THE FUNCTIONS OF THE CEREBELLUM 451 



both at rest and during locomotion. This organ is the cerebellum, 

 associated with the grey matter in immediate connection with it 

 in the upper part of the fourth ventricle, and situated at the point 

 of entry of the vestibular nerves. The cerebellum commences in 

 early foetal life as a small elevation in the dorsal wall of the neural 

 tube, in close connection with the point of entry of the eighth 

 nerve. Simple in structure [and small in extent in most of the 

 fishes and amphibia, it grows^in extent with increasing complexity 

 of the animal's motor reactions, and attains its greatest develop- 

 ment in the mammalia. In this class the cerebellum, like the 

 cerebrum, is most highly developed in man and the higher apes. 

 It is generally described in man as consisting of a middle lobe, 

 composed of the superior and inferior vermis, with two lateral 

 hemispheres, and these are subdivided by anatomists according to 

 the situation of the chief sulci. From the physiological point of 

 view the structure of the organ is relatively simple, as is shown by 

 the uniformity of its structure throughout all parts. It may be 

 considered as formed of two main structures, viz. the cortex and the 

 central or roof ganglia. 



The surface of the cerebellum is increased by being thrown into 

 folds or laminae, so that a section of this organ has a tree- like appear- 

 ance. A section through a lamina shows three distinct zones : an 

 outer molecular layer presenting a granular appearance with a few 

 nuclei ; internal to this a granule layer composed of many nuclei of 

 nerve- cells ; and most deeply a central core of white matter. Between 

 the molecular and granular layers are situated the cells of Purkinje, 

 large flask-shaped cells each with one apical dendrite, distinguished 

 above all other dendrites of the central nervous system by the richness 

 of its branching, and with one axon, which leaves the base of the 

 cell and passes down into the central white matter, giving off collaterals 

 in its course. In preparations made by Golgi'a method we are able to 

 distinguish the various elements composing these layers and their 

 relations. The molecular layer, besides neuroglia-cells and the 

 branching dendrites of the cells of Purkinje, contains certain star- 

 shaped cells (a, Fig. 201 A), which give off an axon running parallel 

 with the surface in the molecular layer. From this axon branches dip 

 down towards the cells of Purkinje, where they end in a rich basket- 

 work of fibres around the body and beginning of the axon of these 

 cells. The nuclear or granular layer presents two kinds of cells. 

 The most numerous is a small cell with a few short dendrites, each 

 of which terminates in a claw-shaped arborisation, and a single 

 long axon, which passes straight up into the molecular layer, where 

 it bifurcates. The two branches run parallel with the surface in a 

 direction at right angles to the plane of expansion of the dendrites 



