284 THE ANATOMY OF VERTEBRATED ANIMALS. 



" In the greater number of animals, the principal articu- 

 lation of the astragalus connects it with the tibia, by means 

 of a more or less loose ginglymus, which allows the foot to be 

 bent on the leg. But here the principal and superior facet of 

 the astragalus is a conical fossa, into which the pointed ex- 

 tremity of the fibula penetrates, like a pivot. {See PI. 308, 

 Fig. 2a.) The inner edge of this fossa turns against a very 

 small facet, which occupies only a third of the lower head of 

 the tibia. The result of this arrangement is that the fool 

 turns on the leg, like a weathercock on its support, but that it 

 cannot be flexed. It further follows that the plane of the sole 

 of the foot * is almost vertical when the leg is so, and that the 

 animal can only place the plantar surface of its foot on the 

 ground bj' spreading out the leg so as to make it almost hori- 

 zontal." 



Meckle f has already justly remonstrated against Cuvier'a 

 assertion that only abduction and adduction are possible to 

 the pes of the Ai, affirming that it is capable of flexion and 

 extension, though only to a limited extent. A. Wagner fol- 

 lows Meckel, but Rapp (" Edentaten," p. 46) adopts Cuvier's 

 statement in its fulness : " Extension and flexion of the foot 

 caimot take place, but only abduction and adductior,." How- 

 ever, it is easy to demonstrate on the uninjured dead animal, 

 or, still better, on the limb from which the muscles have been 

 removed, while the ligaments have been left intact, that the 

 pes of the three-toed Sloth is capable of extensive motion in 

 three directions : first, in abduction and adduction ; a move- 

 ment in azimuth, when the leg is vertical; secondly, in flexion 

 and extension ; a more extensive movement in altitude, un 

 der the same circumstances ; and, thirdly, in rotation upon its 

 own axis, by means of which the sole can be moved through 

 90° from a position perpendicular to the axis of the leg to one 

 parallel with it. 



The anatomical arrangements upon which the execution 

 of these movements depend are the following : The astraga- 

 lus presents two facets to the bones of the leg, one of which 

 (when the pes is in the position usual in other quadrupeds) 

 looks inward and upward, while the other looks outward and 

 upward. The former, convex from before backward, as well 

 as from side to side, is by no means a mere rim, though it is 



* Cuvier's words are : " II en r^sulto encore que le plan, le corps du pied^ 

 est presque vertical quaTid la jambe Test." 



f " System der vergleichenden Anatomie," 2te Theil., 2te Abtheilung, 

 p. 467. 



