398 THE INTERMEDIATE PRIMATES 



in this region the beginning mesencephalic appearance of the reticular 

 formation with a large mesial mass and a lateral prolongation which turns 

 outward and backward to encircle the superior cerebellar peduncle as it 

 enters into the mesencephahc tegmentum from the superior medullary 

 velum. Below the appearance of the inferior collicuh a prolongation of the 

 reticular formation passes around dorsally toward the midline. 



Into the space between the superior colliculus and inferior colliculus a 

 similar prolongation of the mesencephalic reticular formation passes laterally 

 from the lateral portion of the reticular formation almost to the midline, 

 thus separating the colliculi from each other. Similarly, above the superior 

 colliculus a rather narrow extension from the most cephalic region of 

 the reticular formation again comes to the surface close to the junction 

 between the mesencephalon and the diencephalon. In the mesencephalon 

 itself, the major portion of the mesial mass of the reticular formation gives 

 place to that most characteristic nucleus of the mesencephalon, the nucleus 

 ruber; while the lateral portion extends upward and outward and at the 

 junction of the mesencephalon with the diencephalon it forms a bed for the 

 mesial surface of the mesial geniculate body and then fuses cephalically with 

 the indifferent reticular formation of the diencephalon. In the upper medullary 

 levels of the brain steni the dorsal surface of the reticular formation is in 

 fairly close contact with the subependymal gray matter of the fourth ventricle 

 and presents at its dorsolateral angle the specialization of the reticular forma- 

 tion, producing the vestibular complex. 



The Pontile Nuclei 



In the pontile nuclei of Macacus rhesus is found for the first time an 

 extension indicating the arciform nuclei which appear as a flattened layer 

 of gray matter applied to the ventral surface of the pyramid, conforming to its 

 convex surface. This arciform nuclear formation is relatively poorly developed 



