692 THE HIGHER ANrHROPOIDS 



by fibers emerging from the inlerior colliculus which collect themselves as a 

 compact biinclle — the inferior Ijrachium — and groove the surface of the 

 reticuhir lorniation as the tract passes upward to the mesial genicuhite body. 



A similar arrangement appears in the upper part of tlie mesencephalon 

 where a mass of libers emerges frona the superior eolhculus. These libers 

 I)roduce a groove on the lateral surlace ot the reticular lormation, passing 

 Irom the lateral extremity ol the superior colliculus to the lateral geniculate 

 body. The libers constitute a secondary tract in the visual pathway, the 

 superior brachiuni. 



The mesencephalic reticular formation can, in its upper parts, be divided 

 into three main masses: (i) a dorsal mass which lies in contact with the 

 central gray matter, ventral to the colliculi; 1 2) a lateral portion which forms 

 the most considerable part of the tegmentum of the mesencephalon; and (3) 

 a central portion which gives rise to the condensation ol the nucleus ruber. 

 From the mesencephalon the reticLilar formation is continued u]jward to 

 merge with the zona incerta of the diencephalon and tlu' inditlerent nuclear 

 masses in the caudal-most part of the inkrlnain. 



The Pontile Nuclei 



The pontile nuclei lirst appear at le\els marked by the lower extremity 

 ol the lourth \entricle as the arcijarm nuclei. Here they are disposed along 

 the mesial and \entral surfaces of llu' p\ rainidal tract. The arcilorm nuclei 

 gradually become more extensive, pass laterally and mcsiall\- around the 

 pyramidal tract and linallx produce a complete iinestment of that mass ol 

 fibers. Proceeding upward they become more extensixe, merging with the 

 gray masses of tlu' intrinsic pontile nuck'us. The\ then assume a typical 

 arrangement in a superficial la\'er and a deep layer. A lati'ial accumulation 

 of nuclear material loiins tlu' so-called lateral buttress, while a similar 

 mesial accumulation forms the mesial buttress. In addition to these, tlu're 



