824 MAN 



ventrolateral jjortion of the section some liht-rs lonninff the dorsal roots of 

 the tri<ieininal ner\e ( N5) penetrate tlie pons. Man\' of tliese ma\- be traced 

 inward and backward to a small attenuated mass of ^ray matter, the (.(.'phalic 

 pole of the substantia m'latmosa of Rolando. The reticidar lormation is 

 clearly bouiuk'd but is relatively small in its general jjroportions and shows 

 no nuclear specializations of note. 



In general, the significance of this portion of the stem derives importance 

 from its bearing upon the organization of skilled performances as indicated 

 bv the size of the pons Varolii. In particular it denotes an extensive expan- 

 sion of the iH'opallium and a marked increase in the \()luine ol tliost' pallio- 

 ponto-cerebellar tracts which afford communication between the cerebral 

 hemispheres and the lateral lolx's of the cerebellum. 



LEVEL SHOWING THE EMERGENCE OF THE FOURTH OR TROCHLE.AR 



NERVE (fig. 351 ) 



At this le\el the contour of the section has undergone those changes 

 incident to the appearance of the caudal orifice of the aqueduct ( Aq) and 

 the general approach to the next higher segment of the brain stem. The 

 central gray matter (Cen) entirel\- surroLinds the floor and lateral wall 

 while its roof is formed by a thin la\er of the posterior medullary \t'liim. 

 Escaping from \\u- roofplate are tin- libtrs of the trochlear nerve (N4) 

 which make their way around the lateral aspect of the stem to enter the 

 cavernous sinus in common with the third and sixth cranial nerves. The 

 boundary between the central gra\ matter and the subjacent reticular 

 formation (Ref) is formed l)\ the posterior longitudinal fasciculus (PL) 

 and the predorsal bundk' ( PD), as well as the scattered bundles of the 

 fibers constituting the mesencephalic root of tlu' fifth nerve (Tmt). The 

 lateral portion of the tegmental area at this le\el is occupied by two impor- 



