CerebellumExternal Appearances. - The cerebellum consists of three lobes - one median, known as the vermis or worm, and two lateral, the cerebellar hemispheres. On the upper surface the vermis presents as a slight median elevation between the hemispheres; on the under surface it lies at the bottom of a deep groove and is partly hidden by the overlapping of the hemispheres.

The surface of the cerebellum is incised by a series of slightly curved transverse fissures, practically parallel to one another and lying very close together (Fig. 51).

Some of the fissures extend for a considerable distance into the cerebellar substance, and are used for descriptive purposes to divide the cerebellum into lobes which, however, are of no specific physiological importance.

The cerebellum is connected with the rest of the brain by three roots or peduncles - superior, middle, and inferior - on either side. The superior peduncles run upwards towards the mid-brain, gradually converging towards one another as they ascend.

They are connected to one another by a thin plate of nervous tissue - the valve of Vieussens - and disappear from surface view under the inferior corpora quadrigemina.

The middle peduncles are the great transverse strands which give form to the the continuations of the restiform bodies of the medulla (rig. 52).

Internal Structure. - If the cerebellum be cut across in vertical antero-posterior sections the laminated or foliated appearance produced by the fissures of the surface is well demonstrated. Further it will be seen that the central part consists largely of white fibres, amongst which, however, are some independent masses of grey matter; the largest of these latter are known as the dentate nuclei (one in each hemisphere). The whole cerebellar surface is formed each individual lamina and lines all the fissures (Fig. 53).

In the grey matter of the cerebellum many of the already described fibres of the medulla terminate. Thus, entering by the inferior peduncle are: the direct cerebellar tract, fibres from the gracile and cuneate nuclei, and fibres from the inferior olivary nuclei.

An important set of fibres entering by this route are those from the auditory nerve, which will be described later. Running in by the middle peduncle are fibres derived from the small nuclei pontis.