502 Observations on the Anatomy of the Sloth. 



direction, must' offer considerable conveniences to an animal 

 which feeds on the leaves of trees in its immediate vicinity, and 

 would also enable the animal to direct his visual organs to any. 

 position, without changing that of its body. 



But the most remarkable peculiarity in the skeleton of this 

 species, and which alone distinguishes it from that of all others, 

 and admirably adapts it for its characteristic mode of locomo- 

 tion, is to be observed in the form, structure, and articulation 

 of its posterior extremities. We have already alluded to the 

 widely separated state of the thighs at the acetabulum, which 

 enables it the more readily to embrace any foreign object ; the 

 knee-joint is large, strong, and flexible ; the femur is long, stout, 

 and depressed, with a considerable concavity on its inner edge ; 

 the bones of the legs are both convex externally,, all admitting 

 of the attachment of powerful muscles, and the joints, though 

 supplied with firm ligaments, are unusually flexible. Baron 

 Cuvier has already dwelt with great interest, on the very extra- 

 ordinary and unique manner in which the foot is articulated 

 with the tibia and fibula ; the astragalus, in addition to the pul- 

 ley-like surface, by which it moves on the end of the tibia, pre- 

 sents, on its exterior and upper surface, a deep conical pit, which 

 receives a corresponding projecting bone of the inferior head of 

 the fibula, admitting the greatest latitude of rotatory motion, 

 together with the usual ginglymus motion of the ankle, at the 

 same time rendering dislocation impossible ; but the powerful 

 lateral ligaments prevent lateral motion at this joint ; this, how- 

 ever, is more than compensated, by the unusual degree of mo- 

 tion existing between the calcis and astragalus, or rather of the 

 latter on the former bone ; producing a rocking motion from side 

 to side, two distinct transverse pulley-like surfaces on the infe- 

 rior aspect of the astragalus, being received into two correspond- 

 ing cavities in the upper surface of the calcis, and to render the 

 joint more secure, the anterior articulating surface of the astra- 

 galus, presents a deep conical pit which receives a pyramidal 

 process, projecting from the usual articulating surface of the 

 cuboid bone : a complication of structure, attended with equally 

 complicated motions, witnessed in no other quadruped, and ut- 

 terly useless and inconvenient to an animal moving on a plane 

 surface ; yet admirably adapted to the habits of the Sloth, as it 

 enables the animal, in any position of the body, to apply the 



