240 THE LOWER PRIMATES 



The Substantia Nigra 



In the reconstruction, the substantia nigra appears in the reticular forma- 

 tion of the mesencephalon, apparently as a cephahc specialization in the deep 

 layer of the pontile nucleus. Mesially it is continuous with the undifTeren- 

 tiated interpeduncular gray matter. Vcntraliy its surface is somewhat cor- 

 rugated by the passage of the libers of the pes peduncuH. Dorsally it is in 

 contact with the reticular formation of the mesencephalon which is demar- 

 cated from it by the circumferential fiber bundles of this nuclear mass. 

 Ventrolaterally in its upper portion it is hollowed out to a marked extent by 

 a special nuclear accumulation which takes place in this part of the substantia 

 nigra. This nuclear mass presents a tangle of myelinated nerve fibers which 

 seem to rise in the nucleus and pass into the reticular formation of the mesen- 

 cephalon. Cephalically it is continuous with the subthalamic gray structures 

 of the diencephalon and laterally it merges with the reticular formation in 

 which are embedded the mesial geniculate bodies. 



The Colliculi 



The reconstruction of the inferior colliculus appears as a rapidly increas- 

 ing mass of gray matter supported laterally by the dorsal extension of the 

 reticular formation. Dorsally it is continuous with the dorsal gray matter of 

 the tectum which separates it from its fellow of the opposite side and also 

 provides a pathway for the inferior collicular commissure. At the mid- 

 mescncephalic level the inferior colliculus diminishes in thickness and 

 gradually disappears. In this region the dorsal extension of the tegmental 

 reticular formation comes to the surface of the tectum of the midbrain. 

 Immediately succeeding this, the superior colliculus appears, situated in an 

 essentially similar position to that occupied by the inferior colliculi. Its 

 dorsal and ventral extremities receive the same type of support as that of the 



