MlDBRAIX 63 



more scattered near the posterior end of the decussation of 

 the brachia conjunctiva, some of its fibres as they continue 

 back being mingled with the medial longitudinal bundle and 

 with the medial lemniscus (Papez). 



The central gray matter surrounding the aqueduct of 

 Sylvius contains several more or less distinct cell-masses. 

 Among the most important of these are the nuclei of the 

 fourth and third nerves, which lie in the ventral part of the 

 central gray, directly dorsal to the medial longitudinal bundle. 

 The trochlear nucleus is a small group of large cells in the 

 posterior part of the midbrain (PI. XIII.). In the rat, it lies 

 actually between the fascicles of the medial longitudinal 

 bundle, suggesting the intimacy of its relation to this tract, 

 from which it receives most of its afferent fibres. The axons 

 arising from this nucleus may be traced postero-laterally and 

 dorsally, running backwards at the boundary between the 

 central gray and the surrounding tissue (PI. XII.). This 

 course takes the root to the antero-lateral region of the 

 anterior medullary velum, in which the fibres decussate to 

 form the peripheral nerve roots (PI. XXVI.). 



The oculomotor nucleus lies directly in front of the troch- 

 lear, being practically continuous with it but entirely dorsal 

 to the medial longitudinal bundle (PI. XIV.). It extends up 

 to near the anterior end of the midbrain. The root-fibres 

 pass ventrally in small fascicles which emerge along the 

 medial margin of the cerebral peduncles (Pis. XI\'., XXVI.), 

 a few of them first decussating between and dorsal to the 

 nuclei arid joining the root of the other side. In man, the 

 oculomotor nuclei consist of a median centre (nucleus of 

 Perlia) and paired lateral nuclei. The former, however, is 

 not present in rodents. Immediately dorsal and anterior to 

 the oculomotor nuclei proper, is a mass of small cells the 

 axons of which also emerge in the third nerve. This is the 

 nucleus of Edinger-Westphal, which is the origin of the general 

 visceral fibres in the oculomotor nerve. It is continuous 



