(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 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 A, 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 laminæ 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, 720-2.*

*The above is a brief outline of the parts of the brain, as given in the best works on human Anatomy. I shall here append a view of its divisions founded on the comparative Anatomy of the vertebrate series of animals, and with reference to the analysis of the cranium into vertebral sections. It is supposed that the bony parts of the head and face of any animal in this series is made up of four vertebræ, expanded and transformed for the accommodation of the brain, senses, and the other organs that distinguish

INTERNAL STRUCTURE OF THE BRAIN.

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7. We must next attend to the internal structure of the brain, considered as made up of the two kinds of matter, the grey, vesicular, or central substance, and the white, fibrous, or communicating substance. 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. The subject is still a very obscure one, but not so obscure as it has been, and we can even now learn from it a better mode of conceiving the workings of the nervous system than what has come down to us from the times when nothing whatever was known. I still quote from Dr. Sharpey.

the head from the rest of the spine. On a similar supposition, the brain would be looked upon as an expansion of as much of the spinal cord as would extend over the length of four vertebræ. When we descend in the scale, as low as fishes, we find a most apparent division of the encephalon into four segments, corresponding with the four vertebræ, whose expansion makes the head.

Proceeding upon this hint, Professor Owen makes a classification of the parts of the brain, which he considers applicable alike to the highest and lowest members of the vertebrate class. Beginning from behind, where the encephalon joins the spinal cord, he enumerates as follows, specifying at the same time what he considers the functions of the several segments.

I. Encephalon (Hind-brain). This includes the hinder parts of the mass, namely, the medulla oblongata, the pons Varolii, and the cerebellum. These parts together form an aggregate centre for sustaining the functions of Respiration and Digestion, and for performing combined and rhythmical movements. The two first functions, Respiration and Digestion, are commonly conceived as attaching to the medulla oblongata. The pons Varolii is far more of a connecting organ than a centre. To the cerebellum belongs, as is supposed, the function of harmonizing complex movements, of the instinctive kind, such as walking on all fours. Of the functions of the brain, we shall, however, speak particularly again.

II. Mesencephalon (Middle-brain). The parts here intended, are those next in order to the previous. They are the enclosed space, called the third ventricle, the corpora quadrigemina, with its connected organ, the pineal gland, and another small round mass in the same region named the Pituitary body or gland. This is considered the Centre of Vision, and of the movements prompted and regulated by vision.

III. Prosencephalon (Fore-brain). The hemispheres, including the corpora striata and thalami optici. This is reckoned the seat of the higher functions of mind, namely, Consciousness, Volition, and Intelligence. It is the portion whose enlargement distinguishes the human subject. In the fish and reptile it is surpassed in size by the members of the middle brain, the corpora quadrigemina being the chief of these.

IV. Rhinencephalon (Nose-brain). The olfactory lobe and crura. In man this is a very insignificant mass, lying over the nose and between the eyes. In the lowest vertebrate animals, it stands forward as the terminating segment of the brain. It is the Centre of Smell.

'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.

'It may suffice here to remark, that one large body of fibres can be traced upwards from the spinal cord to the grey matter situated in different regions of the encephalon; some of these fibres reaching as high as the cortical layer on the surface of the cerebrum and cerebellum, others apparently terminating in the corpus striatum, thalamus opticus, corpora quadrigemina, and other special deposits of grey substance. These fibres are generally believed to be continued by their lower ends into the spinal nerves, though it is also supposed that part of them may terminate below in the grey matter of the cord. Other fibres pass between different parts of the encephalon itself, serving most probably to connect its different masses of grey substance; among the most conspicuous examples of these may be adduced, the fibres connecting the cerebrum and cerebellum, forming what are called the superior cerebellar peduncles; fibres passing up from the grey matter in the medulla oblongata and pons Varolii, in company with those from the spinal cord, and having probably a similar connexion superiorly: fibres radiating from the corpus striatum to the cortical grey matter of the cerebrum; fibres between adjacent or distant convolutions; and, lastly, the vast body of fibres belonging to the commissures of the cerebrum and middle crura of the cerebellum which pass from one side of the encephalon to another.'-QUAIN, Introduction, p. ccii.

The following is an interesting classification of the dif ferent fibres of the cerebrum, and will serve to enhance the effect of the foregoing extract.

'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

FIBRES OF THE CEREBRUM.

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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.'-QUAIN, p. 736.

This general classification is followed out by the author into minute details, full of interest in themselves, but too technical and too little instructive as regards the workings of mind, to be farther dwelt upon here. We shall now give an extract on the distribution of the grey matter, and then pass to the general view of the mechanism and mode of working of the brain, suggested by these descriptions of its component structure.

'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 regret to have to omit a portion of the connected account of the spread of the grey matter in the parts in the interior and base of the 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 of nerve force, as well as being 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.'-p. 744

8. Plan of Structure indicated by the above arrangement of white and grey substance.-The object of the present chapter being to ascertain, as far as possible, the mode of working of the brain and the connexion of its mechanism with the mental functions, we may here take a summary view of the plan of structure indicated by the foregoing description. We shall thus prepare the way for discussing, at a later stage, the precise kind of action that seems to be maintained throughout the different parts of the nervous system.

It would appear, then, 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, being made of cells or vesicles; 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.

Beginning with the spinal cord,-which we have seen to be a rod or column of white matter or fibres, enclosing a