RECONSTRUCTION OF PITHECUS RHESUS 401 



The dorsal cochlear nucleus, on the other hand, appears below the lateral 

 recess of the fourth ventricle as a more or less triangular mass of gray matter 

 in the extreme dorsolateral angle of the brain stem. At this point it is located 

 dorsal to the nucleus. A certain amount of nuclear material lies interspersed 

 between the fibers of the cochlear nerve extending between and connecting 

 the dorsal and ventral cochlear nuclei. The dorsal and ventral cochlear 

 nuclei appear below the level of the cochlear nerve and extend to a level 

 higher than that of the nerve itself. 



The Substantia Nigra 



The substantia nigra appears in the reconstruction as a well de- 

 veloped and extensive mass of gray matter situated dorsal to the cerebral 

 peduncle and supported by the deep layer of the pontile nucleus and the 

 mesial and lateral buttresses. Between these two structures it rests upon the 

 deep layer of the pontile nucleus from which it seems to develop. It extends 

 upward throughout the extent of the mesencephalon, diverging laterally and 

 somewhat dorsally to terminate in a rounded, blunt extremity which is 

 separated from the mesial geniculate body by a relatively small intervening 

 mass of the reticular formation and the fiber capsule of the mesial geniculate 

 body itself. At the junction of the mesencephalon and the diencephalon it 

 comes to an end, separated from the corpus of Luys by a thin lamina 

 of white matter. 



The Colliculi 



The inferior colliculus constitutes a poorly developed structure lying 

 as a crescentic mass of gray matter in the tectum of the mesencephalon. It is 

 supported laterally by the lateral extension of the mesencephalic reticular 

 formation, while mesially it lies against the undifferentiated dorsal gray 

 matter. It is separated from the superior colliculus by a deep narrow^ 



