448 THE INTERMEDIATE PRIMATES 



directed somewhat obliquely from before backward and inward. It rests in a 

 space hollowed out of the reticular formation which surrounds it upon its 

 dorsal, ventral and mesial surfaces. Dorsomesially the substantia gelatinosa 

 trigemini is in relation first with the nucleus of Goll and, more cephahcally, 

 with the nucleus of Burdach. 



In the gibbon, the constriction in the substantia gelatinosa trigemini 

 described in connection with the other primate brain stems is well defmed, 

 occurring at about the level of the junction of the upper and middle thirds of 

 the inferior ohvary nucleus. Above this point the substantia gelatinosa again 

 enlarges and passes upward in the lateral portion of the tegmentum to the 

 upper metencephalic levels. At this point it expands to form its caput and is 

 associated with the motor nucleus of the trigeminal nerve on its mesial aspect. 



The Inferior Olivary Nucleus 



In Hylobates hoolock the inferior olivary nucleus has I^ecome a relatively 

 massive and prominent structure. It presents a great number of secondary 

 plications in its surface. The fundus of the inferior olive is deep and wide, 

 with a number of secondary loops. The accessory olivary nuclei are well 

 developed. The ventral accessory nucleus begins below as a flat, elongated 

 band applied to the inner half of the ventral branch of the main olivary 

 nucleus. It extends upward in this position as a compressed lamina of gray 

 matter lying between the inferior olivary nucleus and the pyramidal tract. 

 As the ventral accessory nucleus is traced upward it gradually diminishes 

 in size until it comes to an end apparently by fusing with the mesial extremity 

 of the ventral branch of the principal nucleus. 



The dorsal accessory olive begins at about the middle of the main olivary 

 nucleus and appears as a narrow layer of gray matter applied to the inner 

 portion of the dorsal branch of the olivary nucleus. It rapidly reaches its 

 maximum transverse diameter and then, gradually diminishing, ends below 



