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The accentuation of the pontine flexure at this stage brings the two cerebellar rudiments into the transverse direction and in line one with the other and with the roof-plate, which is now being thickened by immigrant neuroblasts from the medial extremities of the two cerebellar rudiments. When one organ is thus formed by the union in the roof-plate of the originally separate rudiments, it presents the form of a dumb-bell shaped mass (Fig. 503). Upon the inferior aspect of this mass there is a slight ridge, to which the tela chorioidea ventriculi quarti is attached. Opposite the lateral cerebellar rudiments (but not in the median plane) the attachment of the tela becomes thickened to form the posterior medullary velum.
Early in the third month the growth of the cerebellar rudiment begins to manifest itself by lateral bulgings of its surface.
ENCEPHALON IN A HUMAN EMBRYO.
The rhombic lip, the inferior part of which has been seen to play an important part in the development of the nuclei pontis and nucleus olivaris inferior, is also continued upwards beyond the pontine flexure on to the cerebellar rudiment, FIG. 503.-DORSAL ASPECT OF THE RHOMBwhere it forms a marginal fringe. Thus, even in the second month, a groove can be detected upon the cerebellum separating off a band which is continuous with the tuberculum acusticum. The part nearest to the tuberculum represents the rudiment of the flocculus and the medial extremity the nodulus (Fig. 503). During the third month the cerebellum appears as a rounded bar transversely placed across the upper part of the roof of the fourth ventricle, and as the lateral extremities of this bar expand (Fig. 504), it assumes a dumb-bell shape not unlike that presented a few weeks earlier (Fig. 503) on its ventricular aspect. As these lateral bosses (lobi laterales) develop, a mass of transverse fibres in connexion with them also becomes apparent. It represents the fibræ transversæ of the pons. They arise from the cells (nuclei pontis) which have wandered into the basal lamina of the metencephalon from the rhombic lip of the myelencephalon (Fig. 499); and the fibres which enter each cerebellar boss come mainly from the nuclei pontis of the other side. Towards the end of the fourth month, or even a month earlier in some cases, a little bud grows out from the cerebellum on each side immediately above the flocculus. It is the paraflocculus or flocculus secundarius. In man it never attains a large size, but in most mammals it develops into a large lobe, even as big as one-third the size of a cerebellar hemisphere (in the manatee), and in many animals a deep fossa is formed in the temporal bone to lodge this part of the cerebellum.
Nodulus. Obex. Taenia ventriculi quarti.
FIG. 504. THE POSTERIOR ASPECT OF A FOETAL (FOURTH MONTH) CEREBELLUM, MEDULLA OBLONGATA AND FOSSA RHOMBOIDEA.
As the cerebellum grows the lateral hemispheres expand much more rapidly than the median part-the handle of the dumb-bell. But the superficial area of the latter becomes increased by means of transverse folds which begin to make their appearance at the close of the
third month. Earlier in that month the median part of the cerebellum presents in sagittal section almost a semicircular outline (Fig. 507, A) with a slight notch near its inferior margin (fissura postnodularis) demarcating the nodulus. development proceeds during the third month the nodular region becomes bent forwards upon the rest of the cerebellum (Fig. 507, B), thus starting the posterior diverticulum of the fourth ventricle, which ultimately assumes a tent-like outline Fig. 519).
At the close of the third month the irregular growth of the surface of the
median bridge, which can now be called the vermis, leads to the appearance of
a transverse depression upon the superior surface. This is the fissura prima (Fig. 507, B, C, and D), which becomes the deepest and most complex of all the multitude of fissures that ultimately cut into the i cerebellum (Fig. 519). Soon afterwards the fissura secunda makes its appearance (Fig. 507, C) and with the fissura prima subdivides the vermis into anterior, median, and posterior lobes.1
Öther transverse fissures appear in rapid succession until the vermis becomes cut up into the following parts, named from above (at the velum medullare anterius) downwards: lingula, lobulus centralis, culmen, declive, pyramid, uvula, and nodule.
Quite unnecessary importance is usually attached to the subdivisions of the part here called
Horizontal fissure of cerebellum
FIG. 505.-INFERIOR SURFACE OF THE CEREBELLUM OF A HUMAN FETUS
FIG. 506.-CEREBELLUM OF A HUMAN FOETUS WHICH HAS REACHED THE END
velum medullare ant.
uvula fiss. postnodularis
FIG. 507.-MEDIAN SAGITTAL SECTIONS OF FETAL CEREBELLA IN FOUR STAGES OF DEVELOPMENT. A and B, third month; C, fourth month; D, fifth month. declive, which is described as consisting of three parts (declive, folium vermis, and tuber vermis).
1 The term median is used advisedly because, the anterior and posterior lobes having quite insignificant lateral connexions, the rest of the vermis is virtually the medial continuation of (or bridge between) the lateral lobes.
There is no justification for such a subdivision, nor is any useful purpose served by linking together two parts so distinct, morphologically and physiologically, as the culmen and declive and giving the name monticulus to the complex.
Only some of the fissures of the vermis become prolonged laterally beyond the limits of the vermis, but as the boss-like lateral lobes begin to expand, their surface becomes folded and a series of independent lateral fissures are formed. [The anterior d: lobe, however, is prolonged laterally upon each side into tapering wings and all the fissures in them are merely prolongations of the fissures of the vermis.]
After the limiting fissures of the flocculus and paraflocculus, the first independent fissure to make its appearance is one which develops behind and almost parallel to the lateral prolongations of the fissura le prima. Kölliker called the intervening strip of cerebellum lobulus lunatus posterior and the fissure may be called fissura postlunaris. These postlunar fissures begin far out on the lateral swelling in the fourth monthand gradually approach the median plane, where they may meet and become confluent on the vermis. But it often happens that they do not meet, in which case no folium vermis is cut off the declive.
line of floor of fissura horizontalis
At the end of the fourth or beginning of the fifth month an oval swelling makes its appearance upon each side of the uvula upon the inferior surface of each lateral lobe (Fig. 505). This is called the tonsilla cerebelli or tonsil, and the fissure which develops behind it and delimits it is called posttonsillar. As a rule the two post-tonsillar fissures become confluent with the fissura secunda upon the vermis and the whole furrow in the adult may be called fissura secunda. At the middle of the fifth month a lateral fissure, called parapyramidal, makes its appearance some distance behind the post-tonsillar, from which it is separated by an area called the lobulus biventer. As a rule, these parapyramidal fissures become confluent with the suprapyramidal fissure. The whole furrow is known in the adult by the latter name. The fissure to which most importance is usually attached develops quite late in the human cerebellum, and not at all in that of the great majority of other animals. It is called the fissura horizontalis cerebelli. In the adult it begins upon the front, where the brachium pontis plunges into the cerebellum, and the furrow is formed in a more or less mechanical way by the bulging forwards (above and below the cerebellar peduncles) of the exuberant mass of the cerebellar hemispheres. The actual infolding is preceded by the appearance of several irregular depressions (Fig. 508) in the place where the horizontal fissure will develop. This fissure begins in front and
lips of developing fissura horizontalis
FIG. 508.-THE LEFT LATERAL ASPECT OF THE FETAL RHOMBENCEPHALON
passes continuously round the circumference of the organ, cutting deeply into its lateral and posterior margins. In front, its lips diverge to enclose the three cerebellar peduncles as they pass into the interior of the cerebellum. horizontal fissure divides the organ into a superior and an inferior part, which may be studied separately.
In some cases it meets the corresponding fissure of the other side upon the vermis, but very often such a confluence does not occur. The folium vermis in such cases is not distinguished from the tuber vermis.
The cerebellum is subdivided somewhat arbitrarily into a median portion termed the vermis, and two much larger lateral portions, called the hemispheres. The demarcation between these main subdivisions of the organ is not very evident from every point of view. In front, and also behind, there is a marked deficiency or notch. The incisura cerebelli posterior (O.T. marsupial notch) is smaller and narrower than the anterior notch. It is bounded at the sides by the hemispheres, whilst its bottom is formed by the axial lobe or vermis. It is occupied by a fold of dura mater called the falx cerebelli. The incisura cerebelli anterior (O.T. semilunar notch) is wide, and, when viewed from above, it is seen to be occupied by the inferior quadrigeminal bodies and by the brachia
FIG. 509.-SUPERIOR SURFACE OF THE CEREBELLUM.
Mesencephalon occupying the incisura anterior
Lobulus lunatus oud en
conjunctiva. As in the case of the posterior notch, its sides are formed by the hemispheres, and the bottom by the vermis.
On the superior surface of the cerebellum there is little distinction to be noted between the median lobe and the superior surface of each hemisphere. On this aspect the median lobe receives the name of superior vermis, and it forms a high median elevation, from which the surface slopes gradually downwards on each side to the margin of the hemisphere. The superior vermis is highest in front, immediately behind the anterior notch, and from this it shows a somewhat sharp descent towards the posterior notch. This elevation of the superior vermis is frequently called the monticulus. The folia on the surface of the superior vermis are thicker and fewer in number than those on the upper surface of the hemisphere. It is this which gives it the worm-like appearance from which it derives its name.
On the inferior surface of the cerebellum the distinction between the three main constituent parts of the organ is much better marked (Fig. 510). On this aspect the hemispheres are full, prominent, and convex, and occupy the cerebellar fossæ in the floor of the cranium. They are separated by a deep median hollow, which is continued forwards from the posterior notch. This hollow is termed the vallecula cerebelli, and in its anterior part the medulla oblongata is lodged. When the medulla oblongata is raised and the hemispheres are pulled apart, so as to expose the bottom of the vallecula, it will be seen that this is formed by the vermis
inferior, or inferior aspect of the median lobe, and, further, that the vermis is separated on each side from the corresponding hemisphere by a distinct furrow, termed the sulcus valleculæ.
Lobes on the Superior Surface of the Cerebellum.-When examined from before backwards, the superior vermis presents the following subdivisions: (1) the lingula cerebelli; (2) the lobulus centralis; (3) the culmen; (4) and the declive. With the exception of the lingula, each of these is continuous, on each side, with a corresponding district on the upper surface of the hemisphere. Thus, the central lobule is prolonged laterally on each side in the form of a small, flattened, winglike expansion called the ala lobuli centralis. The culmen together with its lateral prolongations can be called the lobulus culminis of the hemispheres; the declive stands in the same relation to the lobulus lunatus; and the postero-superior lobules of the hemispheres may be linked by a folium vermis.
The lingula can be seen only when the part of the cerebellum which forms the bottom of the anterior notch is pushed backwards. It consists of four or five small flat folia, continuous with the gray matter of the vermis superior, which are prolonged forwards on the superior surface of the anterior medullary velum in the interval between the two brachia conjunctiva.
The lobulus centralis lies at the bottom of the anterior cerebellar notch, and is seen only to a very small extent on the superior surface of the organ. It is a little median mass which is prolonged laterally for a short distance round the anterior notch in the form of two expansions, termed the ala lobuli centralis.
The culmen constitutes the highest part or summit of the monticulus of the vermis superior. It is bounded behind by a deep and strongly marked fissura prima, and is prolonged laterally on each side into the hemisphere. This is the most anterior subdivision on the superior surface of the hemisphere.
The declive lies behind the culmen, from which it is separated by the fissura prima, and it forms the sloping part or descent of the monticulus of the vermis superior. On each side it is continuous with the hemisphere, and the three parts are included under the one name of lobulus lunatus (Fig. 509).
Lobes on the Inferior Surface of the Cerebellum.-The connexion between the several parts of the inferior vermis and the corresponding districts on the inferior surface of the two hemispheres is not so distinct as in the case of the vermis superior and the lobules on the superior surface of the hemispheres. A groove, called the sulcus valleculæ, intervenes between the vermis inferior and the hemisphere on each side.
From behind forwards the following subdivisions of the vermis inferior may be recognised: (1) the tuber vermis; (2) the pyramis; (3) the uvula; (4) the nodulus.
On the inferior surface of the hemisphere there are four main lobules mapped out by intervening fissures. From behind forwards these are: (1) the postero-inferior lobule, a large subdivision which bounds the horizontal fissure on its inferior aspect; (2) the biventral lobule, which lies in front of the postero-inferior lobule, and is partially divided into two parts by a curved fissure which traverses its surface; (3) the tonsil, a small rounded lobule which bounds the anterior part of the vallecula, and is lodged in a deep concavity on the medial aspect of the biventral lobule; (4) the flocculus, a minute lobule situated on the brachium pontis of the cerebellum in front of, and partially overlapped by, the anterior border of the biventral lobule.
These lobules, with the corresponding portions of the vermis inferior, constitute the lobes on the inferior surface of the cerebellum. Still, it should be noted that, just as in the case of the superior surface of the organ, this subdivision is to some extent artificial, and is not in every particular provided with a sound morphological basis.
The tuber vermis (usually not definitely marked off from the declive) forms the most posterior part of the vermis inferior, and is composed of several transversely arranged folia which, on either side, run directly into the postero-inferior lobule.
The postero-inferior lobule, which is wider towards the vallecula than it is more