THE MEDULLA OBLONGATA.

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and the Medulla Oblongata. The cerebrum, which is the highest and by far the largest part of the human encephalon, occupies the upper and larger portion of the cranial cavity.' 'The cerebellum is placed beneath the hinder part of the cerebrum, by which it is completely overlapped.' The pons Varolii is in the base of the brain near the entrance of the spinal cord, and connects together the three other parts,the cerebrum, the cerebellum, and the medulla oblongata. The medulla oblongata connects the spinal cord with the brain.

6. In giving a more detailed description of those four parts, it will be convenient to take them in an inverse order, beginning from below, or where the brain joins the spinal cord.

(1.) The Medulla Oblongata.-This portion is continuous below with the spinal cord, of which it seems an expansion; lying wholly within the cranial cavity, its upper end passes into the pons Varolii. See Figs. 3 and 4, D.

'It is of a pyramidal form, having its broad extremity turned upwards, from which it tapers to its point of connexion with the spinal cord; it is expanded laterally at its upper part. Its length from the pons to the lower extremity of the pyramids is about an inch and a quarter; its greatest breadth is about three quarters of an inch; and its thickness from before backwards about half an inch.

In form and general anatomical characters, the medulla oblongata very much resembles the cord, of which it is a prolongation upwards to the brain. It is not our purpose here to enter into the minute anatomy of the part, or to set forth the points of difference between it and the cord; we need only observe that in it the white and grey constituents of the cord are both increased in size and altered in arrangement. The grey matter especially becomes more abundant, and additional deposits occur. The medulla oblongata has thus more of the character of an independent centre of nervous

Sylvius, which separates the anterior and middle lobes. B, Cerebellum. C, Pons Varolii. D, Medulla oblongata. a, Peduncles of cerebrum; b, Superior; c, Middle; and d, Inferior peduncles of cerebellum.'—QUAIN.

action, as well as of a grand junction, than belongs to the cord. It gives origin to nerves of a very special and important nature.

(2.) The Pons Varolii, or annular protuberance (tuber 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 nerves 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 circumstance.

'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.' 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 simple 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

THE CEREBRAL HEMISPHERES.

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portion of the encephalon. It is of an ovoid (or egg) shape, but is regularly 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 hemispheres, 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.

* 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. 2, Optic. 3. Motor nerve of eye. 4, Pathetic. 5, Trifacial. 6, Abducent nerve of eyc. 7, Auditory; and 7', Facial. 8, Glosso-pharyngeal. 8', Vagus. 8", Spinal accessory nerve. 9, Lingual or hypoglossal nerve.

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

The complete description of the cerebrum includes an account of the external surface with its convolutions, and of 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 to distinguish completely 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

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

THE GANGLIA OF THE BRAIN.

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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 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 side 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 eminencos, 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: