76 THE LOWER PRIMATES 



The ventral accessory (paleo-olivc) nucleus appears somewhat below the 

 mid-decussational level of the oblongata as an oval mass directed mesially 

 and shghtly dorsally. It presents no secondary pHcations and after appearing 

 as a small oval collection of nerve cells it extends transversely to form a 

 flattened band of nuclear material. Mesially it fuses with the extremity of the 

 dorsal accessory nucleus. The dorsal accessory nucleus appears as a round 

 nuclear accumulation imbedded in this ventral surface of the reticuhir for- 

 mation. It spreads rapidly into a thin lamina which fuses with the mesial 

 extremity of the ventral accessory ohve. The chief oHvary nucleus appears 

 as a loop between the lateral extremities of the accessory olivary nuclei with 

 which it fuses. There are no reduplications in this loop. 



The Reticular Formation 



In the reconstruction, the reticular formation presents an extensive 

 mass of nuclear material traversed by great numbers of scattered nerve 

 libers. It is roughly quadrilateral in shape and forms the main portion of the 

 tegmentum of the oblongata, pons and midbrain. In it develop the various 

 nuclei of termination and origin of the medullary and pontile cranial nerves. 

 Upon its surface it presents the indentations made by the various ascending 

 and descending tracts of the brain stem; while at various levels it is pierced 

 by the wide swinging bundles of decussating fibers constituting the mesial 

 and lateral fillets and the inferior cerebellar peduncles. It forms a matrix 

 along the mesial septum for the posterior longitudinal fasciculus, the predorsal 

 bundle and other longitudinal fasciculi. 



The reticular formation, in affording passage to the various white fiber 

 tracts, seems to serve as a semi-fluid medium which surrounds these tracts 

 on all sides, supporting them in a soft, gelatinous matrix. It begins at about 

 the level of origin of the decussation of the pyramidal tracts and, extending 

 ventrally, dorsally and laterally, receives the cephalic termination of the 



