78 THE LOWER PRIMATES 



in the inferior colliculus. From the inferior colliculus arises the inferior 

 brachium which occupies a groove on the lateral surface of the mesencephalic 

 reticular formation until it reaches and ends in the mesial geniculate body. 

 The reticular formation is most extensive at the level of the inferior olivary 

 nuclei which develop in its ventral portion. It forms the main mass of the 

 gray matter of the brain stem. 



Between the levels of the vestibular complex and the inferior colliculus, 

 the reticular formation sends out a long, slender process on the lateral 

 surface of the pons and midbrain to form an enveloping layer over the 

 superior cerebellar peduncle. 



The reticular formation contains the ventral portions of the twelfth, 

 sixth, fourth and third nerve nuclei, while the dorsal portions of these nuclei 

 project into the central gray matter. In the upper medullar}' and lower 

 pontile regions the reticular formation presents the specialized condensations 

 forming the vestibular nuclei. 



In the mesencephalon the reticular formation approaches but docs not 

 come into actual fusion with the substantia nigra. Laterally it forms a support 

 for the mesial geniculate body, while dorsolaterally it becomes continuous 

 with the inferior and superior colliculi. At its cephalic extremity it is con- 

 tinuous with the zona incerta of the diencephalon and fuses with the thalamic 

 gray matter. 



The Pontile Nuclei 



Reconstruction of this mass of gray matter provides one of the most 

 striking differential features in the brain stem of primates. This nuclear 

 aggregation, forming with the pallio-spinal and pallio-pontile tracts the 

 basilar portion of the metencephalon, presents a relatively simple arrange- 

 ment. The pontile nuclei begin rather suddenly at the trapezoid level and 

 show but little tendency to expansion caudally into the ob ongata. The 



