THE BRAIN 123 



Dimensions. About i inches long, and about 2 inches 

 transversely. 



Anterior surface is convex, grooved along the midline for the 

 basilar artery ; has transverse markings, and openings for the 

 entrance of vessels. 



Posterior surface : smaller than the anterior, and continuous 

 with the posterior surface of the medulla ; part of it forms the 

 upper part of the floor of the 4th ventricle. 



Laterally : transverse fibres pass outwards and backwards 

 from it, forming the middle cerebellar peduncles. 



Upper border : longer than the inferior, with a notch in the 

 median line corresponding to the groove on the anterior 

 surface. 



Lower border : slightly arched, overlapping the upper part 

 of the medulla. 



Structure: Alternating layers of transverse and longitu- 

 dinal fibres, with intermingled grey matter. The transverse 

 fibres come from the middle peduncles of the cerebellum, and 

 pass to the crura cerebri ; the longitudinal are the continua- 

 tion upwards of the fibres of the medulla. 



Most anteriorly is a layer of transverse fibres of the middle 

 peduncles ; behind these, longitudinal fibres prolonged from 

 the anterior pyramids of the medulla with intermingled trans- 

 verse peduncular fibres. More posteriorly is a deep layer of 

 transverse fibres, most marked at the lower part of the pons, 

 where they form the corpus trapezoides, and behind these are 

 longitudinal fibres prolonged from the formatio reticularis of 

 the medulla, the lateral tract and the olivary fillet. A septum 

 of commissural fibres exists at the posterior part of the pons. 



The grey matter of the pons. In addition to the grey matter 

 scattered in the above-mentioned formatio reticularis are the 

 following special deposits : 



(a) Superior olivary nucleus. Placed posteriorly near the 

 outer side of the corpus trapezoides. 



(b) Nucleus of the facial nerve. Lies in formatio reticu- 

 laris, just behind superior olivary nucleus. 



(c) Motor nucleus of the 5th nerve. 



(d) Upper sensory nucleus of the 5th nerve, placed outside 

 motor nucleus. 



(e) Continuation upwards of principal auditory nucleus. 

 (/) Superior auditory nucleus, placed just outside principal 



nucleus. 



Accessory auditory nucleus. 



k) Nucleus of the 6th nerve, corresponding to the part of 

 fasciculus teres, above the auditory s*riae (vide 4th ventricle). 



(g) 

 (h] 



