528 THE BRAIN 



which go to the opposite inferior olivary nucleus ; (3) from the cuneate 

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



Fibrae Arcuatae Externse. 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 in the antero-median fissure between the pyramids, 

 in the groove between the pyramid and the olive, and some- 

 times 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. 



Dissection. The pyramid of one side should now be carefully raised. 

 When dislodged from its bed it should be gently pulled upwards towards 

 the pons. In this way its entrance into the pons is brought very 

 clearly into view. Further, numerous arcuate fibres will be seen 

 running anteriorly 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 is interposed between the medulla ob- 

 longata and the pedunculi cerebri, and lies anterior to the 

 cerebellum. It is convex from side to side, as well as antero- 

 posteriorly, and the transverse streaks on its surface show 

 that, superficially, it is composed of transverse bundles of 

 nerve fibres. On either side these transverse fibres collect 

 themselves together to form a large compact strand which 

 sinks postero-laterally into the corresponding hemisphere of 

 the cerebellum. This strand is termed the brachium pontis 

 (O.T. middle cerebellar peduncle]. 



The ventral surface of the pons is in relation to the basilar 

 portion of the occipital bone and the dorsum sellae of the 

 sphenoid bone. It presents a median groove which gradually 

 widens as it is traced upwards (Fig. 219). The groove 

 lodges the basilar artery, but is not caused by that vessel ; 

 it is due to the prominence produced, on either side, by 

 the passage downwards through the pons of the bundles 

 of fibres which form the pyramids of the medulla oblongata. 



