2i6 THE LOWER PRIMATES 



is evidently a defrnite accession in motor control, particularly in the regula- 

 tion of the movements in the upper extremity. The structures in the dorsal 

 sensory field have occasioned a general broadening in the diameters in this 

 region and also show an increase both in fiber richness and extensiveness of 

 the nuclear collections. The nucleus of Goll (NG) appears to be of about 

 the same size as the nucleus of Burdach (NB). The increase in the latter 

 element indicates the addition of a highly developed hand. A corresponding 

 increase in the mesial fillet (Mf) indicates increments in the function of 

 discriminative sensibility already made apparent by expansions in the 

 dorsal columns of Coil and Burdach. A feature of much importance is the 

 inferior olivary body (10) whose caudal extremity is seen in this section. 

 Here, as elsewhere in the brain stem of mycetes, the olivary body fails to 

 show that clearcut delimitation characteristic of those forms in which the 

 structure attains its highest differentiation. Since the inferior olive is function- 

 ally active in the coordination of simultaneous movements in head, eyes and 

 forearm, and since it facilitates the coordination of all skilled learned perform- 

 ances, it should be more highly specialized in this species than in the lemur 

 or marmoset. Differentiation of the olive depends upon the extent to which 

 cooperative movements of the eyes, head and hand are coordinated. Such 

 movements manifest an increase of effectiveness in proportion as the animal 

 is able, by means of the hand, to bring objects into closer scrutiny by the eyes. 

 This ocular, cephalic and brachial adjustment is also proportional to the 

 degree of accuracy with which head and eye movements are capable of 

 following manipulations of the hand. 



The substantia gelatinosa of Rolando (NR) and its accompanying 

 descending tract of the fifth nerve (Trd) are prominent features at this 

 level although comparatively smaller than in lemur or marmoset. The signifi- 

 cance of this apparent decrease in the sensory representation of the head 



