Lobes of the upper surface.-We may now describe in detail the subdivisions of the upper surface, taking (since they are directly continuous with one another) those of the worm and hemispheres together.

The lingula.—This is usually confined to the worm, and from the surface it is entirely concealed by the next lobe: it is best seen in a median section of the organ (figs. 57 and 59). It consists of a small tongue-shaped group of four or five transverse laminæ, which may be said to lie upon the middle of the superior medullary velum (fig. 58, 1). Its medullary centre is in continuity with the velum, and forms part of the roof or dorsal boundary of the fourth ventricle which here has a tent-shaped projection into the middle of the cerebellum (fig. 57, 4).

The lingula gradually shades off at the sides and is usually unrepresented in the hemispheres, but its lamina are sometimes prolonged laterally for a little distance over the superior cerebellar peduncle (frænulum lingulæ).

The central lobe and its alæ.-The central lobe is largely concealed by the culmen when the cerebellum is in its natural position, but when the organ is cut away from the adjacent structures this lobe is seen in the anterior notch, where its laminæ appear at the surface. They are prolonged beyond the limit of the worn for some distance along the upper and anterior part of the hemispheres, where they form wing-like continuations of the central lobe which are known as the alæ lobuli centralis (fig. 58). The central lobe receives a primary branch of the arbor vitæ which passes upwards and forwards into it from the enlargement of the white centre which is known as the trapezoidal body, but the fissures (precentral and postcentral) which separate it from the lingula and culminate lobe respectively, are not better marked at the surface than those which intervene between the laminæ of the culminate lobe, and, as a glance at the sections (fig. 59) shows, its laminæ all belong to the same (ascending) group of folia as those which constitute the culminate lobe.

The culmen and anterior crescentic lobes: lobus culminis.-The culmen occupies rather more than half of the upper surface of the worm, and, as its name implies, constitutes the most prominent part of the upper worm. Its surface shows three or four well-marked lamellæ, beset by a number of secondary and tertiary folia. It is separated from the next part of the worm by a deep groove which descends to the middle of the organ; this sulcus is prolonged, as we have already seen, on to either hemisphere, having there been termed the antero-superior sulcus, and passing in a curved manner parallel with the general disposition of the laminæ on this surface to the antero-lateral margin, where it runs into the anterior part of the great horizontal fissure. The antero-superior sulci together may conveniently be termed the preclival. The subdivision of the hemisphere which is cut off between this preclival fissure behind and the postcentral sulcus in front has been known as the anterior crescentic lobe (lobus lunatus anterior, Kölliker). The two anterior crescentic lobes, together with the culmen with which they are in complete lateral continuity, form a main subdivision of the upper surface of the cerebellum, which may appropriately be termed the lobe of the culmen. The lateral parts of the lobe each receive three well-marked branches of the medullary centre of the hemispheres; in the central part they come off by a common stem from the corpus trapezoides of the worm (fig. 59).

The clivus and posterior crescentic lobes: lobus clivi.-Behind the antero-superior or preclival fissure, and extending as far as the folium cacuminis (from which it is separated by the postero-superior or postclival fissure), is another considerable group of lamine which receive their branches from the upper aspect of the horizontal stem of the arbor vitæ. In a median section of the worm these laminæ appear to form one group with the folium cacuminis and the laminæ of the tuber valvulæ (fig. 59 A), and this group has been described by Schwalbe as con

stituting a posterior lobe. In fact, however, the great horizontal fissure below the folium cacuminis and the postclival fissure above this folium separate the group into three well-marked divisions, of very unequal size it is true in the worm, owing to the rudimentary character of the central or cacuminate division, but far more distinct and equal in the hemispheres; they may therefore be conveniently thus subdivided in the worm also, and of them, two (the clivus and folium cacuminis) belong to the upper worm, the third, tuber valvula, to the lower.

Two or three secondary laminæ of the clivus reach the surface of the worm, but they are beset with many tertiary folia, and other important folia belonging to the same group lie concealed in the preclival fissure.

The lateral extension of the clivus on to each hemisphere is known as the posterior crescentic lobe (l. lunatus posterior, Kölliker), and the two posterior crescentic lobes with the clivus between them, bounded in front by the preclival, behind by the postclival fissure, may collectively be termed the lobe of the clivus. The lateral parts of this lobe each receive two or three primary branches of the medullary centre of the hemisphere (fig. 59).

The combined anterior and posterior crescentic lobes of each hemisphere were formerly termed the quadrilateral lobe.

The folium cacuminis and postero-superior lobes: lobus cacuminis. -The folium cacuminis is formed by the extremity or apex of the main horizontal stem of the arbor vitæ vermis (figs. 57, 59). As the vertical section shows, it is composed of but a single primary folium, which may be either plain or beset with rudimentary folia. But at the side of the worm it rapidly expands, with divergence of its bounding fissures and a great increase of size of its main branch of the arbor vitæ and the formation of numerous secondary and tertiary folia, a large lobe being thereby produced at the posterior and upper part of each hemisphere which has been termed the postero-superior lobe. The expansion occurs mostly above the horizontal plane, and its branch of the arbor vitæ has a direction no longer directly backwards, but rather upwards and backwards; this upward shifting appears to be due to the great development of the lobes of the lower surface of the hemisphere. The postero-superior lobes are bounded, like the folium cacuminis itself, in front and above by the postclival fissure, below by the great horizontal fissure; joined as they are in the middle line by the folium cacuminis, they form a great winged mass which occupies the posterior third of the upper surface of the cerebellar hemispheres, and forms the rounded postero-lateral border; to this conjoined mass the term lobus cacuminis may be applied.

UNDER SURFACE.-Turning now our attention to the under surface of the organ, we here meet with considerably greater complexity, and the correspondence between the parts of the worm and those of the hemisphere is less clearly apparent. The lower worm extends from the inferior medullary velum to the folium cacuminis (great horizontal fissure), and the parts or lobules which are enumerated in it are four in number, viz.: from before back (1) the nodule, (2) the uvula, (3) the pyramid, and (4) the tuber valvulæ seu posticum. On the hemisphere a greater number of lobes are distinguished, viz.: (1) the flocculus, corresponding with the nodule; (2) the tonsil (amygdala), corresponding with the uvula ; (3) the biventral lobe, corresponding with the pyramid; (4) the slender lobe, formed of an anterior and a posterior part well marked off from one another; and (5) the inferior semilunar lobe, which also shows a tendency to subdivision. This last lobe corresponds with and is directly in lateral continuity with the tuber posticum of the worm. The slender lobes are interpolated in the hemisphere, and have not any obvious prominence of the worm corresponding with them, but they appear to represent a development of certain rudimentary folia which are seen in

sagittal sections of the worm on the lower part of the stalk of the tuber valvulæ, entirely concealed by the pyramid (see fig. 59 B). If this is the case, then the combined slender and inferior semilunar lobes may be reckoned as collectively forming a primary subdivision of the hemisphere, corresponding with the tuber posticum of the worm and to which the term postero-inferior may be applied,' and the two postero-inferior lobes, together with the tuber valvula which unites them in the middle line, may collectively be spoken of as the lobus tuberis. This large lobe occupies at least two-thirds of the inferior surface of the cerebellum; of the remaining third more than one-half is occupied by the combined pyramid and biventral lobes, about one-third by the uvula and amygdala, and the small remaining portion by the nodule and flocculus.

Fissures of the under surface. The lobes of the under surface of the organ are separated from one another by fissures extending deeply into both the worm and hemispheres. In the worm there are three such interlobar depressions, viz.: one between the nodule and the uvula (postnodular), one between the uvula and pyramid (prepyramidal), one between the pyramid and tuber valvula (postpyramidal), besides the middle of the great horizontal fissure separating the tuber valvulæ from the folium cacuminis. In the hemispheres a larger number is apparent. The postnodular sulcus passes laterally into a groove curved with its convexity forwards which first limits the amygdala in front and then passes between the flocculus and biventral lobe to join the anterior end of the great horizontal fissure. The prepyramidal sulcus curves laterally round the outer side of the amygdala, separating it from the biventral lobe: it joins the postnodular in front of the amygdala. From the sides of the postpyramidal sulcus three deep fissures, concentric with one another, curve outwards and forwards over the under surface of the hemisphere. The most anterior of these, and also the shortest, passes at first more forwards, concealed by the amygdala before curving round in the manner described. It lies between the biventral and slender lobes, and may be termed the anterior arcuate or pregracile sulcus. The second of the three concentric fissures subdivides the slender lobe into two nearly equal parts (lobulus gracilis anterior and lobulus gracilis posterior); we may distinguish it as the middle arcuate or midgracile. The third, which is the longest, separates the slender lobe from the inferior semilunar lobe; like the other two it arises at the mesial edge of the hemisphere opposite the depression (postpyramidal sulcus) between the pyramid and the tuber valvulæ, and arches round on the lower surface of the hemisphere with its concavity forward to fall into the great horizontal fissure at the antero-lateral margin. This fissure may be termed the posterior arcuate or postgracile. Behind this again there is a fairly well marked deep fissure, serving to subdivide the inferior semilunar lobe. This may be termed the lower or lesser horizontal sulcus. Lastly, the deep antero-posterior grooves which mark off the lower worm may conveniently be termed the sulci vallecula (dexter et sinister).

Lobes of the under surface.-The lobes of the under surface of the organ as already enumerated may be now more particularly described in order from before back.

The nodule and flocculus: lobus noduli.—The nodule occupies much the same position relatively to the inferior medullary velum and tent of the fourth ventricle that the lingula occupies with regard to the superior medullary velum, but it is usually better developed than the lingula. Although sometimes merely composed of a few transverse laminæ set upon the median part of the inferior medullary velum, it more often appears as a distinct prominence having a separate branch of the

1 In the previous edition of this work, the term postero-inferior was used as a synonym for the inferior semilunar lobe.

arbor vitæ (fig. 59 A). It can only be seen after the cerebellum is separated from the medulla oblongata and pons, or in a median section, being almost entirely concealed by the uvula when the organ is viewed from below. The lateral part of the inferior medullary velum is continued on either side of the nodule as a thin white semilunar band with a thickened concave free lower border (fig. 58 B, v.m.i). Traced laterally towards the hemisphere, the white lamella becomes thickened and covered with grey matter (pedunculus flocculi), and at length expands into a small irregular lobule lying in the groove between the biventral lobe and the middle peduncle of the cerebellum, and composed of a few short irregular laminæ, which tend to diverge from the attachment of the stalk. The nodule, with the inferior medullary 'velum, the stalk of the flocculus, and the flocculus itself constitute collectively a small but distinct subdivision of the cerebellum (lobus noduli).

Other small portions of laminated grey matter sometimes occur attached to the flocculus, usually lying between it and the middle peduncle. These have been termed accessory flocculi.

The uvula and tonsils: lobus uvula.-The uvula forms a considerable portion of the lower worm, being elongated from before back; three considerable secondary laminæ, beset with eight or more tertiary folia, appear at the surface. It is least prominent close to the nodule, and from this part it enlarges rapidly downwards and backwards to attain its greatest prominence next to the pyramid. Like the nodule and the pyramid it is purely a median prominence, being separated from the hemispheres by the deep groove (sulcus vallecula) which prolongs the vallecula on either side of this part of the worm. At the bottom of this groove on either side of the uvula is a low corrugated greyish ridge which connects the narrow stalk of the uvula with the stalk of the tonsil: this concealed connecting ridge is known as the furrowed band. From its outer extremity the rounded amygdala projects downwards and backwards into the vallecular groove, concealing the sides of the uvula mesially, and the narrow connection between the pyramid and biventral lobe posteriorly. The uvula, furrowed band and tonsil constitute collectively another distinct subdivision of the hemisphere (amygdalo-uvular lobe, lobus uvula). The amygdala or tonsil forms a rounded projection of about 10 or 12 folia, which run nearly in a sagittal direction, so that in a sagittal section of the cerebellum passing through the tonsil, its branch of the arbor vitæ appears expanded and not obviously dendritic (fig. 59 C). Above and in front it has a few transverse laminæ which are only seen in sections, being concealed, in the natural position of the parts, by the rest of the projection. Opposite the inferior medullary velum its white matter comes for a certain distance to the surface, covered, of course, by pia mater. The amygdala is lodged in a depression (nidus avis) at the front of the vallecula, which is bounded by the uvula mesially and the biventral lobe laterally; the surface which rests against the uvula is nearly smooth, as is the corresponding concave surface of the uvula; a continuation of the postnodular sulcus separates it from the inferior medullary velum.

The pyramid and biventral lobes: lobus pyramidis.-The pyramid forms the most marked prominence of the lower worm, but as seen from the surface, it shows only three or four transverse laminæ. It is in reality a long clavate projection attached to the stem of the arbor vitæ by a narrow stalk, and it is not only separated from the uvula and tuber valvulæ by deep fissures, but also from the hemispheres by the sulcus valleculæ on either side of it. Its connection with the biventral lobe of the hemisphere is here maintained by a low narrow ridge, which joins the stalk of the pyramid with the mesial pointed extremity of the biventral lobe. This connecting ridge is evidently analogous with the furrowed band of the preceding lobe, but it is an even less distinct structure To see it the amygdala must be

removed, or a vertical section made through the organ, passing just to the outer side of the pyramid. The biventral lobe is roughly triangular in shape, with the base forwards abutting on the postnodular sulcus, and the apex directed backwards and inwards towards the root of the pyramid. The outer side is bounded by the outwardly curved pregracile fissure, the inner by the side of the vallecula, but is concealed by the amygdala, which projects over the lobe. The lamina have a curved direction radiating from the apex towards the base of the triangle. The lobe is partly bisected by a fissure (midventral) deeper than the rest, and becoming better marked towards the base. This fissure divides the lobe into an inner and an outer portion; from this subdivision its name has been derived. Collectively, the pyramid, the connecting ridges, and the biventral lobes constitute a distinct division of the cerebellum, to which the name lobe of the pyramid may be applied.

The tuber posticum and postero-inferior lobes: lobus tuberis.-The tuber valvulæ seu posticum, which constitutes the hindermost division of the inferior worm, exposes about five or six tertiary folia (laminæ transversales inferiores) at the surface between those of the pyramid and the folium cacuminis. It differs from the other constituents of the inferior worm, and resembles those of the superior worm in being obviously prolonged laterally into and gradually enlarging to form the corresponding hemisphere lobes (fig. 58 B, t.v, l.t). These conjoined postero-inferior lobes of the hemispheres with the tuber valvula of the worm collectively form a large alate mass (lobus tuberis), bounded in front by the anterior arcuate or pregracile fissure, which separates it from the pyramidal (biventral) lobe; behind by the great horizontal fissure, which also limits it antero-laterally. Its laminæ run in a curved manner, concentrically with these fissures, and it is separated into four crescentic parts by three concentric fissures two deep and complete, the middle arcuate and the posterior arcuate and a third only slightly less deep, the lesser horizontal fissure. Of these four parts the two anterior, about equal in size, form what has been termed the slender lobe (lobus gracilis); the two posterior, of which the hinder one is the larger, together form what has been termed the inferior semilunar lobe.

The whole cerebellar worm may thus be regarded as subdivided by deep sulci into nine parts or lobes, each of which has a corresponding lobe of the hemisphere continuous with it. This continuity is obvious upon the upper and posterior aspects of the organ, but on the inferior aspect it tends to be rudimentary and is moreover concealed within the sulci valleculæ, The combined lobes which are thus formed by the continuity of the lobes of the worm with those of the hemispheres are as follows:--(1) lobus lingulæ, (2) lobus centralis, (3) lobus culminis, (4) lobus clivi, (5) lobus cacuminis, (6) lobus tuberis, (7) lobus pyramidis, (8) lobus uvulæ, and (9) lobus noduli. The hemisphere-parts of the lobus lingulæ and of the lobus centralis are rudimentary, but otherwise the hemisphere-parts of the lobes are considerably larger than those of the worm, in some cases, as in that of the lobus cacuminis and the lobus tuberis, the difference of size being very great. The former (lobus cacuminis) is, in fact, represented in the worm by a single concealed folium only, while a large portion of the lobus tuberis, viz., the lobus gracilis, can hardly be said to be represented in the worm. These nine lobes are separated from one another by interlobar fissures, which are all nearly equally well marked in the hemispheres, whilst in the worm some are less developed, particularly those above and below the folium cacuminis. This median part of the lobus cacuminis is, therefore, imperfectly marked off from the clivus above and the tuber valvula below, the three together forming a very distinct posterior subdivision of the worm.

The relations between the parts of the worm and those of the hemispheres, and the fissures which separate the several lobes from one another, as well as those which serve to subdivide the lobus tuberis, are indicated in the accompanying table :—