396 THE INTERMEDIATE PRIMATES 



same location and, gradually diminishing, it appears cephalically as a plug 

 in the hilus of the nucleus. Before it completely disappears it seems to fuse 

 with the dorsal extremity of the inferior olivary nucleus. The entire inferior 

 olivary complex extends upward to a level somewhat below that of the 

 middle of the ventricular cavity, where it ends rather abruptly, merging 

 with the reticular formation in whose ventral surface it is embedded. 



The Reticular Formation 



The reticular formation, as shown in the reconstruction, begins in the 

 upper cervical levels by the appearance of isolated groups of nerve cells 

 lying dispersed between the fibers of the lateral portion of the decussating 

 pyramidal tract. It here receives the termination of the ventral gray 

 column and then rapidly becomes the outstanding feature in the tegmen- 

 tum of the rhombencephalon. It is quite irregular in outline and presents 

 embedded in its substance and along its surfaces the various condensations 

 of gray matter forming the nuclei of the brain stem and the collections of 

 nerve fibers which form the ascending and descending tracts. At its ventro- 

 lateral angle it presents the constant condensations of nuclear material which 

 form the lateral nucleus of the reticular formation and the superior olive, 

 the two latter structures being somewhat more prominent and extensive 

 than those found in the preceding forms. On its ventral surface, extending 

 from the cephalic limit of the pyramidal decussation to the mid-ventricular 

 level of the stem, it forms a bed for the dorsal accessory olivary nucleus of 

 the olivary complex which at its beginning emerges from, and at its termina- 

 tion disappears in the matrix of the reticular formation. 



Laterally the reticular formation receives the substantia gelatinosa 

 trigemini, while dorsomesially it affords attachment to the stem of the 

 nucleus of the column of Goll and, somewhat more cephalically, to the 

 base of the nucleus of the column of Burdach. It continues upward, 



