NERVE 125 



giddiness and an inability to walk steadily This is due 

 to the fact that the sudden stoppage of rotation induces 

 the sensation of rotation in the opposite direction, while 

 the visual impressions convey the information that this 

 rotation is not actually taking place. 



B. The Cerebellum. 



The cerebellum is developed primarily as the ganglion 

 or receiving mass of cells for the vestibular mechanism, and in 

 it stimuli from that mechanism are associated with those 

 from other receptors, so that they may be harmonised and 

 all the movements of balancing adjusted without implication 

 of consciousness. 



1. Structure. 



The cerebellum lies above the fourth ventricle, and is 

 joined to the cerebro-spinal axis by three peduncles on 

 each side (tig. 58). It consists of a central lobe, the upper part 

 of which is the superior vermis, and two lateral lobes, each with 

 a secondary small lobe, the flocculus. Its surface is raised 

 into long ridge-like folds running in the horizontal plane, 

 and is covered over with grey matter, the cortex. 



In the substance of the white matter, forming the centre 

 of the organ, are several masses of grey matter on each side, 

 the most important of which are — (i.) the group of roof 

 nuclei; and (ii.) the dentate nucleus (fig. 58;. 



The cortex may be divided into an outer somewhat homo- 

 geneous layer (the molecular layer, fig. 57, G.L.) and an 

 inner layer studded with cells (the nuclear layer, N.L.). 

 Between these is a layer of large cells — the cells of 

 Purkinje (P.). 



By Golgi's method the arrangement of fibres and 

 cells in the cerebellar cortex has been shown to be as 

 follows : — 



(i.) Fibres coming into the cortex from the white matter 

 either end in synapses round cells in the nuclear layer, or 

 proceed at once to the outer layer (fig. 57). (ii.) From the 

 cells in the nuclear layer, processes pass to the outer layer 



