PATHS OF THE CRANIAL NEKVES 549 



fugal or motor nerve, begin in the nerve cells of the facial nucleus, 

 a small oval group of large motor cells placed on the dorsal side 

 of the trapezoid body, just dorso-lateral from the superior olive. 

 Their neuraxes soon collect into a compact tract, which, in its first 

 portion, passes dorsalward in the region of the abducens nucleus, 

 around which it makes a sharp turn, the internal genu. It then 

 passes somewhat cephalad, and finally takes a lateral, yet slightly 

 ventral and caudad course, toward its exit at the lower margin of the 

 pons Varolii, just dorsal to the root of the auditory nerve. 



The facial tract thus presents three portions: (a) the proximal, 

 whose direction is dorso-mesial and slightly cephalad ; (b), the in- 

 ternal genu, whose course is first dorso-mesial and cephalad, but later 

 ventro-lateral and cephalad, and (c) the distal, which is directed 

 lateralward, but slightly ventralward, and caudad. 



The central neurones of the facial reach its nucleus in the pons 

 through the pyramidal tracts, probably by passing dorsalward 

 along the median raphe, until at a point dorsal to the trapezoid 

 body, where they decussate and pass directly to the facial nucleus. 



The sixth or abducens nerve (Figs. 385 and 386) is entirely 

 centrifugal or motor. Its peripheral neurones arise from the cells 

 of the compact ovoid abducens nucleus which lies beneath the grey 

 matter of the floor of the fourth ventricle on either side of the 

 median line. It is in close relation to the fasciculus longitudinalis 

 posterior, which lies on its ventro-mesial side, and with the tract 

 of the facial nerve whose internal genu encircles the dorso-mesial 

 angle of the abducens nucleus. 



The neuraxes of the large nerve cells of this nucleus form 

 bundles of considerable size, which pass almost directly ventral- 

 ward, through the tegmentum and crusta of the pons, and emerge 

 at the inferior margin of the rhombencephalon near the median 

 line, where they unite to form the trunk of the abducens nerve. 



The central neurones of the sixth nerve are derived from the 

 pyramidal tracts. Having reached the pons Varolii, they pass dor- 

 salward along the median raphe until near the abducens nucleus, 

 where they decussate and immediately end about the nerve cells of 

 the peripheral neurones. 



The fifth or trigeminus nerve (I ri facial nerve) (Fig. 387) con- 

 tains a large sensory or centripetal, and a smaller motor or cen- 

 trifugal root. Both make their entrance or exit at the lateral sur- 

 face of the pons Varolii, plunging together into the substance of the 

 middle cerebellar peduncles to reach the dorsal half of the tegmen- 



