188 DISSECTION OF THE BRAIN. 



(fig. 58, p}, and is continuous below with that of the accessory nerve. 

 On the surface it forms a pyriform swelling along the inner side of the 

 posterior pyramid, and limits laterally the calamus scriptorius. 



Above, this sinks under the auditory nucleus, and joins a nucleus for 

 the glosso-pharyngeal nerve in a line with it. 



Above the last two nerves is another collection of cells serving as a 

 nucleus for the auditory nerve. This projects on the lateral part of the 

 medulla (fig. 58, r). 



At the front of the medulla. Outside the pyramid is the gray layer of 

 the olivary body (fig. 58, /") already described (p. 185). Behind this is 

 another separate, elongated, and flattened yellowish streak, the accessory 

 olivary nucleus (fig. 43, k) ; while at the inner part of the olive, near the 

 raphe, is a second collection (fig. 58, I), which is broken up into pieces. 

 Both of the last deposits Mr. Clarke considers to be but parts of the cut 



folds of the corpus dentatum. 



\ 



PONS VAROLII. 



The PONS, or ANNULAR PROTUBERANCE (pons Varolii, nodus encephali) 

 (fig. 54), is situate above the medulla oblongata, and between the hemi- 

 spheres of the cerebellum. In its natural position in the skull it lies below 

 the opening in the tentorium cerebelli. It is nearly of a square shape, 

 though it is rather widest from side to side, and measures two inches in 

 the last direction. 



The anterior surface is grooved along the middle line, and is received 

 into the basilar hollow in the base of the skull. By the opposite surface 

 the pons forms part of the floor of the fourth ventricle. 



The upper border is longest and most curved, and^arches over the cere- 

 bral peduncles ; and the lower border overlays the medulla oblongata. On 

 each side is the crus cerebelli, whose fibres radiate over the surface. 



Structure. In the pons are alternating strata of transverse and longi- 

 tudinal fibres (fig. 56) : The transverse set are continuous with the fibres 

 of the crus cerebelli, much gray matter being interspersed : and the longi- 

 tudinal are prolonged from the medulla oblongata. 



Dissection. The transverse superficial fibres of the pons being divided 

 along the line of the pyramidal body of the right side (fig. 5G), may be 

 turned outwards so as to denude the longitudinal fibres (c) of the pyramid ; 

 and this first set of longitudinal fibres, having been cut across already in 

 the medulla oblongata, may be raised as far as the upper border of the 

 pons. Beneath them will appear the second or deep set of transverse fibres 

 of the pons (fig. 57, f). 



The deep transverse fibres may be cut through outside the pyramidal 

 (fig. 57) ; then the deep longitudinal fibres from the lateral column and 

 the posterior pyramid (e?) will appear. Amongst this last set of longitu- 

 dinal fibres is the fillet of the corpus olivare, which the dissector should 

 trace upwards from that body. 



The superficial fibres of the pons can be seen on the side that is un- 

 touched. 



The transverse fibres of the annular protuberance (fig. 56), are collected 

 chiefly into two strata a superficial and deep, which are united in the 

 middle line : they are commissural fibres of the cerebellum, and are de- 

 rived from the crus or middle peduncle of that body. There are a few 

 other transverse, which serve also as a commissure. 



