The hemispheres should now be sliced off, to a level with the corpus callosum, when the white substance of that structure will be seen connecting together both hemispheres. The large expanse of medullary matter now exposed, surrounded by the convoluted margin of grey substance, is called the centrum ovale majus of Vieussens.

  The Corpus Callosum is a thick stratum of transverse fibres, exposed at the bottom of the longitudinal fissure. It connects the two hemispheres of the brain, forming their great transverse commissure; and forms the roof of a space in the interior of each hemisphere, the lateral ventricle. It is about four inches in length, extending to within an inch and a half of the anterior, and to within two inches and a half of the posterior, part of the brain. It is somewhat broader behind than in front, and it is thicker at either end than in its central part, being thickest behind. It presents a somewhat arched form, from before backwards, terminating anteriorly in a rounded border, which curves downwards and backwards, between the anterior lobes to the base of the brain. In its course, it forms a distinct bend, named the knee, or genu, and the reflected portion, named the beak (rostrum), becoming gradually narrower, is attached to the anterior cerebral lobe, and is connected, through the lamina cinerea, with the optic commissure. The reflected portion of the corpus callosum gives off, near its termination, two bundles of white substance, which, diverging from one another, pass backwards, across the anterior perforated space, to the entrance of the fissure of Sylvius. They are called the peduncles of the corpus callosum. Posteriorly, the corpus callosum forms a thick, rounded fold, which is free for a little distance, as it curves forwards, and is then continuous with the fornix. On its upper surface, its fibrous structure is very apparent to the naked eye, being collected into coarse, transverse bundles. Along the middle line, is a linear depression, the raphe, bounded laterally by two or more slightly elevated longitudinal bands, called the stria longitudinales, or nerves of Lancisi; and, still more externally, other longitudinal striae are seen, beneath the convolution, which rests on the corpus callosum. These are the strise longitudinales laterales. The under surface of the corpus callosum is continuous behind with the fornix, being separated from it in front by the septum lucidum, which forms a vertical partition between the two ventricles. On either side, the fibres of the corpus callosum penetrate into the substance of the hemispheres, and connect together the anterior, middle, and part of the posterior lobes. It is the increased aggregation of fibres derived from the anterior and posterior lobes, which explains the great thickness of the two extremities of this commissure.

  An incision should now be made through the corpus callosum, on either side of the raphe, when two large irregular cavities will be exposed, which extend throughout the entire length of each hemisphere. These are the lateral ventricles.

  The Lateral Ventricles are serous cavities, formed by the upper part of the general ventricular space in the interior of the brain. They are lined by a thin diaphanous lining membrane, covered with ciliated epithelium, and moistened by'a serous fluid, which is sometimes, even in health, secreted in considerable quantity. These cavities are two in number, one in each hemisphere, and they are separated from each other by a vertical septum, the septum lucidum.

  Each lateral ventricle consists of a central cavity, or body, and three smaller cavities, or cornua, which extend from it in different directions. The anterior cornu, curves forwards and outwards, into the substance of the anterior lobe. The posterior cornu, called the digital cavity, curves backwards into the posterior lobe. The middle cornu, descends into the middle lobe.

  The Central Cavity, or body of the lateral ventricle, is triangular in form. It is bounded, above, by the under surface of the corpus callosum, which forms the roof of the cavity. Internally, is a vertical partition, the septum lucidum, which separates it from the opposite ventricle, and connects the under surface of the corpus callosum with the fornix. Its floor is formed by the following parts, enumerated in their order of position, from before backwards, the corpus striatum, tienia semicircularis, thalamus opticus, choroid plexus, corpus fimbriatum, and fornix.

  The Anterior Cornu is triangular in form, passing outwards into the anterior lobe, and curving round the anterior extremity of the corpus striatum. It is bounded, above and in front, by the corpus callosum; behind, by the corpus striatum.

  The Posterior Cornu, or digital cavity, curves backwards into the substance of the posterior lobe, its direction being backwards and outwards, and then inwards. On its floor is seen a longitudinal eminence, which corresponds with a deep sulcus between two convolutions: this is called the hippocampus minor. Between the middle and posterior horns, a smooth eminence is observed, which varies considerably in size in different subjects. It is called the eminentia collateralis.

  The Corpus Striatum (superior ganglion of the cerebrum), has received its name from the striated appearance which its section presents, from white fibres diverging through its substance. The intra-ventricular portion is a large pear-shaped mass, of a grey colour externally; its broad extremity is directed forwards, into the fore-part of the body, and anterior cornu of the lateral ventricle; its narrow end is directed outwards and backwards, being separated from its fellow by the thalami optici; it is covered by the serous lining of the cavity, and crossed by some veins of considerable size. The extra-ventricular portion is imbedded in the white substance of the hemisphere.

  The Taenia Semicircularis is a narrow, whitish, semi-transparent band, of medullary substance, situated in the depression between the corpus -striatum and thalamus opticus. Anteriorly, it descends in connexion with the anterior pillar of the fornix; behind, it is continued into the descending horn of the ventricle, where it becomes lost. Its surface, especially at its fore-part, is transparent, and dense in structure, and was called by Tarinus the horny band. It consists of longitudinal white fibres, the deepest of which run between the corpus striatum and thalamus opticus. Beneath it is a large vein (vena corporis striati), which receives numerous smaller veins from the surface of the corpus striatum, and thalamus opticus, and terminates in the venae Galeni.

  The Choroid Plexus is a highly vascular, fringe-like membrane, occupying the margin of the fold of pia mater (velum interpositum), in the interior of the brain. It extends, in a curved direction, across the floor of the lateral ventricle. In front, where it is small and tapering, it communicates with the choroid plexus of the opposite side, through a large oval aperture, the foramen of Monro. Posteriorly, it descends into the middle horn of the lateral ventricle, where it joins with the pia mater through the transverse fissure. In structure, it consists of minute, and highly vascular villous processes, the villi being covered by a single layer of epithelium, composed of large, round corpuscles, containing, besides a central nucleus, a bright yellow spot. The arteries of the choroid plexus enter the ventricle at the descending cornu, and, after ramifying through its substance, send branches into the substance of the brain. The veins of the choroid plexuses terminate in the venae Galeni.

  The Corpus Fimbriatum, or Taenia Hippocampi, is a narrow, white, tape-like band, situated immediately behind the choroid plexus. It is the lateral edge of the posterior pillar of the fornix, and is attached along the inner border of the hippocampus major as it descends into the middle horn of the lateral ventricle. It may be traced as far as the pes hippocampi.   The Thalami Optici and Fornix will be described when more completely exposed, in a later stage of the dissection of the brain.

  The middle cornu should now be exposed, throughout its entire extent, by introducing the little finger gently into it, aud cutting through the hemisphere, between it and the surface, in the direction of the cavity.

  The Middle, or Descending Cornu, the largest of the three, traverses the middle lobe of the brain, forming in its course a remarkable curve round the back of the optic thalamus. It passes, at first, backwards, outwards, and downwards, and then curves around the crus cerebri, forwards and inwards, nearly to the point of the middle lobe, close to the fissure of Sylvius. Its superior boundary is formed by the medullary substance of the middle lobe, and the under surface of the thalamus opticus. Its inferior boundary presents for examination the following parts: The hippocampus major, pes hippocampi, pes accessorius, corpus fimbriatum, choroid plexus, fascia dentata, transverse fissure.

  The Hippocampus Major, or Cornu Ammonis, so called from its resemblance to a ram's horn, is a white eminence, of a curved elongate form, extending along the entire length of the floor of the middle horn of the lateral ventricle. At its lower extremity it becomes enlarged, and presents a number of rounded elevations with intervening depressions, which, from presenting some resemblance to the claw of an animal, is called the pes hippocampi. If a transverse section is made through the hippocampus major, it will be seen that this eminence is the inner surface of the convolution of the corpus callosum, doubled upon itself like a horn, the white convex portion projecting into the cavity of the ventricle; the grey portion being on the surface of the cerebrum, the edge of which, slightly indented, forms the fascia dentata. The white matter of the hippocampus major is continuous through the corpus fimbriatum, with the fornix and corpus callosum.

  The PCS Acr.essorius, or Eminentia Collateralis, has been already mentioned, as a white eminence, varying in size, placed between the hippocampus major and minor, at the junction of the posterior with the descending cornu. Like the hippocampi, it is formed by white matter corresponding to one of the sulci, between two convolutions protruding into the cavity of the ventricle.

  The Corpus Fimbriatum (Taenia Hippocampi), is a narrow, tape-like band, attached along the inner concave border of the hippocampus major, and reaching down as far as the pes hippocampi. It is a continuation of the posterior pillar of the fornix, prolonged from the central cavity of the lateral ventricle.

  Fascia Dentata. On separating the inner border of the corpus fimbriatum from the choroid plexus, and raising the edge of the former, a serrated band of grey substance, the edge of the grey substance of the middle lobe, will be seen beneath it: this is the fascia dentata. Correctly speaking, it is placed external to the cavity of the descending cornu.

  The Transverse Fissure is seen on separating the corpus fimbriatum from the thalamus opticus. It is situated beneath the fornix, extending from the middle line behind, downwards on either side, to the end of the descending cornu, being bounded on one side by the fornix and the hemisphere, and on the other by the thalamus opticus. Through this fissure the pia mater passes from the exterior of the brain into the ventricles, to form the choroid plexuses. Where the pia mater projects into the lateral ventricle, beneath the edge of the fornix, it is covered by a prolongation of the lining membrane, which excludes it from the cavity.

  The Septum Lucidum forms the internal boundary of the lateral ventricle. It is a thin, semi-transparent septum, attached, above, to the under surface of the corpus callosum; below, to the anterior part of the fornix; and, in front of this, to the prolonged portion of the corpus callosum. It is triangular in form, broad in front, and narrow behind, its surfaces looking towards the cavities of the ventricles. The septum consists of two lamince, separated by a narrow interval, the fifth ventricle.

  Each lamina consists of an internal layer of white substance, covered by the lining membrane of the fifth ventricle; and an outer layer of grey matter, covered by the lining membrane of the lateral ventricle. The cavity of the ventricle is lined by a serous membrane, covered with epithelium, and contains fluid. In the foetus, and in some animals, this cavity communicates, below, with the third ventricle; but in the adult, it forms a separate cavity. In cases of serous effusion into the ventricles, the septum is often found softened and partially broken down.

  The fifth ventricle may be exposed by cutting through the septum, and attached portion of the corpus callosum, with the scissors; after examining which, the corpus callosum should be cut across, towards its anterior part, and the two portions carefully dissected, the one forwards, the other backwards, when the fornix will be exposed.

  The Fornix is a longitudinal lamella, of fibrous matter, situated beneath the corpus callosum, with which it is continuous behind, but separated from it in front by the septum lucidum. It may be divided along the middle line into two symmetrical halves, one for either hemisphere. These two portions are joined together in the middle line, where they form the body, but are separated from one another in front and behind; in front, forming the anterior crura, and behind, the posterior crura.

  The body of the fornix is triangular in form; narrow in front, broad behind. Its upper surface is connected, in the median line, to the septum lucidum in front, and the corpus callosum behind. Its under surface rests upon the velum interpo-situm, which separates it from the third ventricle, and the inner portion of the optic thalami. Its lateral edges form, on each side, part of the floor of the lateral ventricles, and are in contact with the choroid plexuses.

  The anterior crura arch downwards towards the base of the brain, separated from each other by a narrow interval. They are composed of white fibres, which descend through a quantity of grey matter in the lateral walls of the third ventricle, and are placed immediately behind the anterior commissure. At the base of the brain, the white fibres of each crus form a sudden curve upon themselves, spread out and form the outer part of the corresponding corpus albicans, from which point they may be traced upwards into the substance of the corresponding thalamus opticus. The anterior crura of the fornix are connected in their course with the optic commissure, the white fibres covering the optic thalamus, the peduncle of the pineal gland, and the superficial fibres of the tamia semicircular is.

  The posterior crura, at their commencement, are intimately connected by their upper surfaces with the corpus callosum; diverging from one another, they pass downwards into the descending horn of the lateral ventricle, being continuous with the concave border of the hippocampus major. The lateral thin edges of the posterior crura have received the name corpus Fimbriatum, already described. On the under surface of the fornix, towards its posterior part, between the diverging posterior crura, may be seen some transverse lines, and others longitudinal or oblique. This appearance has been termed the lyra, from the fancied resemblance it bears to the strings of a harp.

  Between the anterior pillars of the fornix and the anterior extremities of the thalami optici, an oval aperture is seen on each side, the foramen of Monro. The two openings descend towards the middle line, and joining together, lead into the upper part of the third ventricle. These openings form a transverse communication between the lateral ventricles, and below with the third ventricle.

  Divide the fornix across anteriorly, and reflect the two portions, the one forwards, the other backwards, when the velum interpositum will be exposed.

  The Velum Interpositum is a vascular membrane, reflected from the pia mater into the interior of the brain through the transverse fissure, passing beneath the posterior rounded border of the corpus callosum and fornix, and above the corpora quadrigemina, pineal gland, and optic thalami. It is of a triangular form, and separates the under surface of the body of the fornix from the cavity of the third ventricle. Its posterior border forms an almost complete investment for the pineal gland. Its anterior extremity, or apex, is bifid; each bifurcation being continued into the corresponding lateral ventricle, behind the anterior crura of the fornix, forming the anterior extremity of the choroid plexus. On its under surface are two vascular fringes, which diverge from each other behind, and project into the cavity of the third ventricle. These are the choroid plexuses of the third ventricle. To its lateral margins are connected the choroid plexuses of the lateral ventricles. The arteries of the velum interpositum enter from behind, beneath the corpus callosum. Its veins, the venae Galeni, two in number, run along its under surface; they are formed by the vena} corporis striati and the venae plexus choroides: the venae Galeni unite posteriorly into a single trunk, which terminates in the straight sinus.

  The velum interpositum should now be removed. This must be effected carefully, especially at its posterior part, where it invests the pineal gland; the thalami optici will then be exposed with the cavity of the third ventricle between them (fig. 24-6).

  The Thalami Optici {Superior Ganglia of the Cerebrum) are two large oblong masses, placed between the diverging portions of the corpora striata; they are of a white colour superficially, internally they are composed of white fibres intermixed with grey matter. Each thalamus rests upon its corresponding crus cerebri, which it embraces. Externally, it is bounded by the corpus striatum and tnenia semicircularis, and is continuous with the hemisphere. Internally, it forms the lateral boundary of the third ventricle; and running along its upper border is seen the peduncle of the pineal gland. Its upper surface is free, being partly seen in the lateral ventricle; it is partly covered by the fornix, and marked in front by an eminence, the anterior tubercle. Its under surface forms the roof of the descending cornu of the lateral ventricle; into it the crus cerebri passes. Its posterior and inferior part, which projects into the descending horn of the lateral ventricle, presents two small round eminences, the internal and external geniculate bodies. Its anterior extremity, which is narrow, forms the posterior boundary of the foramen of Monro.

  The Third Ventricle is the narrow oblong fissure placed between the thalami optici, and extending to the base of the brain. It is bounded above by the under surface of the velum interpositum, from which are suspended the choroid plexuses of the third ventricle, and laterally by two white tracts, one on either side, the peduncles of the pineal gland. Its floor, somewhat oblique in its direction, is formed, from before backwards, by the parts which close the interpeduncular space, viz., the lamina cinerea, the tuber cinereum and infundibulum, the corpora albicantia, and the locus perforatus; its sides, by the optic thalami; in front, by the anterior crura of the fornix and part of the anterior commissure; behind, by the posterior commissure and the iter a tertio ad quartum ventriculum.

  The cavity of the third ventricle is crossed by three commissures, named, from their position, anterior, middle, and posterior.

  The Anterior Commissure is a rounded cord of white fibres, placed in front of the anterior crura of the fornix. It perforates the corpus striatum on either side, and spreads out into the substance of the hemispheres, over the roof of the descending horn of the lateral ventricle.

  The Middle or Soft Commissure consists almost entirely of grey matter. It connects together the thalami optici, and is continuous with the grey matter lining the anterior part of the third ventricle.

  The Posterior Commissure, smaller than the anterior, is a flattened white band of fibres, connecting together the two thalami optici posteriorly. It bounds the third ventricle posteriorly, and is placed in front of and beneath the pineal gland, above the opening leading to the fourth ventricle.

  The third ventricle has four openings connected with it. In front are two oval apertures, one on either side, the foramina of Monro, through which the third communicates with the lateral ventricles. Behind, is a third opening leading into the fourth ventricle by a canal, the aqueduct of Sylvius, or iter a tertio ad quartum ventriculum. The fourth, situated in the anterior part of the floor of the ventricle, is a deep pit, which leads downwards to the funnel-shaped cavity of the infundibulum (iter ad infundibulum).

  The lining membrane of the lateral ventricles is continued through the foramina of Monro into the third ventricle, and extends along the iter a tertio into the fourth ventricle; at the bottom of the iter ad infundibulum it ends in a cul-de-sac.

  Grey Matter of the Third Ventricle. A layer of grey matter covers the greater part of the surface of the third ventricle. In the floor of this cavity it exists in great abundance, and is prolonged upwards on the sides of the thalami, extending across the cavity as the soft commissure; below, it enters into the corpora albicantia, surrounds in part the anterior pillars of the fornix, and ascends on the sides of the septum lucidum.

  Behind the third ventricle, and in front of the cerebellum, are the corpora quadrigemina, and resting upon these the pineal gland.

  The Pineal Gland (Conarium), so named from its peculiar shape (pinus, the fruit of the fir), is a small reddish-grey body, conical in form, placed immediately behind the posterior commissure, and between the nates, upon which it rests. It is retained in its position by a duplicature of pia mater, derived from the under surface of the velum interpositum, which almost completely invests it. The pineal gland is about four lines in length, and from two to three in width at its base, and is said to be larger in the child than in the adult, and in the female than in the male. Its base is connected with the cerebrum by some transverse commissural fibres derived from the posterior commissure, and by four slender peduncles, formed of medullary fibres. Of these, the two superior pass forwards upon the upper and inner margin of the optic thalami, to the anterior crura of the fornix, with which they become blended. The inferior peduncles pass vertically downwards from the base of the pineal body, along the back part of the inner surface of the thalami, and are only seen on a longitudinal vertical section through the gland. The pineal gland is very vascular, and consists chiefly of grey matter, with a few medullary fibres. In its base is a small cavity, said by some to communicate with that of the third ventricle. It contains a transparent viscid fluid, and occasionally a quantity of sabulous matter, named acervulus cerebri, composed of phosphate and carbonate of lime, phosphate of magnesia and ammonia, with a little animal matter. These concretions are almost constant in their existence, and are found at all periods of life. When this body is solid, the sabulous matter is found upon its surface, and occasionally upon its peduncles.

  On the removal of the pineal body and adjacent portion of pia mater, the corpora quadrigemina are exposed.

  The Corpora or Tubercula Quadrigemina (optic lobes) are four rounded eminences placed in pairs, two in front, two behind, and separated from another by a crucial depression. They are situated immediately behind the third ventricle and posterior commissure, beneath the posterior border of the corpus callosum, and above the iter a terlio ad quartum ventriculum. The anterior pair, the nates, are the larger, oblong from before backwards, and of a grey colour. The posterior pair, the testes, are hemispherical in form, and lighter in colour than the preceding. They are connected on each side with the thalamus opticus and commencement of the optic tracts, by means of two white prominent bands, termed brachia. Those connecting the nates with the thalamus (brachia anteriora) are the larger, and pass obliquely outwards. Those connecting the testes with the thalamus, are called the brachia posteriora. Both pairs, in the adult, are quite solid, being composed of white matter externally, and grey matter within. These bodies are larger in the lower animals than in man. In fishes, reptiles, and birds, they are only two in number, and called the optic lobes, from their connection with the optic nerves; and are hollow in their interior; but in mammalia they are four in number, as in man, and quite solid. In the human foetus they are developed at a very early period, and form a large proportion of the cerebral mass; at first they are only two in number, as in the lower mammalia, and hollow in their interior.

  These bodies, from below, receive white fibres from the olivary fasciculus or fillet; they are also connected with the cerebellum, by means of a large white cord on each side, the processus ad testes, or superior peduncles of the cerebellum, from the corpora quadrigemina, these tracts pass upwards to the thalami.

  The Valve of Vieussens is a thin translucent lamina of medullary substance, stretched between the two processus e cerebello ad testes; it covers in the canal leading from the third to the fourth ventricle, forming part of the roof of the latter cavity. It is narrow in front, where it is connected with the testes; and broader behind, at its connection with the vermiform process of the cerebellum. A slight elevated ridge (the frenulum) descends upon the upper part of the valve from the corpora quadrigemina, and on either side of it may be seen the fibres of origin of the fourth nerve. Its lower half is covered by a thin transversely grooved lobule of grey matter prolonged from the anterior border of the cerebellum; this is called the linguetta laminosa.

  The Corpora Genicnlata are two small flattened oblong masses, placed on the outer side of the corpora quadrigemina, and on the under and back part of each optic thalamus, and are named from their position, corpus geniculatum externum and internum. They are placed one on the outer and one on the inner side of each optic tract. In this situation, the optic tract may be seen dividing into two bands, one of which is connected with the external geniculate body and nates, the other being connected with the internal geniculate body and testis.

  Structure of the Cerebrum. The white matter of each hemisphere consists of three kinds of fibres. I. Diverging or peduncular fibres, which connect the hemion the anterior to that on the posterior border. It is divided into three lobes; the lobulus centralis, a small lobe, situated in the incisura anterior; the monticulus cerebelli, the central projecting part of the process; and the commissura simplex, a small lobe near the incisura posterior.

  The under surface of the cerebellum is subdivided into two well marked convex lateral hemispheres by a depression, the valley, which extends from before backwards in the middle line. The lateral hemispheres are lodged in the inferior occipital fossae; the median depression, or valley, receives the back part of the -medulla oblongata, is broader in the centre than at either extremity, and has, projecting from its floor, part of the median lobe of the cerebellum, called the inferior vermiform process. The parts entering into the composition of this body are, from behind forwards, the commissura brevis, situated in the incisura posterior; in front of this, a laminated conical projection, the pyramid; more anterior, a larger eminence, the uvula; placed between the two rounded lobes which occupy the sides of the valley, the amygdala; and connected with them by a commissure of grey matter, indented on the surface, and called the furrowed band. In front of the uvula is the nodulus: it is the anterior pointed termination of the inferior vermiform process, and projects into the cavity of the fourth ventricle; it has been named by Malacarne the laminated tubercle. On each side of the nodule is a thin layer of white substance, attached externally to the flocculus, and internally to the nodule, and to a corresponding part on the opposite side; they form together the posterior medullary velum, or commissure of the flocculus. They are usually covered in and concealed by the amygdalae, and cannot be seen until these are drawn aside. This band is of a semilunar form on each side, its anterior margin being free and concave, its posterior being attached just in front of the furrowed band. Between it and the nodulus and uvula behind, is a deep fossa, called the swallow's nest (nidus hirundinis).

  Lobes of the Cerebellum. Each hemisphere is divided into an upper and a lower portion by the great horizontal fissure, which commences in front at the pons, and passes horizontally round the free margin of either hemisphere, backwards to the middle line. From this primary fissure numerous secondary fissures proceed, which separate the cerebellum into lobes.

  Upon the upper surface of either hemisphere there are two lobes, separated from each other by a fissure. These are the anterior or square lobe, which extends as far back as the posterior edge of the vermiform process, and the posterior or semi-lunar lobe, which passes from the termination of the preceding to the great horizontal fissure.

  Upon the under surface of either hemisphere there are five lobes, separated by Kulci; these are from before backwards; the flocculus or sub-peduncular lobe, a prominent tuft, situated behind and below the middle peduncle of the cerebellum; its surface is composed of grey matter, subdivided into a few small laminse: it is sometimes called the pneumogastric lobule, from being situated behind the pneu-mogastric nerve. The amygdala or tonsil is situated on either side of the great median fissure or valley, and projects into the fourth ventricle. The digastric lobe is situated on the outside of the tonsil, being connected in part with the pyramid. Behind the digastric is the slender lobe, which is connected with the back part of the pyramid and the commissura brevis: and most posteriorly is the inferior posterior lobe, which also joins the commissura brevis in the valley.