24 ARBOREAL MAN 



of the carpus-a pre-axial bone (radiale), an intermediate 

 bone (Jermedinm), and a post-axial bone (ulnare. 

 The second row is composed of five small and fairly 

 uniform bones (Carpalia L-V.), one being situated at the 

 base of each metacarpal. Between the two rows, and 

 situated in the middle of the carpus, is a central element 

 (centrale) (We may, in modern morphology, be forced 

 to depart slightly from the classical scheme of Gegenbaur, 

 in admitting the possible presence of more than one 

 centrale, but this possibility does not detract from the 

 simplicity of the main plan.) 



As to the number of rays in the distal segment of the 

 limb, we know that among the lowest Vertebrates which 

 lead aquatic lives they may be extremely numerous; in 

 the most primitive of the higher land-living classes this 

 is changed, and the possession of five terminal elements 

 has become the rule. This change is undoubtedly 

 associated with the development of extra-neural ribs and 

 the formation of intra-costal limb plexuses, the number 

 of epiblastic segments entering into the limb plexuses 

 being now restricted to five, represented by the five 

 roots entering into the formation of the brachial plexus. 

 1 These five terminal digits are composed of a series of 

 separate jointed elements, metacarpals and phalanges, of 

 which there are primitively (or at any rate in very primi- 

 tive types, if not in basic form) one metacarpal and three 

 phalanges, or four separate elements, in each digit 

 (see Fig. 6). 



Taking the Mammalia as a whole, and selecting from 

 any and every type the most unaltered feature of every 

 segment of the limb, regardless of the condition of the 

 other segments, we may estimate the amount of minimal 

 departure from this archsetype consistent with that stage 

 of evolution represented by the Mammalia. The skeletal 

 elements of the first and second segments may persist 

 quite unaltered in a large number of Mammals. 

 The bones of the first row of the carpus and the os 



