SUMMARY' OF STRUCTURES 469 



passing from the lower to the higher group there has been a definite incre- 

 ment in these nuclear structures. The gibbon, for example, as compared 

 with the lemur, shows a pontile nuclear aggregation nearly four times as 

 large. These nuclei in the gibbon are nearly twice as large as they appear 

 in either the marmoset or mycetes. 



Whatever difficulties, therefore, may seem to exist in estimating the 

 relative importance of these nuclei so far as this is revealed by their dimensions 

 in the intermediate group, there can be no doubt of the expansion in the 

 pontile nuclei in passing from the lower to the intermediate primates. 

 This may be accepted as indicative of a progressive functional increment in 

 manual specialization. 



vir. The Midbrain Colliculi in Relation to the Functions of 

 Sight and Hearing 



The comparison of the inferior colliculi of the intermediate primates 

 indicates a further decrease in functional prominence in these auditory relay 

 centers. The inferior colliculus in gibbon is smaller and less well developed 

 than in either the macacus or the baboon. That some of the primordial audi- 

 tory function is still vested in the inferior colliculus and acts in the interest of 

 immediate reflex adjustment to sudden noises or sounds is probably true. 

 But the sense of hearing in gibbon appears to have undergone such amplifi- 

 cation in its associational values that the auditory area of the cerebral cortex 

 has taken supersedence over the more primitive midbrain region originally 

 active in auditory function. Both the macacus and baboon seem to have 

 retained more of the immediate reflex organization based upon the sense of 

 hearing than is the case in gibbon; they seem to depend less than the gib- 

 bon upon the transmission of auditory stimuli to the cerebral cortex before 

 determining upon specific courses of action. This structure thus indicates the 



