242 THE LOWER PRIMATES 



smooth and presents little modelling of its surface in the lower half of the 

 fourth ventricle. The walls are formed successively by the nuclei of Goll and 

 Burdach and the nucleus of Deiters. In the upper half of the ventricle the 

 door shows a single well-marked and rounded medial emmence produced b\- 

 the mass of gray matter forming the nucleus alxlucentis, close to the mid- 

 line and just above a line joining the two lateral recesses of the fourth 

 ventricle. As the upper portions of the ventricle are approached the walls 

 rapidly contract to form the narrow aqueduct of Sylvius which traverses the 

 mesencephalon. The central gray matter of the mesencephalon is consider- 

 ably thicker than that found in the lemur or the marmoset. It contains the 

 dorsal extension of the trochlear and oculomotor nuclei which lie embedded 

 in the dorsal region of the mesencephalic tegmentum. At the upper extremity 

 of the mesencephalon the central gray matter is directly continuous with the 

 subependymal gray matter of the third ventricle and the mesial thalamic 

 nuclei. The most dorsal portion of the central gray matter in the 

 mesencephalon is continuous with the epithalamic group of structures, 

 while the most ventral portion is continuous with the hypenccphalic 

 structures. 



