biventral has become distinctly bifid. The nucleus dentatus does not extend far into the lateral part of the hemisphere, and the superior cerebellar peduncle is no longer cut; but fibres are seen streaming from the white centre of the hemisphere into the middle and inferior peduncles. The general conformation of the section is somewhat altered, but the number and relations of the sub-divisions of the hemisphere is not materially different, and the several branches of the arbor vitæ and the more important fissures are readily recognizable.

Nuclei in the white matter of the cerebellum.-The dentate nucleus (corpus ciliare, corpus dentatum) of the cerebellum (figs. 59 C, and 60, n.d.), very

Fig. 60.-SECTION ACROSS THE CEREBELLUM AND MEDULLA OBLONGATA SHOWING THE POSITION OF THE NUCLEI IN THE MEDULLARY CENTRE OF THE CEREBELLUM. (Stilling.).

n.d., nucleus dentatus cerebelli; s., band of fibres derived from restiform body, partly covering the dentate nucleus; s.c.p., commencement of superior cerebellar peduncle; com', com", commissural fibres crossing in the median white matter.

similar to that already described in the olivary body of the medulla oblongata, presents the appearance of a waved line of compact yellowish brown substance, containing white matter within. The wavy character is more apparent in horizontal than in vertical sections through the hemisphere. The line is interrupted at its anterior and mesial part (hilum), where the superior cerebellar peduncle emerges from it. The dentate nucleus may be described as consisting of a plicated pouch or capsule of grey substance open at one part and enclosing white matter in its interior, like the dentate nucleus of the lower olivary body.

In addition to the corpus dentatum certain other portions of grey matter, which have been only more recently recognised, are found in the white centre of the cerebellum (Stilling). They are three in number on each side and are termed respectively the nucleus emboliformis, nucleus globosus, and nucleus fastigii (figs. 60. 61). The nucleus emboliformis is a small clavate mass of grey substance lying mesially to and partly covering the hilum of the dentate nucleus. On the inner side of the nucleus emboliformis, between it and the middle line, is a streak of grey matter passing antero-posteriorly and ending behind in an enlarged extremity. This has been named the nucleus globosus. Finally, close to the middle line, where

it is only separated from its fellow by a narrow septum of white matter, is a rather larger portion of grey substance, which lies in the anterior part of the white centre of the worm, and close to the upper wall of the tent-like projection in the roof of the fourth ventricle. It is termed the nucleus of the roof or nucleus fastigii.

Fig. 61.- HORIZONTAL SECTION THROUGH

THE WHITE CENTRE OF THE CEREBELLUM,
SHOWING THE NUCLEI OF GREY MATTER.

(From Henle, after Stilling.)

The section is taken just over the roof of the fourth ventricle. The nuclei are represented lighter than the white matter in which they are embedded.

Cd, corpus dentatum; x, nucleus emboliformis; y, y, nucleus globosus; z, nucleus fastigii. Above the two coalesced roof-nuclei are seen some of the fibres of the superior (anterior) decussation, and above these again the lamina and furrows of the lingula (Lg); whilst below the roof-nuclei one or two laminæ and furrows of the inferior vermiform process are included in the section. Ccq, superior cerebellar peduncle.

These several portions of grey matter are not entirely isolated, but are connected here and there both with one another and with the dentate nucleus.

The structure of the corpus dentatum resembles that of the olivary body. Stellate cells th to 26th inch (6μ to 10μ) in size, lie in grey matter which is traversed by bundles of nerve-fibres, passing in various directions but chiefly from without inwards.

The nucleus emboliformis agrees closely in structure with the nucleus dentatus, to which it seems to bear the same relation as do the accessory olivary nuclei to the chief olivary nucleus. The nucleus fastigii and nucleus globosus differ somewhat in structure from the dentate nucleus, and chiefly in the much larger size of their cells, which, according to Meynert, are very similar to those of the nucleus of Deiters in the medulla oblongata (see p. 56).

Commissural fibres in the white matter of the cerebellum.-Two chief sets of decussating commissural fibres were described by Stilling in the middle line of the cerebellum; one at the superior part of the worm at the base of the central lobule-the superior commissure (fig. 60, com'); the other at the inferior part (inferior commissure (com")). Commissural fibres also pierce the nuclei of the roof. In addition to these crossing fibres, which connect the two halves of the white centre, other association fibres connect one lamina with another, passing in the white substance of the laminæ across their general direction, and arching round the fissures between the laminæ.

Peduncles of the cerebellum.-The cerebellar peduncles are constituted by white fibres which pass out from or into the white medullary substance of the hemispheres.

The superior peduncles (crura ad cerebrum) emerge from the upper and mesial part of the medullary substance of the hemispheres, and run upwards and forwards towards the corpora quadrigemina, under which they eventually pass and thus disappear from the surface. They are situated at first at the side, but subsequently in the roof, of the upper part of the fourth ventricle. These peduncles are concealed by the upper part of the cerebellum, so that to see them properly this must be

divided in the middle line and turned aside. When this is done the superior crura, with the superior medullary velum stretched out between them, are brought into view. Their further course in the mid-brain will be subsequently traced.

The fibres of the superior peduncle pass almost entirely out of the interior of the dentate nucleus (intraciliar fibres), but some fibres curve round the outer side of this without passing into it (extraciliar), and some of the mesial fibres are traceable directly into the white substance of the worm, Probably many of the fibres of these peduncles which emerge from the dentate nucleus are connected with its cells, but others pass in bundles through the grey lamina which composes it, without

Fig. 62.-FIGURE SHOWING THE THREE PAIRS

OF CEREBELLAR PEDUNCLES. (From
Sappey after Hirschfeld and Leveillé.)

On the left side the three cerebellar peduncles have been cut short; on the right side the hemisphere has been cut obliquely to show its connection with the superior and inferior peduncles.

1, median groove of the fourth ventricle; 2, the same groove at the place where the auditory striæ emerge from it to cross the floor of the ventricle; 3, inferior peduncle or restiform body; 4, funiculus gracilis ; 5, superior peduncle; on the right side the dissection shows the superior and inferior peduncles crossing each other as they pass into the white centre of the cerebellum; 6, fillet at the side of the crura cerebri; 7, lateral grooves of the crura cerebri ; 8, corpora quadrigemina.

being thus connected. They appear to go eventually to the superficial grey matter of the laminæ.

From the superior medullary velum longitudinal fibres can be traced passing into the white centre of the worm. These are chiefly fibres belonging to the anterolateral ascending cerebellar tract (see pp. 25 and 65).

Many, if not most, of the fibres of the superior peduncle originate in cells within the cerebellum, and undergo degeneration as the result of lesions of that organ (see p. 93). But in a case, reported by Mendel, of lesion of the left thalamus opticus, a well-marked bundle of degenerated fibres was traceable through the tegmentum of the left side mesial to the nucleus tegmenti, across to the right side at the decussation of the superior peduncles, and along the outer side of the right superior peduncle to the right hemisphere.

The middle peduncles (crura ad pontem), distinguished by the small size of their fibres, coming from the pons Varolii, enter the lateral part of the white matter in two main bundles. One of these, composed of the superior transverse fibres of the pons which pass obliquely downwards over the others (fig. 30, i), radiates into the lateral and lower parts of the medullary centre of the hemispheres. The other bundle, which is formed of the lower transverse fibres of the pons, is joined at its passage into the white centre by the restiform body or inferior peduncle (fig. 30, k), and the fibres of both turn upwards and radiate into the upper parts of the medullary centre of the hemisphere, and partly into the upper part of the worm (but most of the pons fibres enter the hemisphere). Those peduncular fibres which pass into the worm are derived chiefly from the restiform body, and include the large fibres of the dorso-lateral cerebellar tract, most of which go to the same side but some pass across to the opposite side. Those which enter the hemisphere curve over the corpus dentatum, and are termed by Stilling the semicircular fibres (fig. 60, s). They come mainly from the opposite olivary through the restiform body. A small part of the fibres of the restiform body is said by Stilling to end in the corpus dentatum.

The fibres of the middle peduncle, when traced ventralwards into the pons, reach the middle line and there undergo decussation. After this intercrossing many of their fibres appear to end in the grey matter which is so abundant in the ventral part of the pons (nuclei pontis, fig. 49, n.p.). A certain number of fibres, however, take a sagittal direction in the raphe and pass towards the reticular formation, where they appear to give fibres to the posterior longitudinal bundle; by which means a direct connection seems to be established between the cerebellum and the nuclei of the third, fourth, and sixth nerves (Mingazzini).

The inferior peduncles (crura ad medullam) issue from the white matter of the lateral hemispheres, between the other two, and pass forwards immediately outside the superior peduncles to reach the lateral wall of the fourth ventricle. Here they turn sharply downwards, at a right angle, and become the restiform bodies of the medulla oblongata.

The restiform body consists of several sets of fibres having a distinct origin, and obtaining their medullary sheath at different periods of development. These fibres are as follows:-(1) Fibres of small size derived from the contralateral lower olives. These, which are the last to become medullated, are seen passing as arched fibres through the corresponding half of the medulla oblongata and across the raphe to enter the hilum of the opposite olivary nucleus. After passing through the band of grey matter, whether joining its cells or not is not certainly known, these fibres appear to pass longitudinally upwards in the reticular formation of the medulla oblongata and pons, and in the tegmentum of the cerebral peduncle, and thus to reach the cerebral hemisphere without again crossing: ultimately they are in all probability connected with the cerebral cortex (? of the psychomotor region only). The existence of this connection may probably explain those cases in which atrophy of one of the cerebral hemispheres, especially of the psychomotor region, has been found associated with atrophy of the inferior olive of the same side and of the restiform body and cerebellar hemisphere of the opposite side. (2) Fibres which emerge from the adjacent cuneate nucleus (especially its outer portion), and perhaps also from the gracile nucleus, and pass directly into the restiform body of the same side. These fibres may represent a bulbar ascending cerebellar tract homologous with the dorso-lateral cerebellar tract of Flechsig which is seen in the spinal cord, in which case the outer cuneate nucleus may very probably represent Clarke's cell-column of the cord.

According to some authorities, the restiform body also receives a contribution through the arched fibres from the contra-lateral nucleus gracilis and nucleus cuneatus.

(3) Fibres of the dorso-lateral ascending cerebellar tract of Flechsig, which are traceable along the whole length of the cord from the lumbar region upwards, and which pass into the restiform body, and through this mainly into the same side of the worm.

(4) Fibres of the descending cerebellar tract (see p. 25) which, after removal of the cerebellar hemisphere, undergo degeneration down the whole length of the antero-lateral column of the cord near its periphery (see p. 32, and fig. 71, A, B, C). (5) Fibres which are passing to or are derived from the root of the auditory nerve and perhaps others to or from some of the other cranial nerves (see p. 93).

MICROSCOPIO STRUCTURE OF THE LAMINÆ.1

Each lamina of the cerebellum has a central part or core of white substance which is an offshoot (secondary or tertiary) from the white centre of the organ,

1 Our knowledge of the actual relationship of cells and nerve-fibres in the cerebellar cortex has been only quite recently entirely remodelled, owing to the introduction of the method invented by Golgi, and its fruitful application firstly by Golgi himself and subsequently by Ramón y Cajal.

and a cortex of grey matter consisting of two layers, an inner and outer, the latter being covered superficially by pia mater. Between the inner and outer layers of grey matter is an incomplete stratum of large nerve-cells, the corpuscles of Purkinje. The fibres of the white matter are medullated, and are disposed in bundles which have a parallel course as they pass from the principal offshoots of the white centre

Fig. 63.-SECTION OF CORTEX OF CEREBELLUM.

(Sankey.)

a, pia mater; b, external layer; c, layer of corpuscles of Purkinje; d, inner or granule layer; e, medullary centre.

of the organ into the secondary laminæ.
This parallelism is maintained in their
passage through the centre of the
laminæ, but the fibres gradually turn
off obliquely into the grey matter, so
that the white core gradually thins off
towards the extremities of the laminæ.
Owing to the turning outwards and
passage into the grey matter of these
bundles of white fibres, the white core
is not sharply marked off under the
microscope from the grey cortex ; but
it is more distinctly marked off at the
bottom of the fissures which separate
the lamina than in the laminæ them-
selves. As the fibres pass radially into
the grey matter they lose their parallel
arrangement, and tend to branch
amongst the small nerve-cells of the
adjacent inner layer of the grey matter;
many pass through this and end in the
axis-cylinder processes of the cells of
Purkinje, whilst others pass beyond
these cells into the outer or
"mole-
cular" layer of the grey matter.

The grey matter of the cerebellar

cortex is disposed, as already intimated,

in two distinct layers. The inner or granule layer is so called because it contains numerous small nerve-cells known as "granules:" this layer has a reddish or yellowish-brown colour in the fresh condition, hence it is sometimes termed the "rust-coloured" layer. The granules are more closely packed in the outer part of the layer; near the medullary centre of the lamina they are separated by the entering bundles of white fibres, between which they may penetrate for some distance within the white centre. Besides small nerve-cells the granule-layer includes a few glia-cells. The outer or molecular layer has, under the microscope, a finely punctated (molecular) appearance. It is of fairly uniform thickness, whereas the granule-layer is thicker near the extremities of the lamina than in the furrows. It contains nerve-cells, but they are neither so numerous nor so small as the "granules" of the inner layer, many nerve-fibres, mostly running parallel to the surface, and also a number of fibres which run vertically to the surface (Bergmann's fibres), and end below the pia mater. These fibres are derived from cells which are situated in the granule-layer, and which are usually regarded as glia-cells (see p. 92).