RECONSTRICTION OF IlLMAN 855 



thrn nu-siall\ to |i)ia its fellow oi the opposite side at the mid-line, thus 

 separating the superior and nilerior eoHieuli Irom eoutaet w ith one another. 

 The superior eollieulus is eonsideral)l\ niori- extensive eephalo-caudad 

 than the ulterior eollieuhis and ajjpears as a nuelear specialization in the 

 reticular formation whieh surrounds it on all sides and sejjarates it from 

 the iiilenor eollieulus eaudally, its lellow ot the op|)()site side niesialiy and the 

 thalamic nuclei ee])halically. Mesiall\ the superior eollieuli are separated 

 Ironi one another by a continuation ii])\vard ol the indillerent reticular lor- 

 mation which se|xu"ates the inlerior Irom the superior colliculi. Mesialix the 

 superior eollieulus rests upon a mass of fibers which arise Irom the interior of 

 the superior eollieulus, converge toward each other at the midline and cross 

 to join the superior eollieulus ol the other side, thus lorming the commissure 

 ol the superior eollieulus. The de\ elo])nient ot this mass ot libers in this region 

 definitely separates the superior eollieulus Irom the gray matter surrounding 

 the aqueduct ot Sylvius. Laterally and ventrally the superior eollieulus is 

 supported by the gradually increa^sing bulk of the reticular formation, which 

 appears cephalad and dorsad to the intertegmental course of the superior 

 cerebellar peduncle. Cephalically, the superior eollieulus gradually decreases 

 in thickness and losing its characteristic appearance, it becomes a part of a 

 thin lamina ot indillerent gray matter which iuses with the habenular region 

 ot the dience|)halon. 



The SLBsr.\.\Ti.\ Nigka 



The substantia nigra appears as a relatively massive collection of gray 

 matter in the ventral portion of the mesencephalon and seems gradually to 

 dilTercntiate itself from the fused nuclear matt'rial forming the ventral layer 

 of the reticular lormation and the dorsal layer ot the pontile nucleus. This 

 change occurs very gradually and it is diflicult to say at which point the 

 fused reticuIo-])ontilc gray matter ceases and the distinctixc, characteristic 



