TARSI us SPECTRUM 125 



ena, as they affect respectively the appearance of the pyramid and the 

 mesial fillet, denote the slight inlluence exerted by the pontile libers upon the 

 course of such suprasegmental systems. Such a pons as that of tarsius could 

 not belong to an annual with more than a meager neokinetic endowment. 

 As a behavioral index it gives a rating lower than the Felidac and about equal 

 to some of the rodents. A capacity so low as this implies a low cerebral 

 organization which might well serve as the stepping stone in the progressive 

 ascent from lower mammalia toward the primates. The surface appearance 

 of the mesencephalic tectum strongly suggests that the inferior colliculus 

 is a functionally important structure. Its general and microscopic characters 

 support this view. It is pronounced in size even when compared with many 

 subprimate forms. Mensuration also indicates that the inferior colliculus 

 plays an important role in the sense of hearing. That part of the animal's 

 behavior dictated by auditory stimuli is characterized by prompt reflex 

 responses. Tarsius consumes little time in reflecting upon the nature of 

 sounds which it hears. It employs such stimuli with almost automatic direct- 

 ness to produce such reactions as guarantee its immediate safety or pro- 

 voke fundamental activities. Furthermore, these auditory stimuli call forth 

 at best only a most limited series of motor reactions. This conception is 

 substantiated by the meager development of auditory areas in the temporal 

 lobe of the brain, as well as by the auditory suprasegmental connections 

 between the midbrain and the endbrain. 



Tarsius also retains in the microscopic appearance of its inferior collic- 

 ulus much of that organization characteristic of lower forms. In the cortex 

 mesencephalica posterior it is possible to identify nine distinct strata. This 

 arrangement harks back to the lower vertebrates, so that the large size of the 

 inferior colliculus, in conjunction with its architectonic specialization signi- 

 fies a retention of the primordial midbrain control over the function of 

 hearing. 



