84 THE LOWER PRIMATES 



In the oblongata the rounded dorsal surface of the hypoglossal nucleus 

 passes backward and invades to a limited extent the central gray matter. 

 In the metencephalon the dorsal surface of the abducens nerve passes some- 

 w hat further dorsally, while in the mesencephalon the trochlear and oculo- 

 motor nuclei extend deeply into the central gray matter. The central gray 

 matter in the regions of the mesencephalon forms a complete investment for 

 the central canal and as the diencephalon is approached, it becomes contin- 

 uous with the central gray matter underlying the ependymal lining of the 

 ventricle of the diencephalon. Dorsally at the junction of the tectal region 

 with the habenular region, the central gray matter is penetrated by the 

 habenular and the posterior commissures. 



