CALLITHRIX JACCHUS, THE MARMOSET 179 



These movements have much of the quality of immediate reflex responses. 

 They resemble but little the motor reactions characterized by any marked 

 degree of deliberate rcllection in selecting alternative courses of action which 

 may arise in connection with any given situation. For this reason apparently 

 the superior colliculus retains much of its primitive conspicuity as well as much 

 of its original structural organization. The evidence afforded by marmoset 

 indicates that vision has attained only partial telencephalization in this form. 

 Also of importance is the appearance of the large substantia nigra 

 (Sbn) which again points to the possibility of the animal's large endowment 

 in automatic associative control of movement. In the most ventral portion 

 of the basis are the fibers constituting the cerebral peduncle (CP). A lateral 

 protrusion in juxtaposition to the substantia nigra is the mesial geniculate 

 body (Mgb) along whose periphery is situated the brachium conjunctivum 

 posticum containing auditory fibers on their way from the inferior colliculus. 

 Dorsal to the mesial fillet ( M f ) is a collection of large motor cells, the nucleus 

 ruber (NRu), which gives origin to the rubrospinal tract, the connecting 

 link between the cerebellum and the final common pathway of the cerebro- 

 spinal axis. The central gray matter ( C e n ) is now greatly increased in size and 

 surrounds a small aqueduct of Sylvius. At its ventromesial angle it contains 

 the nucleus oculomotorius (Noc) which gives rise to the third cranial nerve 

 supplying all the muscles of the eyeball with the exception of the external 

 rectus and the superior oblique. An important feature of this nucleus is the 

 fact that it is very poorly supplied with internuclear or commissural fibers. 

 The two nuclei maintain a certain degree of independence. This perhaps is no 

 more marked than in lemur, but it is distinctly more pronounced than in the 

 higher apes and man. It is probable that marmoset possesses but a small degree 

 of power to converge the visual axes and in consequence has a limited amount 

 of stereoscopic vision. Passing forward from the nucleus of the third nerve 

 are its emergent fibers. 



