PONS VAROLII.-THE CEREBRUM.

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annulare). (See Figs. 3 and 4 c.) This is a comparatively small portion of the encephalon, which occupies a central position on its under surface, above and in front of the medulla oblongata, below and behind the crura cerebri a, and between the middle crura of the cerebellum c, with all which parts it is connected.' By the term 'crura cerebri,' introduced in this description, is meant the 'legs' or roots of the cerebrum, or the two bundles of nerve that unite it with the parts below. The crura of the cerebellum express in like manner the several connexions of that centre with the other centres. On account of the intermediate and connecting position of the pons, it has also been called the middle-brain (meso-cephalon). From its embracing as in a ring the medulla oblongata and stems of the cerebrum, it has derived the name of annular protuberance; the other name, 'pons,' or bridge, expresses the same circum

stance.

'The substance of the pons Varolii consists of transverse and longitudinal white fibres, interspersed with a quantity of diffused grey matter. The transverse fibres, with a few exceptions, enter the cerebellum under the name of the middle crura or peduncles, and form a commissural (or connecting) system for its two hemispheres. The longitudinal fibres are those which ascend from the medulla oblongata into the crura cerebri, augmented, it would seem, by others which arise within the pons from the grey matter scattered through it.'-Quain, p. 689. The pons is thus mainly a grand junction between the medulla oblongata and spinal cord below, the cerebrum above, and the cerebellum behind. The existence of a considerable amount of the grey or vesicular matter proves that conduction or communication is not the sole function of this part of the brain.

(3.) The cerebrum or brain proper (Figs. 3 and 4, A), as already mentioned, is the highest, and by far the largest portion of the encephalon. It is of an ovoid (or egg) shape, but is irregularly flattened on its under side. It is placed in the cranium with its small end forwards, its greatest width being opposite to the parietal eminences.

'The cerebrum consists of two lateral halves, or hemi

spheres, as they are called, which, though connected by a median portion of nervous substance, are separated in a great

FIG. 4.*

part of their extent by a fissure, named the great longitudinal fissure, which is seen on the upper surface of the brain, and partly also on its base.

The cerebral hemispheres are not plain or uniform upon the surface, but are moulded into numerous smooth and tortuous eminences, named convolutions, or gyri, which are marked off from each other by deep furrows, called sulci, or anfractuosities. These convolutions are coloured externally;

Shows the under surface or base of the encephalon freed from its membranes. A. Cerebrum. f, g, h. Its anterior, middle, and posterior lobes. B. Cerebellum. C. Pons Varolii. D. Medulla Oblongata. d. Peduncle of cerebrum. 1 to 9, indicate the several pairs of cerebral nerves, numbered according to the usual notation, viz.-1. Olfactory nerve. Optic. 3. Motor nerve of eye. 4. Pathetic. 5. Trifacial. 6. Abducent nerve of eye. 7. Auditory, and 7′, Facial. 8. Glosso-pharyngeal. 8'. Vagus. 8". Spinal accessory nerve. 9. Lingual or hypoglossal nerve.

2.

for the surface of the cerebral hemispheres, unlike the parts hitherto described, is composed of grey matter.'-QUAIN, 690.

The complete description of the cerebrum, includes an account of the external surface, with its convolutions and the various masses that make up the interior and in part appear at the base of the brain. Although in the highest degree interesting as a study, no important application to our present subject arises out of such minute knowledge. There are, however, a few particulars that it is of use for us to add, selected out of the elaborate detail of cerebral Anatomy.

A distinction exists between the convoluted mass of the hemispheres and certain enclosed smaller masses of the cerebrum. Of these there are two that are usually named together, partly on account of their proximity, and partly because it has not been practicable hitherto to distinguish their functions. They are the optici thalami and corpora striata, being double and symmetrical on the two sides. They both lie imbedded in the heart of the hemispheres. The peduncles or stems of the cerebrum pass into them before spreading-out into the mass of the hemispheres. The third important mass is termed the corpora quadrigemina (quadruple bodies), from consisting of four rounded masses put together in a square. This portion is more detached than the two others, and finds a place between the cerebrum and cerebellum. In some of the inferior animals it is very large, and takes a prominent position in the general structure of the brain; whereas the two other masses above mentioned for the most part rise and fall according to the degree of development of the convoluted hemispheres. Hence the comparative Anatomist assigns to the quadruple bodies a character and function apart from the rest of the cerebrum. I quote a short description of each of the three centres.

The corpora striata are two large ovoid masses of grey matter, the greater part of which is imbedded in the middle of the white substance of the hemisphere of the brain.' 'The surface

See in Fig. 3, the two rounded eminences behind 6, the superior peduncle of the cerebellum. These represent the corpora quadrigemina in section.

of the corpus striatum is composed of grey matter. At some depth from the surface white fibres may be seen cutting into it, which are prolonged from the corresponding cerebral peduncle, and give it the streaked appearance from which it has received its name.'

'The thalami optici (posterior ganglia of the brain) are of an oval shape, and rest on the corresponding cerebral crura, which they in a manner embrace. On the outer side each thalamus is bounded by the corpus striatum, and is then continuous with the white substance of the hemisphere.' 'The inner sides of the two thalami are turned to each other.' 'The optic thalami are white on the surface, and consist of several layers of white fibres intermixed with grey matter.'

'In front of the cerebellum are certain eminences, which may be reached from the surface of the brain. These are the corpora quadrigemina, and above them is the pineal gland.'

(The pineal gland (conarium) so named from its shape (pinus, conus, the fruit of the fir), is a small reddish body, which rests upon the anterior pair of the corpora quadrigemina.' 'It is about three lines (a quarter of an inch) in length, and its broad part, or base, is turned forwards, and is connected with the rest of the cerebrum by white substance.")

'The corpora or tubercula quadrigemina are four rounded eminences, separated by a crucial depression, placed two on each side of the middle line, one before the other. They are connected with the back of the optici thalami, and with the cerebral peduncles at either side.'

'The upper or anterior tubercles, are somewhat larger and darker in colour than the posterior. In the adult, both pairs are solid, and are composed of white substance outside, containing grey matter within.

'They receive bands of white fibres from below.'-'A white cord also passes up on each side from the cerebellum to the corpora quadrigemina, and is continued onwards to the thalami: 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.'

CORPORA QUADRIGEMINA.

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'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 foetus 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 spaces extending in various directions, and serving as boundaries to the other parts.*

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 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 surfaces 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, PP. 701-2.