CHAPTER XII 



THUMBS AXD BIG TOES 



So far, we have, in considering the question of the develop- 

 ment of the grasp,- dealt only with the j)Ower of (1) adap- 

 ting the palmar surface of the hand and foot to the branch, 

 and (2) flexing the fingers over it, to make the adaptation 

 more perfect. For the perfected grasp another factor 

 comes in, since the hold is made more secure by folding 

 digits over both sides of the object to be grasped. This 

 simple requirement has led to the most divergent develop- 

 ments, when the climbing Vertebrates are looked at as 

 a whole. Among the existing Reptiles, the Chame- 

 leons show the most extreme development of arboreal 

 grasp, and in them the fourth and fifth digits are turned 

 directly backwards, away from the third, second and 

 first, which retain their primitive forward direction. 



In the perching Birds, some variety exists in the 

 arrangement of the clasping digits; and the so-called 

 Zygodactyle foot of the Scansores achieves the same 

 effect as is attained by the Chameleon. 



In the Mammals, and especiall}?- among the Primate 



stock, the arboreal life has led to the specialization of one 



digit upon hand and foot, which opposes the remaining 



four digits. These opposing digits are the thumb (pol- 



lex) and the big toe (hallux). By an opposing digit we 



mean one that can be turned round so that its palmar 



aspect is opposed to the palmar aspect of the remaining 



digits, and therefore can be placed, for example, ujx^n 



the opposite side of a branch. 



The subjects of thumbs and big toes has provided an 



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