370 



PHYSIOLOGY 



section of fibres which run longitudinally and transversely. The transverse 

 fibres are a continuation of the deep arcuate fibres. The longitudinal fibres 

 in the outer part of the formatio reticularis are the representatives of the 

 lateral columns of the cord after the removal of the direct cerebellar and the 

 crossed pyramidal tracts. They include therefore the antero-lateral ascend- 

 ing tract (tract of Gowers) and a number of other fibres corresponding to the 

 lateral basis bundle in the cord. In the mesial part of the formatio reticu- 



4th ventricle 

 Mesenc. root of 5th n 



Postr. long, bundle 



Form, reticularis 

 Nucleus of lateral fillet 



5th ner\ 



Valve of Vieussens 

 Floor of 4th ventricle 



Supr. cerebellar 

 pedunclo 



Lateral fillet 



Commencing de- 

 cussation of supr. 

 cerebellar ped. 



Mesial fillet 



Transverse 

 fibres . 



Pyramids 



Fro. 186. Section across upper part of pons Varolii of the orang. (CUNNINGHAM.) 



laris the longitudinal tracts are the tract of the fillet and the posterior longi- 

 tudinal bundle on eatfh side of the middle line. In the upper part of the pons 

 Varolii a well-marked collection of transverse fibres are to be seen lying 

 dorsally to the tracts of the fillet. This collection is called the corpus 

 trapezoides and is made up of ascending fibres derived from the nuclei of the 

 cochlear nerve, the auditory part of the eighth nerve. 



A little further forward a section will escape the cerebellum altogether, 

 being bounded ventrally by the upper or anterior part of the pons and 

 dorsally by a thin mass of grey matter, the valve of Vieussens (Fig. 186). 

 On each side of the valve of Vieussens may be seen the superior peduncles of 

 t h- <M'ivlM'llimi. As these peduncles are traced upwards they sink gradually 

 deeper into the pons until they lie on the outer side of the tegmental region 

 or formatio reticularis. They are made up of fibres which run from the 

 dentate nucleus of grey matter in the cerebellum to the mid-brain, where 

 . ,they decussate below the Sylvian iter and end in the red nucleus and in the 

 thalamus of the opposite side. They also contain the continuation upwards 

 of the antero-lateral ascending tract which, passing up in the superior 

 peduncles, bends dorsally round the fourth nerve and then, turning back- 

 wards, ends in the superior verrnis of the cerebellum. In a section through 



