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BRAIN (A.S. braegen)

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Originally appearing in Volume V04, Page 401 of the 1911 Encyclopedia Britannica.
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BRAIN (A.S. braegen) , that part of the central nervous system which in vertebrate animals is contained within the cranium or skull; it is divided into the great brain or cerebrum, the hind brain or cerebellum, and the medulla oblongata, which is the transitional part between the spinal cord and the other, 392 two parts already named. Except where stated, we deal here primarily with the brain in man. 1. ANATOMY Membranes of the Human Brain. Three membranes named the dura mater, arachnoid and pia mater cover the brain and lie between it and the cranial cavity. The most external of the three is the dura mater, which consists of a cranial and a spinal portion. The cranial part is in contact with the inner table i. Falx cerebri. 9 and io. Superior and inferior 2. Tentorium. petrosal sinus. 3,3. Superior longitudinal sinus. 11. Cavernous sinus. 4. Lateral sinus. 12. Circular sinus which connects 5. Internal jugular vein. the two cavernous sinuses 6. Occipital sinus. together. 6'. Torcular Herophili. 13. Ophthalmic vein, from 15, 7. Inferior longitudinal sinus. the eyeball. 8. Veins of Galen. 14. Crista galli of ethmoid bone. of the skull, and is adherent along the lines of the sutures and to the margins of the foramina, which transmit the nerves, more especially to the foramen magnum. If forms, therefore, for these bones an internal periosteum, and the meningeal arteries which ramify in it are the nutrient arteries of the inner table. As the growth of bone is more active in infancy and youth than in the adult, the adhesion between the dura mater and the cranial bones is greater in early life than at maturity. From the inner surface of the dura mater strong bands pass into the cranial cavity, and form partitions between certain of the subdivisions of the brain. A vertical longitudinal mesial band, named, from its sickle shape, falx cerebri, dips between the two hemispheres of the cerebrum. A smaller sickle-shaped vertical mesial band, the falx cerebelli, attached to the internal occipital crest, passes between the two hemispheres of the cerebellum. A large band arches forward in the horizontal plane of the cavity, from the transverse groove in the occipital bone to the clinoid processes of the sphenoid, and is attached laterally to the upper border of the petrous part of each temporal bone. It separates the cerebrum from the cerebellum, and, as it forms a tent-like covering for the latter, is named tentorium cerebelli. Along certain lines the cranial dura mater splits into two layers to form tubular passages for the transmission of venous blood. These passages are named the venous blood sinuses of the dura mater, and they are lodged in the grooves on the inner surface of the skull referred to in the description of the cranial bones. Opening into these sinuses are numerous veins which convey from the brain the blood that has been circulating through it; and two of these sinuses, called cavernous, which lie at the sides of the body of the sphenoid bone, receive the ophthalmic veins from the eyeballs situated in the orbital cavities. These blood sinuses pass usually from before backwards: a superior longitudinal along the upper border of the falx cerebri as far as the internal occipital protuberance; an inferior longitudinal along its lower border as far as the tentorium, where it joins the straight sinus, which passes back as far as the same protuberance. One or two small occipital sinuses, which lie in the falx cerebelli, also pass to join the straight and longitudinal sinuses opposite this protuberance; several currents of blood meet, therefore, at this spot, and as Herophilus supposed that a sort of whirlpool was formed in the blood, the name torcular Herophili has been used to express the meeting of these sinuses. From the torcular the blood is drained away by two large sinuses, named lateral, which curve forward and downward to the jugular foramina to terminate in the internal jugular veins. In Its course each lateral sinus receives two petrosal sinuses, which pass from the cavernous sinus backwards along the upper and lower borders of the petrous part of the temporal bone, The dura mater [ANATOMY consists of a tough, fibrous membrane, somewhat flocculent externally, but smooth, glistening, and free on its inner surface. The inner surface has the appearance of a serous membrane, and when examined microscopically is seen to consist of a layer of squamous endothelial cells. Hence the dura mater is sometimes called a fibroserous membrane. The dura mater is well provided with lymph vessels, which in all probability open by stomata on the free inner surface. Between the dura mater and the subjacent arachnoid membrane is a fine space containing a minute quantity of limpid serum, which moistens the smooth inner surface of the dura and the corresponding smooth outer surface of the arachnoid. It is regarded as equivalene to the cavity of a serous membrane, and is named the sub-ducal space. Arachnoid Mater.—The arachnoid is a membrane of great delicacy and transparency, which loosely envelops both the brain and spinal cord. It is separated from these organs by the pia mater; but between it and the latter membrane is a distinct space, called subarachnoid. The sub-arachnoid space is more distinctly marked beneath the spinal than beneath the cerebral parts of the membrane, which forms a looser investment for the cord than for the brain. At the base of the brain, and opposite the fissures between the convolutions of the cerebrum, the interval between the arachnoid and the pia mater can, however, always be seen, for the arachnoid does not, like the pia mater, clothe the sides of the fissures, but passes directly across between the summits of adjacent convolutions. The subarachnoid space is subdivided into numerous freely-communicating loculi by bundles of delicate areolar tissue, which bundles are in-vested, as Key and Retzius have shown, by a layer of squamous endothelium. The space contains a limpid cerebro-spinal fluid, which varies in quantity from 2 drachms to 2 oz., and is most plentiful in the dilatations at the base of the brain known as cisternae. It should be clearly understood that there is no communication between the subdural and sub-arachnoid spaces, but that the latter communicates with the ventricles through openings in the roof of the fourth, and in the descending cornua of the lateral ventricles. When the skull cap is removed, clusters of granular bodies are usually to be seen imbedded in the dura mater on each side of the superior longitudinal sinus; these are named the Pacchionian bodies. When traced through the dura mater they are found to spring from the arachnoid. The observations of Luschka and Cleland have proved that villous processes invariably grow from the free surface of that membrane, and that when these villi greatly increase in size they form the bodies in question. Sometimes the Pacchionian bodies greatly hypertrophy, occasioning absorption of the bones of the cranial vault and depressions on the upper surface of the brain. Pia Mater.—This membrane closely invests the whole outer surface of the brain. It dips into the fissures between the convolutions, and After D. J. Cunningham's Text-book of Anatomy. a wide prolongation, named velum inter positum, lies in the interior of the cerebrum. With a little care it can be stripped off the brain without causing injury to its substance. At the base of the brain the pia mater is prolonged on to the roots of the cranial nerves. This membrane consists of a delicate connective tissue, in which the arteries of the brain and spinal cord ramify and subdivide into small branches before they penetrate the nervous substance, and in which the veins conveying the blood from the nerve centres lie before they open into the blood sinuses of the cranial dura mater and the extradural venus plexus of the spinal canal. Optic chiasma-Optic tract Corpus geniculatum i externum Corpus geniculatum internum Locus perforatus posticus Restiform body Olive Pyramid Anterior superficial arcuate fibres Decussation of pyramids Optic nerve Infundibulum Tuber cinereum Corpora mammillaria Oculo-rnotor nerve (III.) Trochlear nerve(IV.) winding round the crus cerebri Ttigeminal nerve (V.) Abducent nerve (VI.) Facial nerve (VII.) Auditory nerve (VIII.) V a go-gl osso ph aryngea l nerve (IX. and X.) Hypoglossal (nerve XII.) most anterior is the pyramid or motor tract, the decussation of which has been seen. Behind this is the mesial fillet or sensory tract, The Medulla Oblongata rests upon the basi-occipital. It is somewhat which has also decussated a little below the point of section, while pyramidal in form, about I4 in. long, and I in. broad in its widest farther back still is the posterior longitudinal bundle which is coming part. It is a bilateral organ, and is divided into a right and a left half by shallow anterior and posterior median fissures, continuous with the corresponding fissures in the spinal cord; the posterior fissure ends above in the fourth ventricle. Each half is subdivided into elongated tracts of nervous matter. Next to, and parallel with the anterior fissure is the anterior pyramid (see fig. 2). This pyramid is continuous below with the cord, and the place of continuity is marked by the passage across Fasciculus solitarius the fissure of three or four bundles of nerve fibres, Spinal root of trigeminal nerve from each half of the cord to the opposite anterior pyramid; this crossing is called the decussation of the pyramids. To the side of the pyramid, and separated posterior from it by a faint fissure, is the olivary fasciculus, Deep arcuate fibres laetudmal which at its upper end is elevated into the projecting fasciculus oval-shaped olivary body. Behind the olivary body in the lower half of the medulla are three tracts named from before backward the funiculus of Rolandp, the funiculus cuneatus and the funiculus Anterior superficial gracilis (see fig. 3). The two funiculi graciles of opposite sides are in contact in the mid dorsal line and have between them the postero median fissure. When the fourth ventricle is reached they diverge to form the lower limit of that diamond-shaped space and are slightly swollen to form the clavae. All these three bundles appear to be continued up into the cerebellum as the restiform bodies or inferior cerebellar peduncles, but really the continuity is very slight, as the restiform bodies are formed from the direct cerebellar tracts of the spinal cord joining with the superficial arcuate fibres which curve back just below the olivary bodies. The upper part of the fourth ventricle is bounded by the superior cerebellar From Cunningham, Text book of Anatomy. peduncles which meet just before the inferior quadri- FIG. 4,-Transverse Section through the Human geminal bodies are reached. Stretching across between Lower Olivary Region. them is the superior medullary velum or valve of Vieussens, forming the upper part of the roof, while the inferior velum forms the lower part, and has an opening called the foramen of Majendie, through which the sub-arachnoid space communicates with the ventricle. The floor (see fig. 3) has two triangular depressions on each side of a median furrow; these are the superior and inferior fovea, the significance of which will be noticed in the development of the rhombencephalon. Running horizontally across the middle of the floor are the striae acusticae which are continued into the auditory nerve. The floor of the fourth ventricle is of special Medulla Oblongata. Superior quadrigeminal body Inferior quadrigeminal body Crus cerebri Pontine part of floor of ventricle IV. Eminentia Ceres Fovea superior Restiform body Trigonum hypoglossi Clava Rolandic tubercle Funiculus cuneatus From Cunningham, Text-book of Anatomy. interest because a little way from the surface are the deep origins of all the cranial nerves from the fifth to the twelfth. (See NERVE: cranial). If a section is made transversely through the medulla about the apex of the fourth ventricle three important bundles of fibres are cut close to the mid line on each side (see fig. 4). The up from the anterior basis bundle of the cord. External to and behind the pyramid is the crenated section of the olivary nucleus, the surface bulging of which forms the olivary body. The grey matter of the medulla oblongata, which contains numerous multipolar nerve cells, is in part continuous with the grey matter of the spinal cord, and in part consists of independent masses. As the grey matter of the cord enters the medulla it loses its crescentic arrangement. The posterior cornua are thrown outwards towards the surface, lose their pointed form, and dilate into rounded masses named the grey tubercles of Rolando. The grey matter of the anterior cornua is cut off from the rest by the decussating pyramids and finally disappears. The formatio reticularis which is feebly developed in the cord becomes well developed in the medulla. In the lower part of the medulla a central canal continuous with that of the cord exists, but when the clavae on the opposite sides of the medulla diverge from each other, the central canal loses its posterior boundary, and dilates into the cavity of the fourth ventricle. The grey matter in the interior of the medulla appears, therefore, on the floor of the ventricle and is continuous with the grey matter near the central canal of the cord. This grey matter forms collections of nerve cells, which are the centres of origin of several cranial nerves. Crossing the anterior surface of the medulla oblongata, immediately below the pons, in the majority of mammals is a trans-verse arrangement of fibres forming the trapezium, which contains a grey nucleus, named by van der Kolk the superior olive. In the human brain the trapezium is concealed by the lower transverse fibres of the pons, but when sections are made through it, as L. Clarke pointed out, the grey matter of the superior olive can be seen. These fibres of the trapezium come from the cochlear nucleus of the auditory nerve, and run up as the lateral fillet. The Pons Varolii or BRIDGE is cuboidal in form (see fig. 2): its anterior surface rests upon the dorsum sellae of the sphenoid, and is marked by a median longitudinal groove; its inferior surface receives the pyramidal and olivary tracts of the medulla oblongata; at its superior surface are the two crura cerebri; each lateral surface is in relation to a hemisphere of the cerebellum, and a peduncle passes from the pons into the interior of each hemisphere; the posterior surface forms in part the upper portion of the floor of the fourth ventricle, and in part is in contact with the corpora quadrigemina. The pons consists of white and grey matter: the nerve fibres of the white matter pass through the substance of the pons, in either a transverse or a longitudinal direction. The transverse fibres go from one hemisphere of the cerebellum to that of the opposite side; some are situated on the anterior surface of the pons, and form its superficial transverse fibres, whilst others pass through its substance and form the deep transverse fibres. The longitudinal fibres ascend from the medulla oblongata and leave the pons by emerging from its upper surface as fibres of the two crura cerebri. The pons possesses a median raphe continuous with that of the medulla oblongata, and formed like it by a decussation of fibres in the mesial plane. Superior peduncle of the cerebellum Middle peduncle of the cerebellum Striae acusticae Area acusticae Trigonum vagi Taenia thalami Pineal body Mesial accessory olivary nucleus Medulla in the In a horizontal section through the pons and upper part of the fourth ventricle the superficial transverse fibres are seen most anteriorly; then come the anterior pyramidal fibres, then the deep transverse pontine fibres, then the fillet, while most posteriorly and close to the floor of the fourth ventricle the posterior longitudinal bundle is seen (see fig. 5). The grey matter of the pons is scattered irregularly through its substance, and appears on its posterior surface; but not on the anterior surface, composed exclusively of the superficial transverse fibres. The Cerebellum.lobes, of which the most important are the amygdala or tonsil, which forms the lateral boundary of the anterior part of the vallecula, and the flocculus, 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, semilunarshaped plate of nervous matter, called the inferior medullary velum. The whole outer surface of the cere- bellum possesses a characteristic foliated or laminated appearance, due to its sub- division into multitudes of thin plates or lamellae by numerous fissures. The cerebellum consists of both grey and white matter. The grey matter forms the exterior or cortex of the lamellae, and passes from one to 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 in- terior of the several lamellae give to the section an arborescent appearance, known by the fanciful name of. arbor vitae (see fig. 6). Independent masses of grey matter are, however, found in the in- terior of the cerebellum. If the hemi- sphere be cut through a little to the outer side of the median lobe, a zigzag arrange- ment of grey matter, similar in appear- ance and structure to the nucleus of the olivary body in 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, and encloses white fibres, which leave the interior of the corpus at its inner and lower side. On the mesial side of this corpus dentatum lie three smaller nuclei. 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 pe- duncles of the cerebellum. Thus the restiform or inferior peduncles pass from below upward 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 with the Middle peduncle of cerebellum From Cunningham, Text-book of Anatomy. The Cerebellum, LITTLE BRAIN, Or AFTER BRAIN occupies the inferior pair of occipital fossae, and 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 vermis. It is connected below with the medulla oblongata by the two callosum restiform bodies which form its inferior peduncles, and above with the corpora quadrigemina of the cerebrum by two bands, which form its superior peduncles; whilst the two hemispheres are connected together by the trans-verse fibres of the pons, 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 process, which lies at the bottom of a deep fossa or vallecula; this fossa is prolonged to the posterior border of the cerebellum, From Cunningham, Text-book of Anatomy. and forms there a deep notch which separates the two hemispheres from FIG. 6.-Mesial Section through the Corpus Callosum, the Mesencephalon, the Pons, Medulla and each other; in this notch the falx Cerebellum. Showing the third and fourth ventricles joined by the aqueduct of Sylvius. cerebelli is lodged. Extending hori- zontally backwards from the middle cerebellar peduncle, along the grey matter of the corpus dentatum. The superior peduncles, which outer border of each hemisphere is the great horizontal fissure, which descend from the corpora quadrigemina of the cerebrum, form divides the hemisphere into its tentorial and occipital surfaces. connexions mainly with the corpus dentatum. The middle peduncles Each of these surfaces is again subdivided by fissures into smaller form a large proportion of the white core, and their fibres terminate Forni; Foramen of Monro Septum lucidum Anterior commissure Corpus mammillare Lamina cinerea Optic nerve Pituitary body Tuber cinereum Third nerve 1 d Uvula Pons , ~ i l Central lobule Valve of Vieussens , I Nodule Ventricle IV. I bfedulla Choroid plexus in ventricle IV. in the grey matter of the foliated cortex of the hemispheres. It has been noticed that those fibres which are lowest in the pons go to the upper surface of the cerebellum and vice versa. Histology of the Cerebellum.—The white centre of the cerebellum is composed of numbers of medullated nerve fibres coursing to and from the grey matter of the cortex. These fibres are supported in a groundwork of neuroglial tissue, their nutrition being supplied by a small number of blood vessels. The cortex (see fig. 7) consists of a thin layer of grey material forming an outer coat of somewhat varying thickness over the whole external surface of the laminae of the organ. When examined microscopically it is found to be made up of two layers, an outer " molecular " and an inner " granular " layer. Forming a layer lying at the junction of these two are a number of cells, the cells of Purkinje, which constitute the most characteristic feature of the cerebellum. The bodies of these cells are pear-shaped. Their inner ends taper and finally end-in a nerve fibre which may be traced into the white centre. In their course through the granule layer they give off a number of branching collaterals, some turning back and passing between the cells of Purkinje into the molecular layer. Their inner ends terminate in one or sometimes two stout processes which repeatedly branch dichotomously, thus forming a very elaborate dendron in the molecular layer. The branchings of this dendron From Cunningham, Text-book of Anatomy. Kolliker). Treated by the Golgi method. P. Axon of cell of Purkinje. GRI. Axons of granule cells in F. Moss fibres. molecular layer cut K and K'. Fibres from white core transversely. of folium ending in molecular M'. Basket-cells. layer in connexion with the ZK. Basket-work around the dendrites of the cells of cells of Purkinje. Purkinje. GL. Neuroglial cell. M. Small cell of the molecular N. Axon of an association GR. Granule cell. [layer. cell. are also highly characteristic in that they are approximately restricted to a single plane like an espalier fruit tree, and those for neighbouring cells are all parallel to one another and at right angles to the general direction of the folium to which they belong. In the molecular layer are found two types of cells. The most abundant are the so-called basket cells which are distributed through the whole thickness of the layer. They have a rounded body giving off many branching dendrons to their immediate neighbourhood and one long neuraxon which runs parallel to the surface and to the long axis of the lamina. In its course, this gives off numerous collaterals which run downward to the bodies of Purkinje's cells. Their terminal branchings together with similar terminals of other collaterals form the basket-work around the bodies of these cells. The granular layer is sometimes termed the rust-coloured layer from its appearance to the naked eye. It contains two types of nerve cells, the small granule cells and the large granule cells. The former are the more numerous. They give off a number of short dendrites with claw-like endings, and a fine non-medullated neuraxon process. This runs upward to the cortex, where it divides into two branches in the form of a T. The branches run for some distance parallel to the axis of the folium and terminate in unbranched ends. The large granule cells are multipolar cells, many of the branchings penetrating well into the molecular layer. The neuraxon process turns into the opposite direction and forms a richly branching system through the entire thickness of the granular layer. There is also an abundant plexus of fine medullated fibres within the granule layer. The fibres of the white central matter are partly centrifugal, the neuraxons of the cells of Purkinje, and partly centripetal. The position of the cells of these latter fibres is not known. The fibres give rise to an abundant plexus of fibrils in the granular layer, and many reaching into the molecular layer ramify there, especially in the immediate neighbourhood of the dendrites of Purkinje's cells. From the appearance of their plexus of fibrils these are sometimes called moss fibres. 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 the posterior surfaces of the medulla oblongata and pons, already described (see figs. 3 and 6) ; its roof partly by the inferior vermis of the cerebellum, the nodule of which projects into its cavity, and partly by a thin layer, called valve of Vieussens, or superior medullary velum; its lower lateral boundaries by the divergent clavae and restiform bodies; its upper lateral boundaries by the superior peduncles of the cerebellum. The inferior medullary velum, a reflection of the pia mater and epithelium from the back of the medulla to the inferior vermis, closes it in below. Above, it communicates with the aqueduct of Sylvius, which is tunnelled below the substance 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 fasciculi teretes, striae acusticae, and deposits of grey matter described in connexion with the medulla oblongata. Its epithelial lining is continuous with that of the central canal. The Cerebrum. The Cerebrum or GREAT BRAIN lies above the plane of the tentorium, 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 crura cerebri developed in the mesencephalon or middle cerebral vesicle. The cerebrum is ovoid in shape, and presents superiorly, anteriorly and posteriorly a deep median longitudinal 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 forward and upward (see fig. 2). Winding round the outer side of each crus is a flat white band, the optic tract. These tracts con-verge 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, which, from being perforated by several small arteries, is called locus perforatus posticus; (b) two white mammillae, 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 of the third 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. If a transverse section is made at right angles to the surface of the crura cerebri it will pass right through the mesencephalon and come out on the dorsal side through the corpora quadrigemina (see fig. 8). The ventral part of each crus forms the crusta, which is the continuation forward of the anterior pyramidal fibres of the medulla and pons, and is the great motor path from the brain to the cord. Dorsal to this is a layer of pigmented grey matter, called the substantia nigra, and dorsal to this again is the tegmentum, which is a continuation upward of the formatio reticularis of the medulla, and passing through it are seen three important nerve bundles. The superior cerebellar peduncle is the most internal of these and decussates with its fellow of the opposite side so that the two tegmenta are continuous across the middle line. More externally the mesial fillet is seen, while dorsal to the cerebellar peduncle is the posterior longitudinal bundle. If the section happens to pass through the superior corpus quadrigeminum a characteristic circular area appears between the cerebellar peduncle and the fillet, which, from its tint, is called the red nucleus. More dorsally still the section will pass through the Sylvian aqueduct or passage from the third to the fourth ventricle, and this is surrounded by a mass of grey matter in the ventral part of which are the nuclei of the third and fourth nerves. The third nerve is seen at the level of the superior corpus quadrigeminum running from its nucleus of origin, through the red nucleus, to a groove on the inner side of the crus called the oculomotor groove, which marks the separation between the crusta and tegmentum. Dorsal to the Sylvian aqueduct is a layer called the lamina quadrigemina and on this the corpora quadrigemina rest. The superior pair of these bodies is overlapped by the pineal body and forms part of the lower visual centres. Connexions can be traced to the optic tract, the higher visual centre on the mesial surface of the occipital lobe, the deep origin of the third or oculo-motor nerve as well as to the mesial and lateral fillet. The inferior pair of quadrigeminal bodies are more closely in touch with the organs of hearing, and are connected by the lateral fillet with the cochlear nucleus of the auditory nerve. Surface of the Brain. The peripheral part of each hemisphere, which consists of grey matter, exhibits a characteristic folded appearance, known as gyri (or convolutions) of the cerebrum. These gyri are separated from each other by fissures and sulci, some of which are considered to subdivide the hemisphere into lobes, whilst others separate the gyri in each lobe from each other. In each hemisphere of the human brain five lobes are recognized: the temporo-sphenoidal, frontal, parietal, occipital, and the central lobe or Island of Reil; it should, however, be realized that these lobes do not exactly correspond to the outlines of the bones after which they are named. Passing obliquely on the outer face of the hemisphere from before, upward and backward, is the well marked Sylvian fissure (fig. 9, s), 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. As soon as it appears on the external surface of the brain the fissure divides into three limbs, anterior horizontal (si), ascending (s2), and posterior horizontal (s3), the latter being by far the longest. The place whence these diverge is the Sylvian point and corresponds to the pterion on the surface of the skull (see ANATOMY: Superficial and Artistic). Between these three limbs and the vallecula or main stem of the fissure are four triangular tongues or opercula; these are named, according to their position, orbital (fig. 9, C), frontal (pars triangularis) (B), fronto-parietal (pars Superior quadrigeminal basilaris) (A) and temporal. The frontal lobe body is separated from the parietal by the fissure of Rolando (fig. 9, r) which extends on the outer face of the hemisphere from the Sylvian grey matter longitudinal fissure obliquely downward and forward towards the Sylvian fissure. About 2 in. 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 Nucleus of third nerve 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 downward, and named superior, middle and inferior temporal gyri. The fissure which separates the superior and Fibres of superior middle of these convolutions is called the parallel fissure (fig. 9, tf). The Occipital Lobe also consists from above downwards of three parallel gyri, 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 or pre-central, which ascends obliquely backward and upward from the Sylvian to the longitudinal fissure. Springing from the front of this gyrus, and passing forward to the anterior end of the cerebrum, are three gyri, arranged in parallel tiers from above downwards, and named superior, middle and inferior frontal gyri, which are also prolonged on to the orbital face of the frontal lobe. The Parietal Lobe is also complex; its most anterior gyrus, named ascending parietal or post-central, ascends parallel to and immediately behind the fissure of Rolando. Springing from the upper end of the back of this gyrus is the supra-parietal lobule, 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 gyrus is the supra-marginal, which forms the From Cunningham, Text-book of Anatomy. superior Quadrigeminal Body. Po End of Article: BRAIN (A.S. braegen)
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