496 THE NEKVOUS SYSTEM 



Thus the floor of the fourth ventricle in its upper half contains 

 that dorsal grey matter which is continued upward from the chief 

 nuclei of the ninth and tenth nerves ; it is thus homologous with 

 the dorsal horns of the spinal cord. This grey mass at the lower 

 border of the pons forms the chief or mesial nucleus of the eighth 

 cranial nerve. 



Just ventral to this nucleus is an ovoid compact group of large 

 motor nerve cells, the nucleus of the sixth or abducens nerve. The 

 nuclei of the sixth pair are thus in the same vertical plane as 

 those of the twelfth, but are somewhat more separated from one 

 another by a median longitudinal fibre bundle, the medial or pos- 

 terior longitudinal fasciculus, which, in its upward course, curves 

 dorsalward in the medulla oblongata, so that in the pons Varolii 

 it lies on either side of the median raph6 and beneath the grey 

 matter of the floor of the fourth ventricle. 



The deeper part of the pontal tegmentum contains a continua- 

 tion of the formatio reticularis of the medulla. Ventral to this 

 and separating it from the pons fibres are the longitudinal fibre 

 bundles of the mesial fillet, which in transection form an oval 

 bundle with its long axis at nearly a right angle to the median 

 raph6. 



Lateral to the formatio reticularis and above, but in the same 

 vertical plane as the inferior olivary body, is the superior olivary 

 nucleus, a small oval mass of grey matter which forms a cell station 

 in the path of the cochlear division of the auditory nerve. In some 

 of the lower mammals this nucleus is more highly developed than 

 in man. Connecting the superior olivary body with a similar region 

 of the opposite side is a transverse bundle of fibres, the trapezoid 

 body, which crosses the pons just ventral to the mesial fillet, but 

 dorsal to the pyramidal tracts and transverse fibres of the pons 

 Varolii. 



In close proximity to the superior olivary nucleus, on its dorso- 

 lateral side, is the nucleus of the seventh cranial nerve, a slender 

 rounded cell column whose position is above but homologous with 

 that of the nucleus ambiguus, the lateral nucleus, and the ventral 

 horns of the spinal cord. 



The eighth cranial nerve enters just lateral to the nucleus of 

 the seventh, its vestibular division lying at a somewhat lower level 

 than its cochlear portion. The vestibular nerve near its point of 

 entrance is surrounded by a ganglionic mass of grey matter which 

 lies in the extreme lateral portion of the metencephalon. This 



