148 THE LOWER PRIMATES 



ventrally and laterally into the lateral prolongation which continues forward 

 toward the mesial geniculate body. The superior collicuhis continues upward 

 to the point of junction of the mesencephalon with the diencephalon, 

 where it rapidly contracts and comes to a termination. 



The Substantia Nigra 

 This mass of gray matter appears at about the junction of the meten- 

 cephalon with the mesencephalon in the ventromesial aspect of the I^rain 

 stem, as a direct continuation upward of a thin lamina of reticular formation 

 which is found to underHe the metencephahc pontile nuclei. The substantia 

 nigra is situated obliquely from before, backward and outward, as a rather 

 definite lamina of gray matter of mediocre dimensions. It continues upward 

 in this position relatively unchanged, being in relationship mesially with the 

 development of the interpeduncular gray matter, and dorsally with the 

 reticular formation of the mesencephalic tegmentum. Laterally the sub- 

 stantia nigra is in relationship with the origin of the mesial geniculate body 

 and somewhat more cephalically, with the beginning of the lateral geniculate 

 body. In the lateral portion of this lamina of gray matter there develops the 

 lateral nucleus of the substantia nigra which has been found to be present in 

 the brain stems of all of the primates. This forms a rather complicated tangle 

 of nerve fibers arising from a definite nuclear formation, which seems to turn 

 backward into the mesencephalic tegmentum. The substantia nigra con- 

 tinues upward to a point somewhat above the beginning of the superior col- 

 liculus, and then comes to an end being replaced by various developments of 

 the reticular formation, which in this region assumes very considerable pro- 

 portions and extreme complexity. 



The Central Gray Matter 

 The central gray matter appears in the lowest levels of the model as a 

 more or less v-shaped mass of nuclear material, presenting at its ventrolateral 



