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laterally, is traversed by two or it may be three curved fissures. The most anterior of these cuts off a narrow, curved strip of cerebellar surface, which presents a more or less uniform width throughout its whole length. This is the so-called lobulus gracilis.
The pyramid is connected with the biventral lobule on each side by an elevated ridge which crosses the sulcus valleculæ. The term lobus pyramidis is applied to the three lobules, which are thus associated with each other.
The uvula is a triangular elevation of the vermis inferior. It lies between the two tonsils, and is connected with each of these by a low-lying band-like ridge of gray matter scored by a few shallow furrows, and in consequence termed the furrowed band. The two tonsils and the uvula form the lobus uvulæ.
FIG. 510.-INFERIOR SURFACE OF THE CEREBELLUM.
The right tonsil has been removed so as to display more fully the posterior medullary velum and
The nodule and the flocculus of each side are linked by a delicate connecting lamina which is formed by the posterior medullary velum.
THE STRUCTURE AND CONNEXIONS OF THE CEREBELLUM.
Arrangement of the Gray and White Matter of the Cerebellum.-The white matter of the cerebellum forms a solid compact mass in the interior, and over this is spread a continuous and uniform layer of gray matter. In each hemisphere the white central core is more bulky than in the vermis, in which the central white matter is reduced to a relatively thin bridge thrown across between the two hemispheres. When sagittal sections are made through the cerebellum, the gray matter on the surface stands out clearly from the white matter in the interior. Further, from all parts of the surface of the central core stout stems of white matter are seen projecting into the lobes of the cerebellum. From the sides of these white stems secondary branches proceed at various angles, and from these again tertiary branches are given off. Over the various lamellæ of white matter thus formed the gray cortex is spread, and the fissures on the surface show a corresponding arrangement, dividing up the organ into lobes, lobules, and folia. When the cerebellum is divided at right angles to the general direction of its fissures and folia, a highly arborescent appearance is thus presented by the cut surface. To this the term arbor vitæ is applied.
Nucleus Dentatus and other Gray Nuclei in the White Matter of the Cerebellum.-Embedded in the midst of the mass of white matter which forms the central core of each hemisphere there is an isolated nucleus of gray matter, which presents a strong resemblance to the inferior olivary nucleus of the medulla. It is called the nucleus dentatus, and it consists of a corrugated or plicated lamina
gray matter, which is folded on itself so as to enclose, in a flask-like manner, a portion of the central white matter (Figs. 511 and 512). This gray capsule is not completely closed. It presents an open mouth, termed the hilum, which is directed medially and upwards, and out of this stream the fibres of the
Three small additional masses of gray matter are also present on each side of the median plane in the central white matter of the cerebellum. These are termed the nucleus emboliformis, the nucleus globosus, and the nucleus fastigii. The nucleus emboliformis or embolus is a small lamina of gray matter which lies just medial to the hilum of the nucleus dentatus, being thus related to it some
what in the same manner that the medial accessory olivary nucleus is related to the main inferior olivary nucleus. The nucleus globosus lies medial to the nucleus emboliformis and on a somewhat deeper horizontal plane. The nucleus fastigii or roof nucleus is placed in the white substance of the vermis close to the median plane and its fellow of the opposite side. It is, therefore, situated on the medial aspect of the nucleus globosus.
FIG. 511.-SAGITTAL SECTION THROUGH THE LEFT HEMISPHERE
Showing the "arbor vitæ" and the nucleus dentatus.
Velum medullare anterius
FIG. 512.- From a dissection by Dr. Edward B. Jamieson in the Anatomical Department of the University of Edinburgh. The nucleus dentatus is displayed from above and the brachium conjunctivum has been traced from it to the mesencephalon.
The nucleus dentatus and the emboliform nucleus present a structure very similar to that of the inferior olivary nucleus. In the nucleus globosus and the nucleus fastigii the cells are somewhat larger in size.
Cerebellar Peduncles.-These are three in number on each side, viz., the
middle, the inferior, and the superior (Fig. 519, p. 585). The fibres of which they are composed all enter or emerge from the white medullary centre of the cerebellum.
The middle peduncle or brachium pontis is much the largest of the three, and has already been described on pp. 565 and 566. It is formed by the transverse fibres of the pons, and it enters the cerebellar hemisphere on the lateral aspect of the other two peduncles. The lips of the anterior part of the horizontal fissure are separated widely from each other to give it admission (Fig. 510). Within the cerebellar hemisphere its fibres are distributed in two great bundles. Of these, one, composed of the superior transverse fibres of the pons, radiates out in the inferior part of the hemisphere; whilst the other, consisting of the inferior transverse fibres of the pons, spreads out in the superior part of the hemisphere.
The inferior peduncle is simply the restiform body of the medulla oblongata. After leaving the medulla oblongata it ascends for a short distance on the dorsal surface of the pons and then turns sharply backwards, to enter the cerebellum between the other two peduncles.
The superior peduncle or brachium conjunctivum, as it issues from the cerebellum, lies close to the medial side of the middle peduncle (Fig. 512). Its further course upwards on the dorsum of the pons to the inferior quadrigeminal body has been previously described (pp. 548 and 569).
Connexions established by the Peduncular Fibres.—The fibres of the brachium pontis represent the second stage of the connexion between the cerebral hemisphere of one side and the opposite cerebellar hemisphere. The connexions which they establish in the pons are described on p. 566.
The restiform body is also composed of afferent fibres (see p. 563); only the more important connexions which these establish in the cerebellum can be touched on here. The principal afferent strand is the fasciculus spinocerebellaris [posterior]. The fibres of this strand end in the cortex of the superior vermis on both sides of the median plane, but chiefly on the opposite side. The olivo-cerebellar tract (fasciculus olivocerebellaris) are also afferent. It appears that they end in connexion with cells in the cortex of both the vermis and hemisphere, and also with cells in the nucleus dentatus. The numerous external arcuate fibres which enter the restiform body establish connexions with cells in the cortex of the hemisphere and of the vermis.
The brachium conjunctivum is an efferent tract: its fibres come from the cells of the nucleus dentatus, and pass to the red nucleus and thalamus of the opposite side. According to Ramon y Cajal collateral branches springing from these fibres descend to the motor nuclei in the medulla oblongata and spinal medulla.
There is, however, a bundle of fibres passing downwards alongside the brachium conjunctivum from the tegmentum of the mesencephalon and possibly from the thalamus: these fibres cross in the mid-brain and pass inferiorly to the cerebellum, in contact with the lateral margin of (or intermingled with) the fibres of the brachium. They probably convey to the cerebellum fibres from the visual centres of the opposite side.
The fasciculus anterolateralis superficialis of the spinal medulla (O.T. Gowers' tract), for which the better name fasciculus spinocerebellaris anterior is now in common use, also enters the cerebellum alongside the emerging brachium conjunctivum. It has been noticed in connexion with the lateral funiculus of the spinal medulla (p. 537). The fibres which compose it are carried upwards through the formatio reticularis grisea of the medulla oblongata and the corresponding part of the tegmental portion of the pons. In this part of its course the fibres are scattered and do not form a compact strand. Reaching the superior end of the pons the tract turns backwards across the brachium conjunctivum, enters the anterior medullary velum, and proceeds downwards in it into the cerebellum.
Roof of the Fourth Ventricle.-In its superior part the roof of the fourth ventricle is formed by the anterior medullary velum as it stretches across between the two brachia conjunctiva, and also, to some extent, by these brachia themselves as they approach the mesencephalon.
In its inferior part the roof of the ventricle is exceedingly thin and is not all formed of nervous matter. The posterior medullary velum is a mere ridge which can hardly be said to enter into its formation: the epithelial lining of the cavity, supported by pia mater, is carried downwards towards the inferior boundaries of the floor of the ventricle. At the lowest part of the calamus scriptorius, and also along each lateral boundary of the floor, =the epithelial roof becomes thickened at its attachment to the parts of the medulla oblongata. The small semilunar lamina which stretches across between the inferior parts of the two clave at the calamus scriptorius and overhangs the opening of the central canal is termed the obex (Fig. 482, p. 550). A downwardly directed protrusion of the epithelial roof is often found behind -the obex.
FIG. 514.-SECTION THROUGH THE MOLECULAR
AND GRANULAR LAYERS IN THE LONG AXIS
-Floc. -Recessus lateralis vent. quarti. Tuberculum acusticum. Medulla oblongata.
THE HISTOGENESIS AND MINUTE STRUCTURE OF THE CEREBELLUM.
The developmental history of the cerebellum presents certain peculiar features which seem quite enigmatic unless considered from the point of view of the evolution of the connexions and functions of the organ. The cerebellum is derived from part of the alar lamina of the rhombencephalon, and at an early stage of its development the rudiment shows the regular lamination into ependyma, mantle layer, and marginal layer, which has already been described as distinctive of the corresponding place of development in the whole nervous system. The cells of this mantle layer are to be looked upon as an outlying (superior) part of the receptive nucleus of the vestibular nerve, the cells to which information concerning the position and movements of the body as a whole or of the head will be transmitted from the semicircular ducts of the internal ear. But, if such information is to be put to any use in influencing behaviour, it is obvious that the activity of these cerebellar cells must, firstly, be correlated with visual impressions, which also supply information concerning the position and movements of the body, and with all those nerves which are bringing into the encephalon or spinal medulla impressions of touch, pressure, or any other information concerning the state of the muscles, tendons, joints, or other structures which are concerned in movements; and, secondly, they must be brought to bear upon the various motor nuclei and other motor regulating parts of the brain (such as the red nucleus, tectum mesencephali, basal ganglia, co-ordinating influence of the cerebellum is
N. Axon of a granule cell.
N. Axons of granule cells in molecular layer.
The neuroblasts which receive all these extrinsic sensory impulses, visual, tactile, musculo-sensory, et cetera, collect at the threshold of what was originally the vestibular cerebellar rudiment; and they can be seen during the latter part of the second month migrating from the rhombic lip into the marginal layer of the cerebellum, until eventually its whole surface has been invaded by these alien neuroblasts, so
FIG. 513.-THE POSTERIOR ASPECT OF A FETAL
that the originally non-nucleated marginal layer becomes a densely packed granular layer (stratum marginale embryonale). When this stage is reached the cerebellum consists of an inner ependymal layer, a mantle layer crowded with locally developed neuroblasts, a clear layer (the inner part of the original marginal layer), and the superficial layer of neuroblasts which have invaded the outer part of the marginal layer. As development proceeds in the mantle layer the axons of its neuroblasts are directed mainly towards the ventricular surface the reverse of what happens in the spinal medulla; and as these fibre-masses increase in quantity the main part of the mantle layer becomes pushed farther and farther away from the ependyma by the accumulation of their own (and other) axons. Some of the neuroblasts, however, do not become pushed out into the line of the embryonic cerebellar cortex, but remain behind amidst the fibre-mass and receive the axons that come from the cortical cells. These neuroblasts left amidst the fibres gradually assume the form of the dentate, fastigial, globose, and emboliform nuclei already described; and their axons pass out (as the brachia conjunctiva) to the thalamus, mesencephalon, and pons. In the meantime many of the neuroblasts of the mantlei layer have been converted into the large pear-shaped Purkinje-cells.
While these events have been occurring in the true mantle layer a peculiar process has been taking place in the superficial layer of immigrant cells. One by one they begin to leave their places upon the surface and dip into the mantle layer; many of them pass between the Purkinje-cells to a deeper plane, where they cease their wanderings and form a densely packed layer of granule cells (Fig. 514), the axons of which indicate the course of these migrations.
MINUTE STRUCTURE OF A CEREBELLAR FOLIUM.
A cerebellar folium is composed of a central core of white matter, covered with a layer of gray matter. The gray cortex is arranged in two very evident layers, viz., a superficial molecular layer and a subjacent rust-coloured granular layer. Between these strata a single layer of large cells, termed the cells of Purkinje, are disposed in the form of a very nearly continuous sheet. The cells of Purkinje constitute the most characteristic, and probably the most essential, constituents of the cerebellar cortex.
The cells of Purkinje are most numerous on the summit of the folium. At the bottom of the sulci which intervene between the folia they become fewer in number, and, therefore, looser in their arrangement. Each consists of a large flask-shaped or piriform cell body, the narrow end of which projects into the molecular layer, whilst the thicker, deeper end rests on the granular layer. From the latter arises a single axon, which passes into the granular layer and presents the peculiarity of almost immediately assuming its medullary sheath. From this axon a few collateral branches soon arise, which, taking a recurrent course, enter the molecular layer, to end in connexion with certain of the adjoining cells of Purkinje. They would seem to have the function of binding together adjacent cells, and thus enabling them to carry on their operations in harmony with each other.
The dendritic processes spring from the narrow end of the cell in the form of either one or perhaps two stout stalks. These ascend into the molecular layer, branching and rebranching until an aborescent arrangement of extraordinary richness and extent results. The dendritic branches extend throughout the entire thickness of the molecular layer, and the branching takes place in one plane only, viz., in a plane which is transverse to the long axis of the folium. Consequently, it is only when transverse sections through a folium are made that the full dendritic effect is obtained (Fig. 515); in sections made parallel to the long axis of the folium the cells are seen in profile, and are observed to occupy quite a narrow area (Fig. 514). The branching of the dendrites of a cell of Purkinje may, therefore, be compared to that which takes place in the case of a fruit-tree which is trained against a wall.
In the molecular layer the cells are not very numerous, and of these the most characteristic are the basket-cells which lie in the deeper part of the layer. In addition to numerous dendrites the basket-cell gives off an axon which runs transversely, as regards the long axis of the folium, between the planes of adjacent dendritic arborisations of the cells of Purkinje. At first very fine, these axons gradually become coarse and thick, and at intervals they give off collaterals which run towards the bodies of the cells of Purkinje. Reaching these, they break up into an enormous number of fine terminal