CALLITHRIX JACCHUS, THE MARMOSET 177 



authorities, then it is fair to believe that the marmoset is possessed of a high 

 degree of automatic associative control. Such a presumption would accord 

 well with its recognized inferiority in the development of volitional control 

 of movement. The animal, being poorly provided with such control, would 

 of necessity require a considerable degree of automatic and associative regu- 

 lation to carry on the performances essential to its natural motor adjust- 

 ments. The probability of this supposition is further attested by the large 

 size of the basal ganglia in the forebrain. These structures, comprising 

 the globus pallidus and caudate nuclei, are usually regarded as part of the 

 mechanism essential to the control of automatic associated movements. The 

 pyramidal nerve fibers are now collected in a compact bundle accom- 

 panied by other axons representing the pallio-ponto-cerebellar system. A 

 large mass of decussating libers dorsal to the central gray matter constitutes 

 the commissure of the superior colliculi. The central gray matter (Cen), 

 much enlarged in size, surrounds the small aqueduct of Sylvius. Ventro- 

 mesially it contains the nucleus oculomotorius (Noc). A superficial layer 

 of gray matter forms the stratum griseum superhciale of the superior 

 colliculus, while lateral to this the brachium conjunctivum posticum is 

 extending forward to the mesial geniculate body. 



LEVEL OF THE SUPERIOR COLLICULUS (FIG. 88) 



At the level of the superior colliculus the most distinguishing feature 

 is the appearance of the tectal plate of the midbrain whose surface relief 

 has previously been referred to as particularly prominent. The superior 

 colliculus itself presents a definite degree of stratification reminiscent of the 

 optic lobes in the birds, reptiles, amphibia and fish. The most superficial 

 layer consists of the stratum griseum superficiale. This part of the brain still 

 retains some of its original dominance in visual function. \\ hile it is true 

 that much or most of the higher visual syntheses have their representation 



