46 COMPARISONS OF STRUCTURE IN ANIMALS. 



sile instruments of the existing and extinct 

 sloths, extending as far as was compatible with 

 the different degrees of resistance to be orer- 

 come. In the small climbing sloth, the claws 

 are long and slender, having only to bear the 

 weight of the animal's light body, which is 

 approximated by the action of the muscles 

 towards the grasped branch as to a fixed point. 

 The stouter proportions of the prehensile hooks 

 of the mylodon accord -with the harder task of 

 overcoming the resistance of the object seized, 

 and bringing it down to the body. For the 

 long slender brachial (upper-arm) and anti- 

 brachial (fore-arm) bones of the climbing 

 sloth, we find substituted in its gigantic pre- 

 decessor a humerus, radius, and ulna, of more 

 robust proportions — of such proportions, in- 

 deed, in the mylodon rohiistm, as are unknown 

 in any other existing or extinct animal. 



" The tree being thus partially undermined 

 and firmly grappled with, the muscles of the 

 trunk, the pehds, and hind Hmbs, animated by 

 the nervous influence of the unusually large 

 spinal cord, would combine their forces with 

 those of the anterior members in the efforts at 

 prostration. And now let U6 picture to our- 

 selves the massive frame of the megatherium 



