506 ELEMENTARY PHYSIOLOGY less. 



pons Vai-olii (Fig. 160), and constitute the middle 

 peduncle of each half of the cerebellum. But each 

 half has a further attachment by means of two other 

 bands of fibres. Of these one coming out of the central 

 (medullary) part of the cerebellum on each side, runs 

 forwards towards, and disaj^pears under, the corpora 

 quadrigemina ; this forms the superior peduncle. 

 The otlier runs backwards towards the bulb and merges, 

 as the inferior peduncle, into that part of the bulb 

 wliich is a continuation ujjwards of the lateral columns of 

 white matter of the spinal cord. 



We have seen that the spinal cord consists essentially 

 of a central canal surrounded by grey matter containing 

 nerve cells, external to which is a covering of white 

 matter composed of nerve fibres ; and the arrangement of 

 the grey and white matter is comparatively simple. Now 

 from the description we have so far given of the bi'ain, it 

 is evident that the brain may also be regarded as being 

 built up of structures which arc placed round the sides of 

 a central canal, which is really continuous with the canal 

 of the spinal cord. But, unlike the latter, the canal of 

 the brain, consisting of the ventricles and aqueduct, is not 

 a simple straight tube, but has a very peculiar sliape. 

 Moreover, although tlie brain is made up of grey and 

 white mattei's, which by their greater or less development 

 form the structures of varying size which make up the 

 brain as a whole, the grey and white matters are not ar- 

 ranged in any simple way as they are in the spinal cord. 

 On the contrary, although in the brain a great deal 

 of the grey matter is placed externally to the 

 white, the latter is interspersed with localised de- 

 posits of grey matter, some large, some small, wliich give 

 to the whole an extraordinary complexity. And this 

 complexity is still fui-ther increased by the existence of 

 strands or bundles of nerve fibres, which serve to inter- 

 connect all these various deposits of grey m.itter, so as to 

 ensure the possibility of co-ordinated action between the 



