STRUCTURAL CULMINATION 943 



nuiit in this type of ^ cTtchratc. Animals developing claws, hoofs and paws 

 in the forelimf) dilVerentiation ha\e a forebrain index wliich may rise to, but 

 does not exceed 80 per cent. For tlie most part it is much below this figure. 

 The sloth, the anteater and e\ en the elepliant all lia\e a \ ahie below 76 per 

 cent. The camel, thi' horse and the dog reach the upper limit of 79 or 80 per 

 cent w hich seems characteristic of animals whose forelimbs arc conditioned 

 by de\eIopment as specified. In that group in which manual diHerentiation 

 reaches its final stages, the hand presenting fingers and fingernails, the fore- 

 brain index is always above 80 per cent. It ranges throughout the primates 

 from 81 per cent m lemur to approximately i)o per cent in man. T he signifi- 

 cance of these comparati\ e indices takes its importance from the tact that the 

 forebrain is the most recent accession to the central ner\()us system and 

 attains its greatest expression m mammalian orders, particularly in the jjri- 

 mates. It is the structure essential to the most highly organized behavioral 

 regulation. Animals possessed of the greatest development in this respect 

 manifest the greatest adaptability in meeting the conditions of their einiron- 

 meiit. They are better able to adjust themselves and enjoy a greater range 

 of acti\ity. The primates stand in contrast to all other vertebrates because 

 they possess the most effective organ for the utilization of their environ- 

 mental opportunities. 



The Diametric Indices of the Primate Brain. The diametric in- 

 dices of the brain establish an ordinal character which also distinguishes the 

 primates among mammals. In these animals the ratio of the iiiterj)arietal 

 diameter to the occipito-frontal diameter varies from .81 to .88. Compared 

 with carni\()res, ungulates and marsupials, the contrast is striking. It 

 indicates the type of brain whose longitudinal expansion is concomitantly 

 followed by a transverse expansion. In other words, the brain is becoming 

 broad and losing some of that elongated appearance w hich is characteristic 

 of lower mammalian forms. This condition of broad brainediiess, if such it 



