149 



dependent position of the limb, has the outer edge turned towards the ground, 

 the stout metacarpal bone of the external digit receives the superincumbent 

 weight by three distinct points, from the cuneiform bone, the unciform bone, and 

 the fourth metacarpal : the middle metacarpal, by its peculiarly expanded base, 

 transmits to the fourth metacarpal a great part of the weight which it receives 

 from the radius, by the medium of the os magnum, the trapezoides, the lunare 

 and the scaphoides : the burthen sustained by the ulna and part of that by the 

 radius are concentrated upon the unciforme, and thus transferred to and divided 

 between the two hoofed digits. The great development of the metacarpal bones 

 of these digits, especially as compared with their stunted phalanges, is called for 

 by the important share which they take in transmitting to the ground the su- 

 perincumbent weight. 



Physiological inferences might seem to be exhausted in the demonstration of 

 the admirable adaptation of the foregoing structure of the carpus and metacarpus 

 of the Mylodon to the support and progressive motion on the ground of so pon- 

 derous a quadruped without detriment to the serviceable condition of long and 

 sharp prehensile claws ; but this is far from being the case. A principle cha- 

 racteristic of organic mechanism, and beautifully set forth by the poet*, is no- 

 where perhaps more strikingly exemplified than in the fore-foot of the extinct 

 Megatherioid quadrupeds. The very same arrangement of the bones, which 

 permits the coexistence of hoofs and claws in the same foot, in a great degree 

 expressly provides for the efficiency of the unguiculate digits in their applica- 

 tion to the violent exertions which the whole skeleton of the Mylodon indicates 

 to have been habitual to the living animal. 



The modification of the middle metacarpal, by which it transmits the weight 

 it receives in progressive motion to the fourth and fifth metacarpals, adapts it to 

 its own proper office of overcoming the enormous resistance it must meet in the 

 act of uprooting and prostrating trees. The dislocation of the middle digit when 

 so appHed, is resisted by the interlocking of both ends of its thick expanded 

 base between the contiguous metacarpal and carpal bones. The fourth meta- 



* " In human works, though labour'd on with pain, 

 A thousand movements scarce one purpose gain : 

 In God's, one single can its end produce ; 

 Yet serve to second too some otiier use." 



Essay on Man. 



