ORIGIN OF THE CEREBRAL NERVES. 481 



fifth nerve, we have here again a manifestation of the lateral 

 position of the sensory columns of origin. At a height of about 

 six millimeters above the striae medullares, in the region where 

 the internal surface of the superior peduncle is covered by 

 fasciculi of the internal division of the peduncle of the cere- 

 bellum (fig. 255, H'\ a furrow commences which presents an 

 obtuse angle internally, and forms externally a rhombic space 

 on the grey floor (fig. 254, where its transverse section is shown 

 at 8). This is the internal auditory nucleus of Clarke, the 

 upper and larger triangular half of which Stilling also recog- 

 nized as such, whilst the half belonging to the lower angle of 

 the floor of the fourth ventricle he regarded as the nucleus 

 of the glossopharyngeus. The broad median portion of this 

 groove is situated in the region of the medullary strise of 

 the auditory nerve. Internally to the nucleus of the auditory 

 nerve, the motor column of the grey matter exhibits above 

 the region of the strise an oval eminence, which Stilling and 

 Clarke agree in considering the common nucleus of the facial 

 and sixth pair (fig. 254, at 0). 



The furrow dividing the upper half of the rhombic nucleus 

 of the auditory nerve and the last-named oval eminence is not 

 seldom rendered still more apparent by the presence of the 

 ascending medullary strioe of J. Engel, which run parallel to 

 the furrow upwards and outwards from the internal extremity 

 of the striae acusticse. These are the crossed prolongation of 

 the auditory roots into the opposite cerebellar peduncle, or 

 more correctly speaking, they are the superficial crossed fasci- 

 culi of origin of the auditory nerve from the cerebellum. In 

 many instances only the innermost fasciculus of the ascending, 

 medullary strice just external to the common nucleus of the 

 facial and abducens nerves is visible, for which the name 

 selected by Bergmann, of " sound rod " (Klangstab), should be 

 restored, since Stilling's explanation of its being an inconstant 

 posterior root of the fifth must be given up. The uniform ab- 

 sence of the medullary strise in animals does not indicate that 

 their fasciculi do not exist, but only that they are not visible, 

 as they may remain as naked axis-cylinders without medullary 

 sheaths. The development of these medullary sheaths occurs 

 when the evolution of the brain is complete, and therefore they 



II 



