LIMBS 169 



all Mammals in which the radius is fixed in a position of pronation 

 (vide infra). In certain Mammals --more particularly the 

 Primates, in which the fore limb is prehensile, the bones of the 

 forearm, instead of being firmly connected together, articulate 

 with one another, the distal end of the radius being capable ot 

 rotation round the ulna. When the two bones lie parallel and the 

 wrist is not bent, the palmar surface of the manus looks inwards, 

 and when rotated on one another towards the body, backwards : the 

 former position is spoken of as that of supination, the latter that 

 of pronation. Indications of these movements are seen even in 

 climbing Marsupials. 1 The radius is the more important in 

 supporting the hand, while the ulna forms the chief connection 

 with the humerus. The ulna extends proximally beyond the 

 elbow joint as the olecranon, on which the extensor muscles are 

 inserted. The ulna may undergo more or less reduction and fusion 

 with the radius, so that in some cases only the olecranon is 

 distinguishable. 



In addition to the power of rotation of the forearm, the 

 Prosimii and Primates proper are characterised by a higher 

 differentiation of the first finger (pollex), which becomes more 

 independent and is capable, not only of abduction and adduction, 

 but also of being brought into opposition with the palm of the 

 hand to a greater or less extent. As regards the pes, the hallux 

 even in Marsupials may be opposable, but never as markedly so 

 as in Lemurs and Monkeys, which are often spoken of as 

 Quadrumana. 2 



A brief account of the mammalian carpus and tarsus must 

 suffice in this place, as considerable differences exist in the various 

 groups, and there is no consensus of opinion as regards the 

 homologies of the various components. 



The carpus and tarsus most nearly correspond with those of 

 Urocleles, Hatteria and Chelonians. Primarily the centrale can be 



1 The rotation of the radius on the ulna has doubtless come about largely 

 owing to the gradual increased differentiation of the muscles during phylogeny ; 

 but this does not sufficiently account for the different relative positions, of the 

 two bones of the fore-arm and shank respectively. The tibia lies on the inner 

 side of the shank, while the corresponding bone of the fore-arm, the radius, 

 owing to secondary shifting, is external when in the position of supination. The 

 reason of this cannot be due to a rotation of the distal end of the humerus, for 

 even in Amphibians the same conditions are plainly seen. The crossing of radius 

 and ulna has rather resulted in consequence of the manus becoming rotated in 

 a contrary direction to that of the limb as a whole as it extends inwards towards 

 the body in order to act as a support for the latter. Consequent!}', the originally 

 parallel position of the two bones of the forearm is not retained, as it is in the 

 case of those of the shank, in which the rotation follows the same direction as 

 that of the entire limb. 



2 In the Marmosets (Arctopithecini) the thumb is not opposable, and the 

 opposable hallux is the only digit which bears a flat nail, all the others having 

 claws. In Ateles the pollex is vestigial and possesses only a single small phalanx, 

 while in Colobus it may even be wanting. In consequence of the erect position 

 of Man, and of the foot being used merely as an organ of support and loco- 

 motion, the prehensile character of the pes has become lost, 



