394 THE INTERMEDIATE PRIMATES 



The substantia gelatinosa Rolandi has become relatively slender, and 

 placed as a cap over the dorsal extremity of the dorsal gray cohimn. As the 

 highest cervical levels are approached, it is transformed, without any change 

 whatsoever in its morphological characteristics, mto the substantia gela- 

 tinosa trigemini which thereafter serves as a reception nucleus for the 

 descending fibers from the Gasserian ganglion. It passes gradually into the 

 lateral position which is characteristic of this mass of gray matter in 

 the primate brain, and at the level of the inferior olivary nucleus it assumes 

 its permanent unchanging position. The nucleus continues upward, embedded 

 more or less irregularly in the lateral surface of the reticular formation, 

 appearing at times on the surface and at other times being submerged from 

 view by the reticular formation. The usual constriction which has previously 

 been noticed is present at about the middle of the inferior olivary nucleus. 

 Here the substantia gelatinosa trigemini reaches its minimum diameters. 

 From this point it continues gradually to increase in size until it reaches its 

 maximum at about the mid-ventricular region of the stem, above which it 

 continues and develops its expanded cephalic extremity called the caput. 

 Just prior to its disappearance it presents at its mesial and dorsal aspects 

 the motor nucleus of the trigeminal nerve. 



The Inferior Olivary Nucleus 



The inferior olivary nucleus first appears in the reconstruction at the 

 level of the cephalic extremity of the pyramidal decussation. Here it is 

 a flattened lamina of gray matter directed mesially and slightly dorsally. 

 This is the ventral accessory olivary nucleus (paleo-olive). It extends 

 mesially and somewhat dorsally forming a flat ribbon with its mesial extrem- 

 ity twisted so as to lie in a more direct ventrodorsal diameter of the stem. 

 It ends cephalically by merging with the dorsal extremity of the dorsal 

 lamina of the inferior olivary nucleus. The main mass of the inferior olivary 



