ANATOMY AND PHYSIOLOGY OF THE BRAIN. 419 



bodies and ascend to the cerebellum, some decussate with 

 their fellows in the median line and pass chiefly to the 

 opposite anterior pyramids, while a certain number pass 

 directly upward through the posterior perforated space to 

 enter the substance of the hemispheres of the brain. Van 

 der Kolk supposes that most of these fibres end in nuclei 

 in the medulla which presides over respiration, and that 

 the ascending fibres are conductors of impulses of volition 

 from the brain to those centres. 



The anterior columns seem to give up a portion of their 

 fibres to the restiform bodies and cerebellum, while others 

 decussate in the anterior pyramids like those of the lateral 

 columns, and ascend through the pons to the corpora quad- 

 rigemina and cerebral hemispheres. 



The pons Varolii is composed of numerous transverse and 

 longitudinal fibres interspersed with much grey matter. 

 The superficial white layer of transverse fibres is continuous 

 with the middle peduncles of the cerebellum. Beneath 

 this is a layer formed of similar transverse fibres, and the 

 longitudinal fibres from the anterior pyramids, largely 

 mixed up with grey matter. Still deeper is a layer of 

 longitudinal fibres forming the continuation of the lateral 

 columns. 



The cerebellum may be looked on as a great ganglion, 

 with the grey or ganglionic matter surrounding the white 

 fibres on all sides except the lower, where the latter pass out 

 as the peduncles. Of these peduncles the median passes 

 chiefly into the two lateral lobes, the anterior into the pos- 

 terior part of the vermiform process, and the posterior into 

 its anterior and middle portion. 



The principal masses of white fibres in the brain are 

 arranged in three sets the ascending, transverse, and longi- 

 tudinal. 



