TARSIUS SPECTRUM 99 



The Brain Stem in Tarsius 

 the ventral surface 



The ventral appearance of the brain stem in tarsius is peculiar because 

 of the relatively small size of the pons Varohi, the pyramids, the inferior 

 olivary eminences and the cerebral peduncles. All of these are evidences of a 

 low degree of development in the neokinetic organization of the animal. 

 Its vohmtary control of motor activity, its dispensation of coordinative 

 control of the muscles, its measure of simultaneous regulation in the move- 

 ments of the head, eyes and hands, could not be other than extremely simple 

 and generahzed on the basis of these structures. The pyramids appear as two 

 narrow bands extending caudad from the lower border of the pons toward the 

 upper cervical region of the spinal cord. The pons itself is a narrow, flat band 

 of very httle surface prominence. Caudal to it the corpus trapezoideum 

 occupies a transverse position upon the surface of the axis. Tarsius is the only 

 primate in which the corpus trapezoideum is entirely exposed in this position. 

 This fact does not imply any greater organization in the auditory apparatus 

 of the animal, but does denote how poorly the pons Varohi has developed. 



The ohve makes a very small protuberance lateral to the pyramid and is 

 separated from it by a wide area hardly justifying the term preohvary 

 sulcus. The emergent fibers of the hypoglossal nerve, however, make their 

 appearance on the surface in close relation to the olivary eminence. Two 

 small peduncles form the caudal boundary of a limited optico-peduncular 

 space whose cephalic boundary is provided by the massive optic chiasm and 

 tracts. 



THE DORSAL SURFACE 



The dorsal surface, upon removal of the cerebellum, shows a poorly 

 defined clava, cuneus and tuberculum trigemini. The floor of the fourth 



