552 THE NEEVOUS SYSTEM 



At the level of the spinal border of the inferior corpora quadri- 

 gemina and isthmus rhombencephali the root of the fourth nerve 

 makes a sharp dorsal turn, passes around the lateral margin of the 

 grey matter to its dorsal surface, where it enters the anterior medul- 

 lary velum, decussates with its fellow of the opposite side, and 

 makes its exit from the dorsal surface. 



The fourth nerve is peculiar in that it is the only cranial nerve 

 to leave the dorsal surface of the central nervous system, and is pos- 

 sibly the only one whose decussation is on the distal side of the 

 trophic center for its peripheral neurones. It is also the only cra- 

 nial nerve whose decussation is dorsal to the axial central canal. 



As to the manner in which the central motor neurones reach 

 the trochlear nucleus but little is known. We should, however, 

 expect to find no decussation in the paths of the central neurones 

 to this nerve. The trochlear nucleus is known to receive fibres 

 from the region of the posterior longitudinal fasciculus, but whether 

 these are central motor neurones or collaterals from the other ocu- 

 lar paths is at present uncertain. The latter deduction would 

 appear to be the more probable. 



The Third or Oculomotor Nerves (Figs. 390 and 413). The 

 nuclei of the oculomotor nerves form a series of cell groups in the 

 mesencephalon. They lie in the grey matter between the aqueduct 

 of Sylvius and the posterior longitudinal fasciculus and occupy the 

 deep median trough which is formed by these fasciculi. 



The nuclei of the oculomotorius include a median cell group 

 (median nucleus, nucleus impar) which is also the most ventral 

 portion, and a lateral group on either side of the median line which 

 has been variously subdivided by diiferent observers. It may be 

 said to contain a ventro-medio-anterior nucleus (the Westphal- 

 Edinger nucleus) and a dorso-latero-posterior portion (ventral and 

 dorsal lateral nuclei}. 



The nerve fibres coming from the anterior two-thirds of the 

 group of oculomotor nuclei are uncrossed, but those from the pos- 

 terior or spinal third decussate to the opposite side and enter the 

 lateral fasciculi of the oculomotor nerve roots. The root bundles 

 from the various nuclei of the third nerve pass ventralward through 

 the posterior longitudinal fasciculus, and through or around the 

 red nucleus, to converge to a point near the median line ; they 

 make their exit at the posterior perforated space. 



The course of the central neurones of the oculomotor nerves is 

 not yet known. The nuclei of these nerves are closely connected 



