288 



NORMAL HISTOLOGY. 



The fillet-tracts are also known as the interolivary stratum, since they form 

 a compact and compressed field between the inferior olivary nuclei. Between 

 the fillet-tract and the hypoglossal fibres is (5) the mesial accessory nucleus. 

 (6) The posterior longitudinal fascicuhts appears in cross-section as a com- 

 pact strand, next the raphe and immediately beneath the gray matter of the 

 floor of the fourth ventricle. The remaining space of the ventral field, 

 between the pyramid and the ventricular gray matter, is occupied by (7) the 



7 



Nerve- Longitudinal Transverse 

 cell fibres fibres 



\ 



Median raphe 



FIG. 335. Portion of formatio reticularis alba of medulla. X 130. 



formatio reticularis alba, so designated, in distinction to the formatio grisea, 

 on account of the meagre amount of its gray matter and small number of its 

 nerve-cells, since, with the exception of in the immediate vicinity of the mid- 

 line, where the nucletis raphe is found, these are almost wanting. 



Although at higher levels additional and important masses of gray 

 matter appear, especially those related to the auditory nerve, for the pur- 

 pose of these pages the foregoing general description of the medulla, as 

 seen in typical section, must suffice. 



THE PONS VAROLII. 



Viewed from in front, the pons appears as a quadrilateral prominence, 

 from 2528 mm. long, interposed between the medulla below, the cerebral 

 peduncles above, and the cerebellar hemispheres at the sides. Its lower 

 and upper limits are well defined, but at the sides the narrowed mass of the 

 pons is directly continued, downwards and backwards, into the cerebellum as 

 middle cerebellar peduncles. The free portion of the dorsal surface of the 

 pons forms the upper half of the floor of the fourth ventricle and is, there- 

 fore, not visible until the roof of that cavity is removed. Above the middle 

 peduncles, the sides of the pons are blended with the overlying superior 

 cerebellar peduncles, which, with the membranous -superior medullary 

 velum, complete the ring of tissue surrounding the narrow upper end of the 

 fourth ventricle. 



In transverse section (Fig. 336), the pons is seen to include two clearly 

 defined areas, the ventral and the dorsal. The ventral part, or pars 



