TARSIUS SPECTRUM 129 



permitting a certain degree of latitude, might be considered an "optic 

 lobe." The disposition of the central canal in this midbrain region would 

 carry further suggestiveness in this line of interpretation. The large lateral 

 diverticulum extending in the direction of the collicuhis is strongly reminis- 

 cent of the hiteral mescncephaHc extension characterizing the brain of many 

 lower vertebrates. 



One notable advance in the brain of tarsius toward primate differ- 

 entiation is the definite progress which it has made in extending the visual 

 cortex. On the other hand, it is improbable that such advance has gone so 

 far as to relieve the superior colhculus of much of its primitive visual func- 

 tion. Such a view attributes to the optic portion of the mesencephalic roof a 

 large retention of activities related to vision, a condition which again 

 declares the primitive organization of the bram in this animaL 



Accompanying these strikingly primitive characters in the cerebral 

 peduncle and the superior colhculus are other features at this level which 

 bear testimony of similar moment. The substantia nigra (Sbn), for example, 

 is an even more extensive structure than it is in lemurs. It thus stands out 

 in contrast to this structure in all others of the primate order. The reticular 

 formation (Ref) likewise is more conspicuous and much more diffuse. In 

 it may be distinguished a poorly defined nucleus ruber ( NRu) of relatively 

 small size. Along the ventrolateral border of the substantia nigra are 

 the two divisions of the mesial fillet (Mf), the ventral portion lying in 

 close proximity to the emergent fibers of the nerve, the dorsal portion border- 

 ing the brachium geniculatum and lying ventral to the spinothalamic tract 

 (Spt). The central gray matter (Cen) is more extensive and also more 

 pyriform than in other primates. It contains in its ventral apex the dorsal 

 and ventral divisions of the oculomotor nucleus (Noc) which is charac- 

 terized by the fact that but few fibers enter the oculomotor decussation. 

 This latter observation is in harmonv with the statement made l^y the late 



