482 THE BRAIN 



(i) fibres from the lateral funiculus of the spinal medulla, through the 

 dorsal spino-cerebellar fasciculus ; (2) olivo-cerebellar fibres from the opposite 

 inferior olivary nucleus to the cerebellum ; (3) fibres from the cuneate and 

 gracile nuclei of both sides in the form of the arcuate fibres. 



Fibrae Arcuatse Externae. On the surface of the medulla 

 oblongata, more particularly in the neighbourhood of the 

 lower border of the olive, a number of curved bundles of fibres, 

 termed the external arcuate fibres, may be noticed. They 

 vary greatly in number and in distinctness, and are some- 

 times so numerous as to cover the olive almost entirely. 

 An attentive examination will show that they come to the 

 surface (i) in the antero-median fissure between the pyramids, 

 (2) in the groove between the pyramid and the olive, and (3) 

 sometimes also through the substance of the pyramids. But 

 at whatever point they reach the surface, the majority have one 

 destination, viz., the restiform body a considerable part of 

 which they form. They are derived from the cuneate and 

 gracile nuclei of the opposite side, and end in the cerebellum. 



At the inferior end of the olive there is not uncommonly a curved 

 bundle of fibres, called the circum-olivary fascictclus, which follows the 

 line of the external arcuate fibres but has a different commencement and 

 termination. It consists of fibres descending from the cerebrum, and corre- 

 sponds with the fibres of the pons which end round the pontine nuclei. 



Dissection. The pyramidal fibres of one side should now be 

 carefully raised. When dislodged from their bed they should 

 be gently pulled upwards towards the pons. In that way their 

 entrance into the pons will be brought very clearly into view. 

 Further, numerous arcuate fibres will be seen running forwards 

 upon the medial aspect of the opposite pyramid to reach the 

 surface, and the ventral edge of the medial lemniscus will be 

 exposed also. 



Pons. The pons is the marked prominence, on the base 

 of the brain, which lies anterior to the cerebellum and is 

 interposed between the medulla oblongata and the pedunculi 

 cerebri (Figs. 160, 194). It is convex from side to side, 

 as well as from above downwards, and the transverse streaks 

 on its surface show that, superficially, it is composed of 

 transverse bundles of nerve fibres. On each side the trans- 

 verse fibres collect themselves together to form a large 

 compact strand which sinks, postero-laterally, into the corre- 

 sponding hemisphere of the cerebellum. The strand is 

 termed the brachium pontis (O.T. middle cerebellar peduncle). 



When the brain is in situ the ventral surface of the pons is 

 in relation to the basilar portion of the occipital bone, the 



