1 84 THE LOWER PRIMATES 



descending trigeminal tract irbers. The nuclear mass itself is triangular in 

 form, its apex being mesially directed, while the base is disposed laterally in 

 contact with the descending trigeminal fibers. The nucleus continues upward 

 in its fixed location so constant as to afford an orienting point for other 

 internal structures of the bulb, to the midpontine level where, after expand- 

 ing, it rapidly undergoes contraction and disappears. 



The Inferior Oli\^\ry Nucleus 



Reconstruction of this mass of gray matter resembles very closely the 

 nucleus as described in lemur. The inferior ohve forms a flat, double ribbon 

 with its single plication situated ventrolaterally and close to the surface of 

 the brain stem. Its free extremities are directed dorsomcsially and turn 

 slightly dorsally at their terminations. There is httle indication of the devel- 

 opment of the secondary plications characteristic of this nuclear mass in the 

 higher primates. 



The approach of the nucleus to the surface occasions a slight elevation 

 which corresponds to the relief of the olive so conspicuous in the human brain 

 stem. The chief olivary nucleus, as in the lemur, is poorly developed. The 

 plicated turn of the nucleus appears somewhat thicker and heavier than the 

 two limbs of the nuclear mass formed by the accessory olivary nuclei which 

 stretch dorsomcsially toward the median raphe of the oblongata. 



The inferior olivary nucleus in the marmoset, as in the lemur, consists 

 mainly of the two accessory nuclei. The main inferior olivary nucleus is 

 only suggested. The ventral accessory nucleus appears at the upper decus- 

 sational level of the pyramidal tract as a flattened band directed mesially 

 and slightly dorsally. It ascends, diminishing in width and approaching the 

 midline. The dorsal accessory nucleus is a mass of nuclear material which 

 rapidly flattens out and is embedded in the ventral surface of the reticular 

 formation. It also tends to approach the midline and attenuates as it ascends. 



