332 THE ANATOMY OF VEETEBRATED ANIMALS. 



lation of tlie 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 extremity of the fibula penetrates, like a pivot. 

 (See PI. 208, 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 arrange- 

 ment is that the foot 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 by 

 spreading out the leg so as to make it almost horizontal." 



Meckel,! has already justly remonstrated agaiast Cuvier's 

 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 

 follows Meckel, but Rapp ('Edentaten,' p. 46) adopts 

 Cuvier's statement in its fulness : " Extension and flexion 

 of the foot cannot take place, but only abduction and 

 adduction." However, it is easy to demonstrate on the 

 uninjured dead animal, or stiU 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 ; 1st, in 

 abduction and adduction*; a movement in azimiith, when 

 the leg is vertical ; 2ndly, in flexion and extension ; a more 

 extensive movement in altitude, imder the same circum- 

 stances ; and 3rdly, 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- 



* Cuvier's words are : " II en resulte encore que le plan, le corps du 

 pied, est presque vertical quand la janibe Test." 



t ' System der vergleichendea Anatomie,' 2te Theil., 2te Abthei- 

 lung, p. 457. 



