Medical brain notes 24

  Gross Anatomy of the Cerebellum The cerebellum (or small brain) lies in the posterior cranial fossa. In the adult the weight of the cerebellum is about 150 g. This is about ten per cent of the weight of the cerebral hemispheres. Like the cerebrum, the cerebellum has a superficial layer of grey matter, the cerebellar cortex. Because of the presence of numerous fissures, the cerebellar cortex is much more extensive than the size of this part of the brain would suggest. It has been estimated that the surface area of the cerebellar cortex is about fifty per cent of the area of the cerebral cortex. The cerebellum lies behind the pons and the medulla. It is separated from the cerebrum by a fold of dura mater called the  tentorium cerebelli.  Anteriorly, the fourth ventricle intervenes between the cerebellum (behind), and the pons and medulla (in front). Part of the cavity of the ventricle extends into the cerebellum as a transverse cleft. This cleft is bounded cranially by th...

  Grey matter of the cerebellum          Most of the grey matter of the cerebellum is arranged as a thin layer covering the central core of white matter. This layer is the cerebellar cortex . The subdivisions of the cerebellar cortex correspond to the subdivisions of the cerebellum described above.         Embedded within the central core of white matter there are masses of grey matter which constitute the  cerebellar nuclei.  These are as follows (Fig. 7.4). 1.                The  dentate nucleus  lies in the centre of each cerebellar hemisphere. Cross sections through the nucleus have a striking resemblance to those through the inferior olivary nucleus. Like the latter it is made up of a thin lamina of grey matter that is folded upon itself so that it resembles a crumpled purse. Both the nuclei have a hilum directed m...

  White matter of the cerebellum          The central core of each cerebellar hemisphere is formed by white matter. The peduncles are continued into this white matter. The white matter of the two sides is connected by a thin lamina of fibres that are closely related to the fourth ventricle. The upper part of this lamina forms the superior medullary velum , and its inferior part forms the inferior medullary velum. Both these take part in forming the roof of the fourth ventricle. The white matter consists of: a.                Afferent fibres entering the cerebellum from outside. b.                Projection fibres from the cerebellar cortex to the cerebellar nuclei. c.                 Association fibres interconnecting different par...

  Cerebellar Peduncles          The fibres entering or leaving the cerebellum pass through three thick bundles called the cerebellar peduncles: superior, middle and inferior. Inferior cerebellar peduncle          This peduncle is also called the  restiform body . This is a thick bundle of fibres that connects the posterolateral part of the medulla with the cerebellum. The peduncle passes upwards and laterally along the inferolateral margin of the rhomboid fossa (floor of fourth ventricle, Fig. 20.12). Near the upper end of the medulla the peduncle lies between the superior cerebellar peduncle (on its medial side) and the middle cerebellar peduncle (laterally). The inferior peduncle then turns sharply backwards to enter the while core of the cerebellum.         Over the medial part of the inferior cerebellar peduncle there are fibres that pass through the vestibula...

  DEVELOPMENT OF THE CEREBELLUM        The cerebellum develops from the dorsolateral part of the alar lamina of the metencephalon (Fig. 7.5A). Obviously, there are at first two primordia of the cerebellum, right and left. These extend medially in the roof plate of the metencephalon to eventually fuse across the midline (Figs. 7.5B, C). As the cerebellum increases in size, fissures appear on its surface. The lateral lobes and vermis can soon be distinguished, as a result of differential growth.         The developing cerebellum can be divided into: (a) an  intraventricular part  that bulges into the cavity of the developing fourth ventricle, and (b) an  extraventricular part  that is seen as a bulging on the surface (Fig. 7.5C). At first the intraventricular part is the larger of the two, but at a later stage, the extraventricular part becomes much larger than the intraventricular part and constitut...