nations of the cerebral arteries, and those of the corresponding veins; forming at the surface of the brain a vascular network, which passes into the cavities; and, in the ventricles, forms the plexus choroides and tela choroidea. The dura and pia mater were so called by the older anatomists, because they were conceived to be the origin of all the other membranes of the body.

The cerebrum or brain proper has the form of an oval, larger behind. On its outer surface are various undulating eminences, called convolutions, because they have been thought to resemble the folds of the intestines. They are separated from each other by depressions called anfractuosities. They form the hemispherical ganglion of Mr. Solly. In the brain of man, these convolutions are larger than in animals; and the anfractuosities deeper. In different brains, the number, size, and arrangement of these vary. They are not the same, indeed, in

Fig. 188.

the same individual; those of the right hemisphere being disposed differently from those of the left.

The hemispheres, it has been seen, are separated above by the falx cerebri: below, they are united by a white medullary commissure, corpus callosum, mésolobe or great commissure,-great transverse commissure of Mr. Solly. If we examine the brain at its base, we find that each hemisphere is divided into three lobes,-an anterior, which rests on the vault or roof of the orbit,-a midille or temporal, filling the middle. and lateral parts of the base of the cranium, and separated from the former by a considerable depression, called fissure of Sylvius,-and a posterior, which rests on the tentorium cerebelli. This part of the cerebrum is divided into two very distinct portions by the medulla oblongata. Anterior to it are the crura cerebri or cerebral peduncles-by most anatomists considered to be a continuation of the anterior fasciculi which form the spinal marrow and 9. Its place of junction anteriorly. 10. Pos- medulla oblongata, and proceeding to terior point of union. 11. Middle portion of form the hemispheres of the brain. nia striata. 13. Septum lucidum. 14. Fifth Between the anterior extremities of ventricle. 15. Fornix. 16. Posterior crura. 17. Plexus choroides. 18. Ergot or hippocampus the peduncles are two hemispherical minor. 19. Posterior crura of the lateral projections, called eminentie mammil lares, which are possessed by man exclusively; have the shape of a pea; and are formed of white nervous tissue externally, of gray within. Anterior to these again is the infundibulum; and a little farther forwards, the chiasma of the optic nerves or the part at which these nerves come in contact.

Superior Part of the Lateral Ventricles,
Corpora Striata, Septum Lucidum,
Fornix, &c., as given by a Transverse

Section of the Cerebrum.

1. Section of the os frontis. 2. Section of

the os occipitis. 3. Section of the ossa parietalia. 4, 5. Anterior and posterior extre

mities of the middle fissure of the cerebrum.

6. Anterior extremity of the corpus callosum. 7. Its posterior extremity joining the fornix.

8, 8. Point to where the corpus callosum joins

the lateral medullary matter of the cerebrum.

ventricle.

Laterally, and at the inferior surface of the anterior lobes, is a groove

or furrow, running from behind to before, and from without to within, in which the olfactory nerve is lodged. At the extremity of this furrow is a tubercle, which is trifling in man, but in certain animals is equal to the rest of the brain in bulk. From this the olfactory nerve has been conceived to arise. It is called the olfactory tubercle or lobe.

Fig. 189.

When we examine the interior of the brain, we find a number of parts to which the anatomist assigns distinct names. Of these the following chiefly concern the physiologist. It has been already remarked, that the corpus callosum forms at once the bond of union and of separation between the two hemispheres. It is distinctly perceived, in the form of a long and broad white band, on separating these parts from each other. Beneath the corpus callosum is the septum lucidum or median septum, which passes perpendicularly downwards,

and separates from each other the two largest cavities of the brain, the lateral ventricles. It is formed of two lamina, which leave a cavity between them, called the fifth ventricle. The fornix or inferior longitudinal commissure of Mr. Solly, whose office is to connect the anterior and posterior parts of the same hemisphere, as the transverse commissures do those of the opposite hemisphere, is placed horizontally below the last. The band of fibres which runs in each hemisphere above the corpus callosum, on the edge of the longitudinal fissure, is the superior longitudinal commissure of Mr. Solly. Its use is supposed to resemble that ascribed to the inferior longitudinal commissure. The fornix is of a triangular shape; and constitutes the upper paries of another cavity-the third ventricle. Beneath the fornix, and behind, are the pineal gland and its peduncles, forming the pineal commissure of Mr. Solly, respecting which so much has been said, by Descartes,' and others, as the seat of the soul. Within it is a small cavity; and, after six or seven years of age, it always contains some concretions. Again, anterior to the pineal gland, and immediately

Section of the Cerebrum, displaying the surfaces of the Corpora Striata, and Optic Thalami, the cavity of the Third Ventricle, and the upper surface of the Cerebellum.

a, e. Corpora quadrigemina,-a, testes; e, nates. b. Soft commissure. c. Corpus callosum. f. Anterior pilk, k. Corpora striata. 7, 1. Optic thalami. *. Anterior

lars of fornix. 9. Anterior cornu of lateral ventricle. tubercle of the left thalamus. z to 8. Third ventricle. In front of z, anterior commissure. b. Soft commissure. 8. Posterior commissure. p. Pineal gland with its peduncles, n, n. Processus a cerebello ad testes. m, m. Hemispheres of the cerebellum. h. Superior vermiform

process. i. Notch behind the cerebellum.

Tractatus de Homine, p. 5.

Fig. 190.

c. Pons Varolii. d. Middle crus of cerebellum. ee mencement of horizontal fissure. . Flocculus, or subpeduncular lobe. m. Medulla oblongata cut through. q to 8. The inferior vermiform process, lying in the vallecula. p. Pyramid. r. Uvula. n, n. Amygdala. 8. Nodule, or laminated tubercle. x. Posterior velum, partly seen. w. Right and left hemispheres of cerebellum. 3 to 7. Nerves.

Crura cerebri. i. Notch on posterior border. k. Com

3, 3. Motores oculorum. 5. Trigeminal. 6. Abducent nerve. 7. Facial and auditory nerves.

Fig. 191.

3. Its middle fossa. 4. Oblique band of medullary matter

cerebelli. 6. Same portion deprived of outer layer. 7.

Posterior Superior View of the Pons Varolii, Cere-
bellum, and Medulla Oblongata and M. Spinalis.
1, 1. Crura cerebri. 2. Pons Varolii or tuber annulare.
seen passing from its side. 5. External surface of the crus
Nervous matter which unites it to 4. 8. Trigeminus or fifth
The
comes from the corpus restiforme to the trigeminus nerve
in front, and the auditory nerve behind. 10, 11. Superior
portion of the hemispheres of the cerebellum. 12. Lobulus
amygdaloides. 13. Corpus olivare. 14. Corpus pyramidale.

below the fornix, is another cavity-the third ventricle. Its bottom is very near the base of the brain, and is formed by the nervous layer which unites the peduncles of the brain with the eminentiæ mammillares. At the sides, it has the thalami nervorum opticorum.

In the lateral ventricles, situate on each side of the corpus callosum, some parts exist which demand attention. In the upper or anterior half, commonly called anterior cornu, and in the anterior part of this, two pyri form eminences are seen, of a brownish-gray colour, which, Owing to their being formed of an assemblage of alternate layers of white and gray substance, are called corpora stri ata, the anterior cerebral gan glions of Mr. Solly. Behind these, are two whitish medullary bodies called thalami nervorum opticorum-posterior cerebral ganglions-which are situate before the corpora quadrigemina, and envelope the anterior extremities of the crura cerebri.

Three main sets of fibres may be distinguished in the medullary substance, of which the great mass of the cere brum is composed. First, the ascending fibres, which proceed from the sensory tract of the medulla spinalis, and diverge from the thalami optici to the periphery of the brain; secondly, the descending fibres, which converge from the periphery towards the corpora striata, and then pass downwards to the motor tract of the medulla spinalis; and, thirdly, the commissural fibres, which establish a connexion between the va rious parts of the periphery, and of the substance of the brain. The

pair of nerves. 9. Portion of the auditory nerve.
white neurine seen passing from the oblique band which

15. Medulla spinalis.

bulk of the human brain, and of that of the higher animals, is greatly dependent upon the large proportion borne by these last fibres to the

rest.'

The cerebellum occupies the lower occipital fossæ, or the whole of the cavity of the cranium beneath the tentorium cerebelli. It consists of two lateral hemispheres or lobes, composed of a peculiar arrangement of vesicular and tubular substance; and of a central lobe, composed also of these substances, and known by the name of the worm or vermiform process. Its size and weight, like those of the brain, differ according to the individual, and the age of the subject under examination. We do

Analytical Diagram of the Encephalon-in a Vertical Section.

8. Spinal cord. r. Restiform bodies passing to c, the cerebellum. d. Corpus dentatum of the cerebellum. 0. Olivary body. f. Columns continuous with the olivary bodies and central part of the medulla oblongata, and ascending to the tubercula quadrigemina and optic thalami. p. Anterior pyramids. v. Pons Varolii. n, b. Tubercula quadrigemina. g. Geniculate body of the optic thalamus. t. Processus cerebelli ad testes. a. Anterior lobe of the brain. q. Posterior lobe of the brain.

not observe convolutions in it. It appears rather to consist of lamina in superposition, separated from each other by furrows. We shall see,

hereafter, that the number of cerebral convolutions has been esteemed, in some respects, to accord with the intellect of the individual; and Malacarne asserts, that he has observed a similar correspondence, as regards the number of laminæ composing the cerebellum; that he found only three hundred and twenty-four in the cerebellum of an insane individual; whilst in others he had counted upwards of eight hundred. From the medullary part of the cerebellum, two large white cords pass to the pons Varolii, having the same disposition as the crura cerebri. They are the crura cerebelli.

Owing to the peculiar arrangement of the white and gray cerebral substances, when one of the hemispheres of the cerebellum is divided vertically, an arborescent appearance is presented, -the trunks of the arborization being white, the surrounding substance gray. This appearance is called arbor vite. The part where all these arborizations meet, near the centre of the cerebellum, is called corpus denticulatum seu rhomboidale. Gall was of opinion, that this body has great agency in the production of the cerebellum. Lastly, the cerebellum covers the posterior part of the medulla oblongata, and forms with it a cavity, called fourth ventricle.

The medulla oblongata is so called, because it is the continuation of the medulla spinalis in the cavity of the cranium. It is likewise termed mésocéphale, from its being continuous with the spinal marrow in one direction, and sending towards the brain strong prolongations-crura

Anterior View of the Medulla Oblongata.

p, p Pyramidal bodies, decussating at d. o, o. Olivary bodies. r, r. Restiform bodies. a, a. Arciform fibres. v. Lower fibres of the pons Varolii.

Posterior View of the Medulla Oblongata.

p, p. Posterior pyramids, separated by the posterior fissure. r, r. Restiform bodies, composed of c, c, posterior columns, and d, d, lateral part of the antero-lateral columns of the cord. a, a. Olivary columns, as seen on the floor of the fourth ventricle, separated by 8, the median fissure, and crossed by some fibres of origin of n, n, the seventh pair of nerves.

cerebri; and to the cerebellum similar prolongations-crura cerebelli; so that it appears to be the bond of union between these various parts. In its lower portion, it seems to be merely a continuation of the me