PAPIO CYNOCEPHALUS 331 



is the reticular formation (Rcf), to which attention is directed because of 

 its diffuse, indefinite character. In the ventromesial portion of the reticular 

 formation, in the region through which the radiating fibers of the oculomotor 

 nerve are passing, an ill-dcfincd aggregation of gray matter represents the 

 nucleus ruber (NRu). The indefinite character of this nucleus in baboon, 

 as well as in the lower primates, is a fact of considerable significance. The 

 nucleus merges with the adjacent reticular formation, producing the impres- 

 sion that it is but a part of this formation. To this point reference will be 

 made subsequently in considering the reticular lormation as a diffuse 

 matrix in the tegmental portion of the stem out of which certain discrete 

 structures take their origin or seem to emerge by more exact dehmitation of 

 their boundaries. In this hght, the reticular formation throughout the entire 

 series of primates assumes a greater evolutional importance than is usually 

 attributed to it. This region of the brain stem is usually regarded as a terra 

 incognita and hence passed over without especial comment because of its 

 diffuse and nondescript character. To us it appears more as the territory 

 on the outskirts of advancing specialization, out of which, as the evolutional 

 process goes forward, there develop more highly organized structures. It is 

 not unhke a suburban area in the outlying regions of a community whose 

 structural organization is progressively encroaching upon the as yet undevel- 

 oped fields. That so much of reticular formation still remains even in the 

 highest primates might perhaps argue further potentiahties of differentiation 

 in this diffuse neural materiaL However this may be, emphasis is laid upon 

 the gradual recession of a poorly differentiated portion of the axis, and its 

 equally gradual replacement by more highly organized structures as the 

 physical and physiological factors of evohition make their appearance in the 

 progress of the primates. 



Ventrolateral to the formatio reticularis is a large and broad field of 

 gray matter containing cells of several sizes, the substantia nigra ( Sbn), to 



