent that only a small portion of them, namely, the ground (Boden), remains uncovered. In this way the so-called mantle of the brain, in contrast to its trunk, is formed, which, besides the front and hind brains, draws the whole middle brain into its department.

The further development shows this peculiarity for the original bladders, that the frontal and hinder have their central surface rent asunder longitudinally, producing a considerable opening of the hitherto closed inner ventricles.1

In addition to the median division into hemispheres already mentioned, in the course of the fœtal life of

1 Op. cit., p. 822.

the brain many indentations or fissures are formed on its entire surface. Many of the earlier of these are but of a temporary nature, and disappear. In the ninth month of gestation, however, the human brain is said to present a scheme or picture of fissures (sulci) and convolutions (gyri) which is particularly instructive as regards subsequently distinguishing those which are typical or primary from those which are supplementary or secondary. The convolutions are regarded as foldings of the superficies of the brain-to which, in man, they give a twelve-fold increase. They are smaller and more closely packed in the frontal lobe than in any other equal portion of the brain superficies. Convolutions, generally, are presumed to be owing to the growth of the hemispheres being more rapid than that of the bones of the skull, a check to expansion naturally causing foldings. This view especially applies to the secondary convolutions formed after the skullbones have come into contact, attained consistency, and several of them become firmly united. 'The growth of the different parts of the mantle is relative' to quote the words of an anatomist who has specially studied the development-history of the brain,1 and this means, they are not developed simultaneously, nor in equal degrees.

1 Archiv für Anthropologie,' vol. iii. p. 245.

It is an undoubted fact, of which I have had considerable experience, that the brains of men who have been distinguished for mental power display more numerous and asymmetrical1 convolutions, with deeper fissures between them, than do the brains of men of ordinary capacity. There is, in fact, a considerable difference in the hand and the head-working classes in European countries, not only shown in the more numerous convolutions of the latter, but in the relatively larger development of the frontal lobe.2 (See Plate III., figs. 1 and 2.)

I have spoken hitherto of the cerebrum only, but the cerebellum, or little brain, requires a brief consideration. Like the large brain, it consists of two hemispheres of lobes and convolutions, which, however, are very different in form, and more symmetrical on both sides than is the case in the cerebrum. The surface of the cerebellum is grey, and the colour

1 The convolutions in the brains of infants and some Negroes are said to be symmetrical in each hemisphere. Todd's Encyclopædia of Anatomy and Physiology,' vol. iii. p. 697.

2 The views of those political enthusiasts who fancy that by breaking with the past, and introducing thoroughly novel institutions, not only general Łappiness, but equality of mental gifts would be the speedy outcome, are not supported by physiology. Equality in mental life does not, and probably never can exist. However, the descendants of men of ordinary calibre, even of the mental sluggards of to-day, if through a series of generations their mental faculties have been duly cultivated, may in course of time have brains as well developed as those of our present head-workers; and the descendants of the latter, particularly of those whose brains have been over-taxed, may become inferior to the descendants of the former.

deepens somewhat towards the interior. It possesses fibres from all parts of the medulla oblongata—the connection of the spinal cord with the brain-—has its own system of more delicate fibres, and cells of different sizes. The two brains are likewise closely connected by a system of fibres (pons varolii) extending round the frontal and upper portion of the medulla oblongata. (See Plate I., b.).

Professor Aeby says: At present, neither the morphological nor physiological import of the cerebellum is known.' It is said, however, by many authorities in physiology not to be connected with intelligence, but to be the 'co-ordinator of muscular movements.' From its complicated structure many special functions may be inferred, one of which, I am convinced, is connected with sexual love. Its seat in the lowest cavity (fossa) of the occipital vertebra being well marked, and its general size easily estimated, I have found by extensive experiences, positive and negative, that, as Gall taught, the instinct of sexual desire ('instinct de la propagation, instinct vénérien,' &c.) is undoubtedly a function of the cerebellum.

It appears to be likewise a regulator of locomotive movements in general. As bearing on this point, I may add that muscular exercises in youth belong to