142 THE LOWER PRIMATES 



nuclear collections which form the very poorly organized system of the 

 pontile nuclei. Its ventral surface, partially covered by the irregular pontile 

 nuclei, shows two grooves, one which proceeds laterally and dorsally in a 

 somewhat spiral fashion about the lateral surface of the reticular formation 

 toward the inferior colliculus, representing the course of the lateral fillet as 

 this bundle of fibers seeks its nucleus of termination in the inferior colliculus. 

 In its more ventral portion, largely covered n\cv and concealed from view- 

 by the pontile nuclei, the reticular formation presents another groove which 

 indicates the course of the mesial fillet, which in this form maintains an 

 almost undeviating upward course in the ventromesial angle ol the formation 

 throughout the entire length of the brain stem to end in the nuclei of the 

 thalamus. In the upper half of the metencephalon the reticular formation is 

 almost entirely covered by the pontile nuclei vcntrally, and in its lateral 

 aspect in this region it presents a particularly irregular outline. As the 

 reticular formation is followed into the mesencephalon, it can be seen to send a 

 prolongation laterally and dorsally which passes backward in a rather massive 

 lamina of gray matter into the space separating the superior and inferior col- 

 liculi, extending dorsally and mesially in the space separating the two colliculi 

 to join in the midline dorsally. In the mesencephalon the reticular formation 

 is almost entirely obscured from view vcntrally by the fusion of the upper- 

 most part of the pontile nuclei with the indifferent gray matter filling the 

 interpeduncular space. Lateral to this there appears the substantia nigra 

 which is separated from the ventral surface of the reticular formation by 

 ascending and descending bundles of fibers. The lateral aspect of the reticular 

 formation in the mesencephalon is entirely obscured by the exceptionally 

 low appearance of the geniculate bodies which develop considerably below the 

 mid-mesencephalic level. In the mesencephalon the only portions of the 

 reticular formation which can be seen are those which form the dorsolateral 

 expansions, extending into the space between the superior and inferior col- 



