Anatomy The Cerebellum

fissure posterior parietal lobe anterior grey hemisphere surface matter white

ANATOMY THE CEREBELLUM, LITTLE BRAIN, Or AFTER BRAIN (P1. XVIII. fig. 2, c), occupies the inferior pair of occipital foss, and, along with the pons and medulla oblongata, lies below the plane of the tentorium cerebelli. It consists of two hemispheres or lateral lobes, and of a median or central lobe, which in human anatomy is called the vermiform process. It is connected below with the medulla oblongata by the two restiform bodies which form its inferior peduncles, and above to the corpora quadrigemina of the cerebrum by two bands, which form its superior peduncles; whilst the two hemispheres are connected together by the transverse fibres of the pens, which form the middle peduncles of the cerebellum. On the superior or tentorial surface of the cerebellum the median or vermiform lobe is a mere elevation, but on its inferior or occipital surface this lobe forms a well-defined inferior vermiform process, which lies at the bottom of a deep fossa or vallecula; this fossa is prolonged to the posterior border of the cerebellum, and forms there a deep notch which separates the two hemispheresfrom each other; in this notch the falx cerebelli is lodged. Extending horizontally backwards from the middle cerebral peduncle, along the outer border of each hemisphere is the great horizontal fissure, which divides the hemisphere into its tentorial and occipital surfaces. Each of these surfaces is again subdivided by fissures into smaller lobes, of which the most important arc the amygdala or tonsil, which forms the lateral boundary of the anterior part of the vallecula, and the jlocculus, which is situated immediately behind the middle peduncle of the cerebellum. The inferior vermiform process is subdivided into a posterior part or pyramid ; an elevation or uvula, situated between the two tonsils ; and an anterior pointed process or nodule. Stretching between the two flocculi, and attached midway to the sides of the nodule, is a thin, white, semilunar-shaped plate of nervous matter, called the posterior medullary velum.

The whole outer surface of the cerebellum possesses a characteristic foliated or laminated appearance, due to its subdivision into multitudes of thin plates or lamellae by numerous fissures. The cerebellum consists both of grey and white matter. The grey matter forms the exterior or cortex of the lamellre, and passes from one te the other across the bottoms of the several fissures. The white matter lies in the interior of the organ, and extends into the core of each lamella. When a vertical section is made through the organ, the prolongations of white matter branching off into the interior of the several lamellve give to the section an arborescent appearance, known by the fanciful name of arbor vita (Pl. XVIII. fig. 3, c). Independent masses of grey matter are, however, found in the interior of the cerebellum. If the hemisphere be cut through a little to the outer side of the median lobe, a zig-zag arrangement of grey matter, similar in appearance and structure to the nucleus of the olivary body iu the medulla oblongata, and known as the corpus dentatum of the cerebellum, is seen ; it lies in the midst of the white core of the hemisphere, ar.d. encloses white fibres, which leave the interior of the corpus at its inner and lower side. Stilling has described, in connection with the anterior end of the inferior vermiform process, which projects forwards into the valve of Vieussens, and aids in the formation of the roof of the 4th ventricle, two grey masses, named roof nuclei. They possess flask-shaped nerve cells like those of the corpus dentatum. The white matter is more abundant in the hemispheres than in the median lobe, and is for the most part directly continuous with the fibres of the peduncles of the cerebellum. Thus the restiform or inferior peduncles pass from below upwards through the white core, to end in the grey matter of the tentorial surface of the cerebellum, more especially in that of the central lobe ; on their way they are connected both with the grey matter of the corpus dentatum and of the roof nuclei. The superior peduncles, which descend from the corpora quadrigemina of the cerebrum, reach the grey cortical matter, more especially on the inferior surface of the cerebellum, though they also form connections with the corpus dentatum. The middle peduncles form a large proportion of the white core, and their fibres terminate in the grey matter of the foliated cortex of the hemispheres. But, in addition to these peduncular fibres, which connect the cerebellum to other subdivisions of the encephalon, its white matter contains fibres proper to the cerebellum itself. The fibrce proprice have been especially described by Stilling ; some, which he has termed the median fascieuli, lie near the mesial plane, and connect the grey matter on the tentorial aspect of the middle lobe with that of the inferior vermiform process, whilst others cross directly the mesial plane to unite opposite and symmetrical regions of the hemispheres. Further, the auditory -nerve was said by Foville to derive some of its fibres of origin from the cerebellum ; the connection of this nerve with the cerebellum has been strongly insisted on by /sleynert, and this anatomist has also ascribed a cerebellar origin to a portion of the sensory root of the 5th cranial nerve.

The grey matter of the cortex is divided into two well-defined layers, an external grey, and an inner rust coloured layer of about equal thickness. The rust coloured layer is distinguished by containing multitudes of so called "granules," the well-defined nucleus in which, as described by Strachan, is invested by a small quantity of branched protoplasm. These " granules " are, therefore, minute stellate cells. Where the rust coloured layer joins the grey layer the characteristic nerve cells of the cerebellum, named the corpuscles of Purkinje, are situated. A slender central process arising from each cell enters the rust coloured layer, and, as the observations of Hadlich and Koschennikoff show, becomes continuous with the axial cylinder of a medullated nerve fibre ; for the nerve fibres of the white core enter this layer, divide into minute fibres, and ramify amidst the granules. From the opposite aspect of each cell two peripheral processes arise, and ramify in an antler-like manner in the external grey layer. Obersteiner and Iladlich maintain that the finer branches of these processes curve back towards the rust coloured layer, where, according to Boll, they form a network of extreme minuteness, from which it is believed. that nerve fibres may arise. The substratum of the grey layer, in which the branched processes of the cells of Purkiuje lie, consists of a very delicate neuroglia, in which scattered corpuscles are imbedded ; but, in the outer part of this layer, delicate supporting connective tissue-like fibres are also met with.

The Fourth Ventricle is the dilated upper end of the central canal of the medulla oblongata. Its shape is like an heraldic lozenge. Its floor is formed by the grey matter of ANATOMY-CEREBELLUM the posterior surfaces of the medulla oblongata and pons; its roof partly by the inferior vermiform process of the cerebellum, the nodule of which projects into its cavity, and partly by a thin layer, called valve of Vieussens, or anterior medullary velum ; its lower lateral boundaries, by the divergent restiform bodies and posterior pyramids ; its upper lateral boundaries, by the superior peduncles of the cerebellum; the reflection of the arachnoid membrane from the back of the medulla to the inferior vermiform process closes it in below, but allows of a communication between its cavity and the sub-arachuoid space ; above, it communicates with the aqueduct of Sylvius, which is tunnelled through the sub. stance of the corpora quadrigemina. Along the centre of the floor is the median furrow, which terminates below in a pen-shaped form, the so-called calamus scriptorius. Situated on its floor are the faseiculi teretes, strive aeonstiem, and deposits of grey matter described in connection with the medulla oblongata. Its endothelial lining is continuous with that of the central canal.

The CEREBRUM or GREAT BRAIN lies above the plane of the tentorimn, and forms much the largest division of the encephalon. It is customary in human anatomy to include under the name of cerebrum, not only the convolutions, the corpora striata, and the optic thalami, developed in the anterior cerebral vesicle, but also the corpora quadrigemina and erura cerebri developed in the middle cerebral vesicle,. The cerebrum is ovoid in shape, and presents superiorly, anteriorly, and posteriorly a deep median longitu, dinal fissure, which subdivides it into two hemispheres. Inferiorly there is a continuity of structure between the two hemispheres across the mesial plane, and if the two hemispheres be drawn asunder by opening out the longitudinal fissure, a broad white band, the corpus callosum, may be seen at the bottom of the fissure passing across the mesial plane from one hemisphere to the other. The outer surface of each hemisphere is convex, and adapted in shape to the concavity of the inner table of the cranial bones ; its inner surface, which bounds the longitudinal fissure, is flat and is separated from the opposite hemisphere by the faLx cerebri ; its under surface, where it rests on the tentorium, is concave, and is separated by that membrane from the cerebellum and pons. From the front of the pons two strong white bands, the crura cerebri or cerebral peduncles, pass forwards and upwards to enter the optic thalami in their respective hemispheres. 'Winding round the outer side of each crus is a fiat white band, the optic tract. These tracts converge in front, and join to form the optic commissure, from which the two optic nerves arise. The crura cerebri, optic tracts, and optic commissure enclose a lozenge shaped space, which includes - a, a grey layer, called pons Tarini, which, from being perforated by several small arteries, is often called locus perforatus posticus; b, two white mammillx, the corpora albicantia ; c, a grey nodule, the tuber cinereum, from which, d, the infundibulum projects to join the pituitary body. Immediately in front of the optic commissure is a grey layer, the lamina cinerea or lamina tersninalis of the 3d ventricle; and between the optic commissure and the inner end of each Sylvian fissure is a grey spot perforated by small arteries, the locus perforatus anticus.

The peripheral part of each hemisphere, which consists of grey matter, exhibits a characteristic folded appearance, known as the convolutions or gyri of the cerebrum. These convolutions are separated from each other by fissures or sulci, some of which are considered to subdivide the hemisphere into lobes, whilst others separate the convolutions in each lobe from each other. In each hemisphere of the human brain five lobes are recognised: the temporo-sphenoidal, frontal, parietal, occipital, and the central lobe or insula. Passing obliquely on the outer face of the hemisphere from before, upwards and backwards, is the well-marked Sylvian fissure, which is the first to appear in the development of the hemisphere. Below it lies the temporo-sphenoidal lobe, and above and in front of it, the parietal and frontal lobes. The frontal lobe is separated from the parietal by the fissure of Rolando, which extends on the outer face of the hemisphere from the longitudinal fissure obliquely downwards and forwards towards the Sylvian fissure. About two inches from the hinder end of the hemisphere is the parieto-occipital fissure, which, commencing at the longitudinal fissure, passes down the inner surface of the hemisphere, and transversely outwards for a short distance on the outer surface of the hemisphere; it separates the parietal and occipital lobes from each other.

The Temporo-Sphenoidal Lobe presents on the outer surface of the hemisphere three convolutions, arranged in parallel tiers from above downwards, and named superior,middle,and inferior temporo-sphenoidal convolutions. The fissure which separates the superior and middle of these convolutions is called the parallel fissure. The Occipital Lobe also consists from above downwards of three parallel convolutions, named superior, middle, and inferior occipital. The Frontal Lobe is more complex ; immediately in front of the fissure of Rolando, and forming indeed its anterior boundary, is a convolution named ascending frontal, which ascends obliquely backwards and upwards from the Sylvian to the longitudinal fissure. Springing from the front of this con volution, and passing forwards to the anterior end of the cerebrum, are three convolutions, arranged in parallel tiers from above downwards, and named superior, middle, and inferior frontal convolutions, which are also prolonged on to the orbital face of the frontal lobe. The _Parietal Lobe is also complex ; its most anterior convolution, named ascending parietal, ascends parallel to and immediately behind the fissure of Rolando. Springing from the upper end of the back of this convolution is the postero-parietal convolution, which, forming the boundary of the longitudinal fissure, extends as far back as the parieto-occipital fissure; springing from the lower end of the back of this convolution is the supra - marginal convolution, which forms the upper boundary of the hinder part of the Sylvian 787 fissure ; as this gyrus occupies the hollow in the parietal hone, which corresponds to the eminence, it may appropriately be named the convolution of the parietal eminence. Continuous with the convolution of the parietal eminence is the angular convolution, which bends round the posterior extremity of the Sylvian fissure. Lying in the parietal lobe is the intra-parietal .fissure, which separates the convolution of the parietal eminence from the postero-parietal con787 depth and extent of this fissure vary in different brains in proportion to the size of these bridging convolutions. The superior annectent gyms passes between the postero-parietal and the superior occipital convolutions, whilst the second annectent gyrus connects the middle occipital with the angular gyrus. Two annectent gyri also pass from the inferior occipital convolution to the lower convolutions of the temporosphenoidal lobe. These lobes of the cerebrum, though named after the bones which form the vault of the skull, are not exactly co-terminous with them. The frontal lobe not only lies under cover of the frontal bone, but extends backwards under the anterior part of the parietal; for the fissure of Rolando, which forms its posterior boundary, lies from 1 to 2 inches behind the corona suture. The occipital lobe is not limited to the upper tabular part of the occipital bone, but extends forwards under cover of the posterior part of the parietal, for the parieto-occipital fissure lies about inch in front of the apex of the lambdoidal fissure. The temporo-sphenoidal lobe not only lies under the squamous-temporal and great wing of the sphenoid, but passes upwards under cover of the lower part of the parietal, for the Sylvian fissure passes from below obliquely upwards and backwards across the line of the squamous suture near its middle. The area covered by the parietal bone so far, The above view of the brain in situ shows the relations of the surface convolutions to the regions of the skull. R, fissure of Rolando, which separates the frontal from the parietal lobe. P0, parieto-occipital fissure between the parietal and occipital lobes. SS, fissure of Sylvius, which separates the temporo-sphenoidal from the frontal and parietal lobes. SF, MI?, IF, the supero-, mid-, and infero-frontal subdivisions of the frontal area of the skull; the letters are placed on the superior, middle, and inferior frontal convolutions ; the inferior frontal region is separated from the middle frontal by the frontal part of the curved line of the temporal ridge; the mid- from the supero-frontal by an antero-posterior line through the frontal eminence. SAP, the superoantero-parietal area of the skull ; S is placed on the ascending parietal convolution, AP on the ascending frontal convolution. IAP, the inferoantero-parictal area of the skull; I is placed on the ascending parietal, AP on the ascending frontal convolution. SPP, the supero-postero-parietal area of the skull ; the letters are placed on the angular convolution. IPP, the infero-postero-parietal arca of the skull ; the letters are placed on the mid-temporo-sphenoidal convolution; the temporal ridgeseparates the supero- and infero-parietal regions from each other; a vertical line drawn through the parietal eminence separates the antero- and posteroparietal regions. X, the convolution of the parietal eminence, or supra-marginal gyms. 0, the occipital area of the skull; the letter is placed on the mid.occipital convolution. Sq, the squamoso-temporal region of the skull ; the letters are placed on the mid-temporo-sphenoidal convolution. AS, the all-sphenoid region of the skull ; the letters are placed on the tip of the supero-temporo-spheaoidal convolution. The black lines mark the boundaries of different cranial regions.

then, from being coterminous with the parietal lobe of the cerebrum, is trenched on anteriorly by the frontal, posteriorly by the occipital, and inferiorly by the temporosphenoidal lobe. The convolutions of the parietal lobe itself are grouped around the parietal eminence, and in the interval between it and the sagittal suture. The inner table of the cranial bones is an almost exact mould of the convolutions of these lobes ; but this is not so with the exterior of the skull, the configuration of which is modified by the formation of ridges and processes for the attachment of muscles, by variations in the thickness of the diplob, and by the development of the frontal and mastoid air-sinuses. Hence the outer surface of the skull does not correspond in shape to the outside of the brain.

The Central Lobe of the hemisphere, more usually called the insula or island of Bell. does not come to the surface 787 of the hemisphere, but lies deeply within the Sylvian fissure, the convolutions forming the margin of which conceal it. It consists of four or five short convolutions, which radiate from the locus perforatus anticus, situated at the inner end of the fissure. This lobe is almost entirely surrounded by a deep sulcus, which insulates it from the adjacent convolutions. It lies opposite the upper part of the alisphenoid, where it articulates with the parietal and squamous-temporal.

Convolutions also exist on the inner surface of the hemisphere, and on the under surface which rests on the tentorium, but these have no relation to the bones of the cranial vault. They may be studied in connection with the corpus callosum or great transverse commissure, which connects the two hemispheres, and with certain fissures situated on these surfaces of the hemisphere. The small convolutions which lie behind the internal part of the parieto-occipital fissure form the inner convolutions of the occipital lobe, or the occipital lobule (Fig. 73). Those which lie immediately in front of the same fissure belong to the inner face of the parietal lobe, and form the quadrilateral lobule. It is customary, however, to name the convolution which extends forwards from that fissure along the margin of the longitudinal fissure to the anterior end of the hemisphere, and which then turns back to the locus perforatus anticus as the marginal convolution. This is separated by a fissure called calloso-marginal, from the callosal convolution or gyrus fornicatus, which, commencing at the locus perforatus anticus, turns round the anterior end of the corpus callosum, extends parallel to its upper surface, and then turns round its posterior end. It is separated from the corpus canes= by the callosal fissure, at the bottom of which the grey matter of the gyms fornicatus termintes in a well-defined edge.

The callosal convolution encloses the corpus callosum within the concavity of its arch, and from its direction is 787 787 appropriately called fornicatus (arch-shaped). The posterior end of the callosal convolution curves downwards and then forwards, under the name of gyrus hippocampi, to the tip of the inner surface of the temporo-sphenoidal lobe. This gyrus is separated anteriorly by a narrow curved fissure called hippocampal fissure, from a white band, the tcenia hippocampi, which band possesses a free curved border, round which the pia mater and choroidal artery enter the lateral ventricle through the great transverse fissure of the cerebrum. The hippocampal fissure is continuous round the posterior end of the corpus callosum with the callosal fissure, and at the bottom of the hippocampal fissure the grey matter of the gyrus hippocampi terminates in a well-defined dentated border (fascia dentata). The hippocampal fissure on this surface of the hemisphere marks the position of an eminence in the descending cornu of the ventricle called hippocampus major. The gyrus hippocampi is separated posteriorly from the adjacent temporo-sphenoidal convolution by a fissure, named collateral, which marks the position on this surface of the hemisphere of the collateral eminence in the interior of the ventricle. From the lower end of the parieto-occipital fissure an offshoot, called the calcarine fissure, passes almost horizontally backwards in the occipital lobe, which fissure marks on this surface of the hemisphere the eminence named calcar avis, or hippocampus minor, in the posterior cornu of the ventricle.

If a horizontal slice be removed from the upper part of each hemisphere, the peripheral grey matter of the convolutions will be seen to follow their various windings, whilst the core of each convolution consists of white matter continuous with a mass of white matter in the interior of the hemisphere. If a deeper slice be now made down to the plane of the corpus callosum, the white matter of that structure will be seen to be continuous with the white centre of each hemisphere. The corpus callosum does not equal the hemispheres in length, but approaches nearer to their anterior than their posterior ends (Pl. XVIII. fig. 3, B.) It terminates behind in a free rounded end, whilst in front it forms a knee-shaped bend, and passes downwards and backwards as far as the lamina cinerea. If the dissection be performed on a brain which has been hardened in spirit, the corpus callosum is seen to consist almost entirely of bundles of nerve fibres, passing transversely across the mesial plane between the two hemispheres ; these fibres may be traced into the white cores and grey matter of the convolutions, and apparently connect the corresponding convolutions in the opposite hemispheres. Hence the corpus callosum is a connecting or commissural structure, which brings the convolutions of the two hemispheres into anatomical and physiological relation with each other. On direction. If the corpus callosum be now cut through on each side of its mesial line, the large cavity or lateral ventricle in each hemisphere will be opened into.

The lateral ventricle is subdivided into a central space or body, and three bent prolongations or cornua ; the anterior cornu extends forwards and outwards into the frontal lobe ; the posterior cornu curves backwards, outwards, and inwards into the occipital lobe ; the descending cornu curves backwards, outwards, downwards, forwards, and inwards, behind and below the optic thalamus into the temporo-sphenoidal lobe. On the floor of the central space may be seen from before backwards the grey upper surface of the pear-shaped corpus striatum, and to its inner and posterior part a small portion of the optic thalamus, whilst between the two is the curved flat band, the tamia semicircularis. Resting on the upper surface of the thalamus is the vascular fringe of the velum interpositum, named choroid plexus, and immediately internal to this fringe is the free edge of the white posterior pillar of the fornix. The anterior cornu has the anterior end of the corpus striatum projecting into it. The posterior cornu has an elevation on its floor, the hippocampus minor, and between this cornu and the descending cornu is the elevation called eminentia collateralis.

Extending down the descending cornu and following its curvature is the hippocampus major, which terminates below in a nodular end, the pes hippocampi; on its inner border is the white tania hippocampi, continuous above with the posterior pillar of the fornix. If the trenia be drawn on one side the hippocampal fissure is exposed, at the bottom of which the grey matter of the gyrus hippocampi may be seen to form a well-defined dentated border (the so-called ,fascia dentata). The choroid plexus of the pia mater turns round the gyrus hippocampi, and enters the descending coma through the great transverse fissure between the tmnia hippocampi and optic thalamus. The lateral ventricle is lined by a cylindrical endothelium, which is in many parts ciliated, and which rests on a layer of neuroglia. This lining is continuous through the foramen of Monro with that of the third ventricle, which again is continuous with the lining of the fourth ventricle through the aqueduct of Sylvius. A little fluid is contained in the cerebral ventricles, which, under some pathological conditions, may increase greatly in quantity, so as to occasion considerable dilatation of the ventricular cavities.

If the corpus callosum be now divided about its middle 787 by a transverse incision, and the posterior half of this structure be turned back, the body of the fornix on which the corpus callosum rests is exposed. If the anterior half of the corpus callosum be now turned forward, the grey partition, or septum lucidum, between the two lateral ventricles is exposed. This septum fits into the interval between the under surface of the corpus callosum and the upper surface of the anterior part of the fornix. It consists of two layers of grey matter, between which is a narrow vertical mesial space, the fifth ventricle. If the septum be now removed, the anterior part of the fornix is brought into view.

The fornix or arch is an arch-shaped band of nerve fibres extending in the antero-posterior direction. Its anterior end forms the anterior piers or pillars of the arch, its posterior end the posterior piers or pillars, whilst the intermediate body of the fornix forms the summit or crown of the arch. It consists of two lateral halves, one belonging to each hemisphere. At the summit of the arch the two lateral halves are conjoined to form the body; but in front of the body the two halves separate from each other, and form two anterior pillars, which descend in front of the third ventricle to the base of the cerebrum, where they form the corpora albicantia, and then enter the substance of the optic thalamus. Behind the body the two halves diverge much more from each other, and form the posterior pillars; each of which curves downwards and outwards into the descending cornu of the ventricle, and, under the name of teenia hippocampi, forms the free border of the hippocampus major. If the body of the fornix be now divided by a transverse incision, its anterior part thrown forwards, and its posterior part backwards, the great transverse fissure of the cerebrum is opened into, and the velum interpositum lying in that fissure is exposed.

The velum interpositum is an expanded fold of pia mater, which passes into the interior of the hemispheres through the great transverse fissure. It is triangular in shape; its base is in a line with the posterior end of the corpus callosum, where it is continuous with the external pia mater ; its lateral margins are fringed by the choroid plexuses, which are seen in the bodies and descending cornua of the lateral ventricles, where they are invested by the endothelial lining of those cavities. Its apex, where the two choroid plexuses blend with each other, lies just behind the anterior pillars of the fornix. The interval between the apex and these pillars is the aperture of communication between the two lateral ventricles and the third, already referred to as the foramen of Monro. The choroid plexuses contain the small choroidal arteries, which supply the corpora striata, optic thalami, and corpora quadrigemina; and the blood from these bodies is returned by small veins, which join to form the veins of Galen, (Fig. 75). These veins pass along the centre of the velum, and, as is shown in Fig. 63, open into the straight sinus. If the velum interpositum be now carefully raised front before backwards, the optic thalami, third ventricle, pineal gland, and corpora quadrigemina are exposed.

The optic thalamus is a large, somewhat ovoid body situated behind the corpus striatum, and above the crus cerebri. Its upper surface is partly seen in the floor of the body of the lateral ventricle, but is for the most part covered by the fornix and velum interpositum. Its posteroinferior surface forms the roof of the descending cornu of the ventricle, whilst its inner surface forms the side wall of the third ventricle. At its outer and posterior part are two slight elevations, placed one on each side of the optic tract, and named respectively corpus geniculatunz internum and externum.

The third ventricle is a cavity situated in the mesial plane between the two optic thalami. Its roof is formed by the velum interpositum and body of fornix; its floor, by the pons Tarini, corpora albicantia, tuber cinereum, infundibulum, and optic commissure ; its anterior boundary, by the anterior pillars of the fornix, anterior commis-sure, and lamina einerea ; its posterior boundary, by the corpora quadrigernina and posterior commissure. The cavity of this ventricle is of small size in the living head, for the inner surfaces of the two thalami are connected together by intermediate grey matter, named the middle or soft conzmissure ; but in taking the brain out of the cranial cavity this commissure is usually more or less torn through, and the cavity is consequently enlarged. Immediately in front of the corpora quadrigemina, the white fibres of the posterior commissure pass across between the two optic thalami. If the anterior pillars of the fornix be separated from each other, the white fibres of the anterior conzmissure may be seen entering the two corpora striata.

The pineal body is a reddish cone-shaped body, enveloped by the velum interpositum, and situated upon the more anterior pair of the corpora quadrigemina. From its broad anterior end two white bands, the peduncles of the pineal body, pass forwards, one on the inner side of each optic thalamus. Each peduncle joins, along with the twnis semieircularis, the anterior pillar of the fornix of its own side. In its structure this body consists of a vascular stroma of connective tissue, in the meshes of which lymphoid cells are contained. Branched corpuscles are also found not unlike nerve cells. Amylaceous and gritty calcareous particle:, constituting the brain sand, are also found in it. Usually it is hollowed out into two or more small cavities. The function of the pineal body is not understood, but both it and the pituitary body, which possess a certain structural correspondence, are usually referred to the type of the ductless glands.

The corpora quadrigemina or optic lobes are situated behind and between the two optic thalami, and rest upon the posterior surface of the crura cerebri. The division into two lateral halves is marked by a shallow longitudinal fissure, and the subdivision of each half into an anterior and a posterior eminence, by a shallow transverse fissure. The anterior pair of eminences are called nates; the posterior, testes. From each testis a strong white band, the superior peduncle of the cerebellum, passes backwards to the cerebellum, and stretching between the pair of peduncles is the valve of Vieussens or anterior medullary velum. The corpora quadrigemina are tunnelled in the antero-posterior direction by the aqueduct of Sylvius, which opens anteriorly into the third ventricle immediately below the posterior commissure, and posteriorly into the fourth ventricle under cover of the valve of Vieussens. It is lined by a cylindrical ciliated endothelium.

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