these two white cords are the superior peduncles of the cerebellum. At each side, the corpora quadrigemina send off two white tracts, which pass to the thalami and to the commencements of the optic nerves.'

'In the human brain these quadrigeminal bodies are small in comparison with their size in the series of animals. In ruminant, soliped, and rodent animals, the anterior tubercles are much larger than the posterior, as may be seen in the sheep, horse, and rabbit. In the brains of carnivora, the posterior tubercles are rather the larger.'

'In the fœtus this part of the brain appears very early, and then forms a large proportion of the cerebral mass. The eminences are at first single on each side, and hollow. They are constant in the brains of all vertebrate animals, but in fishes, reptiles, and birds, they are only two in number, and hollow. In marsupialia and monotremata, they are also two in number, but solid.'

In this brief allusion to the different parts composing the cerebrum, we have had to exclude the mention of many smaller portions. We have also avoided all allusion to the ventricles of the brain. These are enclosed spades extending in various directions, and serving as boundaries to the other parts.*

(4.) The cerebellum, little brain, or after brain (Figs. 3 and 4, B), consists of a body and three pairs of crura or peduncles, by which it is connected with the rest of the encephalon. They are named superior, middle, and inferior peduncles.

* The following passage may assist in giving a connected view of the cerebrum, and also of the nature of the ventricular cavities or space.

'The hemispheres are connected together in the middle by the corpus callosum, and it is obvious that the structures filling up the interpeduncular space, serve also as connecting media. Between the corpus callosum above and the peduncles below, the two hemispheres are partially separated from each other, so as to leave an interval, the general ventricular space, across which some slighter connecting portions of nervous substance pass from one hemisphere to another.

'Again, as seen in a transverse vertical section of the cerebrum, the peduncles diverge as they ascend towards the hemispheres, and pass on each side through two large masses of grey matter, sometimes called ganglia of the brain, at first through the thalamus opticus, and afterwards through a much larger mass named corpus striatum. These two masses of grey matter project

'The superior peduncles (Fig. 3, b) connect the cerebellum with the cerebrum through the corpora quadrigemina, as already stated. The inferior peduncles d, pass downward to the back part of the medulla oblongata. The middle peduncles, c, pass from the middle of the cerebellum around the outer side of the crura of the cerebrum, and meet in front of the pons Varolii, constituting its transverse fibres. They connect the two halves of the cerebellum below. All these peduncles consist of white fibres only; and they pass into the interior of the cerebellum at its fore part.'

'The body of the cerebellum B, being covered with cortical substance, is of a grey colour externally, but is rather darker on the surface than the cerebrum. Its greatest diameter is transverse it is about three and a half or four inches wide, about two or two and a half from before backwards, and about two inches deep in the thickest part, but is much thinner all round its outer border.'

'It consists of two lateral hemispheres, joined together by a median portion called the worm, or vermiform process, which in birds, and in some animals still lower in the scale, is the only part existing.'

'The body of the cerebellum at the surface, and for some depth, consists of numerous nearly parallel lamina or folia, which are composed of grey and white matter, and might be compared with the gyri or convolutions of the cerebrum, but are smaller and not convoluted. These are separated by sulci of different depths.'-QUAIN.

somewhat, as smooth convex eminences, on the upper and inner surface of the diverging fibres of the peduncles. Immediately above the thalami and corpora striata, the hemispheres are connected together across the median plane by the corpus callosum; and it is between the under surface of the latter, and the upper surface of the eminences mentioned and the interpeduncular structures, that the general ventricular space is situated in the interior of the cerebrum. The upper part of this space is again divided by a median vertical partition, so as to form the two lateral ventricles: below this, it forms a single cavity named the third or middle ventricle, which communicates with both the lateral ventricles above, and, below, with the ventricle of the cerebellum or fourth ventricle. The median vertical partition, which separates the lateral ventricles from each other, consists at one part (septum lucidum) of two layers, between which is contained the fifth and remaining ventricle of the brain.'-QUAIN.

7. We must next attend to the internal structure of the brain, considered as made up of the two kinds of matter, the grey and the white. The distribution and arrangement of those two kinds of matter throw light upon the mode of action, or the peculiar kind of activity that distinguishes the brain.

'White Part of the Encephalon.-The white matter of the encephalon consists of tubular fibres. The general direction which they follow is best seen in a brain that has been hardened by immersion in spirits, although it is true that we do not then trace the single fibres, but only the fine bundles and fibrous lamella which they form by their aggregation.'

The fibres of the cerebrum, though exceedingly complicated in their arrangement, and forming many different collections, may be referred to three principal systems, according to the general course which they take, viz.-1. Ascending or peduncular fibres, which pass up from the medulla oblongata to the hemispheres, and constitute the two crura or peduncles of the cerebrum. They increase in number as they ascend through the pons, and still further in passing through the optic thalami and striated bodies, beyond which they spread in all directions into the hemispheres. These were named by Gall the diverging fibres. 2. Transverse or commissural fibres, which connect the two hemispheres together. 3. Longitudinal or collateral fibres, which, keeping on the same side of the middle line, connect more or less distant parts of the same hemisphere together.'

'Grey Matter of the Encephalon.-Considering the imputed physiological importance of the grey nervous substance, it may be well to mention connectedly the different positions in which it is found in the several parts of the encephalon.'

'By far the larger amount is situated upon the convoluted surface of the cerebrum and the laminated surface of the cerebellum, forming, in each case, the external cortical layer of cineritious matter.'

I omit a portion of the connected account of the spread of the grey matter in the parts in the interior and base of the

DISTRIBUTION OF GREY MATTER.

27

brain, as including a number of terms that the reader has not been prepared for in the present sketch of the nervous system. We must rest satisfied with perusing, in addition to the above, the account of the distribution of grey substance in the larger portions, and in the parts already in some degree known to us.

'In the crura cerebri, the grey matter is collected into a dark mass; below this it is continuous with that of the pons and medulla oblongata, and through them with the spinal cord.' Thus, though the crura cerebri are, in the main, connexions of white matter between the hemispheres and the parts below, yet, like the medulla oblongata and spinal cord, they contain in the interior a portion of the grey matter, and are to that extent centres and junctions, as well as conductors.

'In the centre of each of the corpora quadrigemina, grey matter is also found, and it occurs in the pineal gland (and in the corpora geniculata). These last bodies appear to be appendages of the large masses of grey matter, situated in the interior of the cerebrum, named the optic thalami; which again are succeeded by the still larger collections of this substance, and indeed the largest situated within the brain,—— viz., the corpora striata.'

8. Plan of Structure indicated by the above arrangement of white and grey substance.—It would appear, thus, that the cerebro-spinal centre, or the brain and spinal cord taken together, is an aggregate of distinct nervous masses or parts, each made up of a mixture of white and grey matter. The grey matter is the vesicular substance, consisting of cells or corpuscles; the white matter is the fibrous substance, being made up of fibres bundled together. The grey matter is a terminus; to it the fibrous collections tend, or from it commence. The fibrous matter contained within any of the cerebral masses is placed there as a means of communicating with some portion or other of the layers, or other collections, of grey substance.

Assuming that one class of nerve fibres (the sensory or incarrying)-those distributed to senses, viscera, &c.—are

employed in conveying influence from without inwards; and the other class (motory or outcarrying)-distributed to muscles, in conveying influence from within outwards,-we find that both classes are usually mixed together in the same ramifying branches, and in the common stem of white matter in the spinal cord. Let us imagine, however, the two kinds separated; the sensory nerves all emerging from the centres on one hand, and the motory nerves emerging apart on the other. We can then express the plan of the brain thus:— The sensory or incarrying fibres arising from the extremities enter the cord, proceed a certain way there, and begin to drop into corpuscles; from these corpuscles fresh fibres arise and proceed, some onwards and some laterally, to other cells; and so on. Thus, in the spinal cord, medulla oblongata, pons Varolii, &c.-up to the cerebral hemispheres, there is a repeated system of fibres passing into cells, and new fibres emerging, and going on to other cells; giving birth to an endless system of cross communications, like the railway network of England. Adverting now to the enormous connecting mass of fibres-ascending, diverging, and transverse-that make up the white substance of the brain, we must consider how the multiplication has been effected. There is only one conceivable process, when we consider that the entire mass is in communication, through cells, with the diminutive mass of the spinal cord. The process

is this. For one fibre coming up from the sense organs and dropping into a cell, two, three, four, or more must emerge; each of these again, proceeding onwards to a new cell, and there replaced by three, four, &c., new fibres; and so on, until the expansion or multiplication is completed. Within the spinal cord, where there is no increase of bulk, the multiplying process is not begun; but in the upper course of the cord, where it enters the brain, there is an arithmetical necessity for the multiplication. We can hardly avoid the supposition that the corpora striata and the thalami optici, through which the great stem of the brain diffuses itself (by the ascending fibres) in the white matter of the hemi