THE ANKLE-JOINT OE THE SLOTHS. 333 



lus presents two facets to the bones o£ tlie leg, one of wliicli 

 (when the pes is in the position usual in othei- quadrupeds) 

 looks inwards and upwards, whUe the other looks outwards 

 and upwards. The former, convex from before backwards, 

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

 though it is not so wide as the other. It is the proper 

 proximal surface of the astragalus, and articulates with the 

 tibia. The other surface is excavated by a deep conical pit. 

 Into this is received a correspondingly conical process of 

 the distal end of the fibula, which is directed from above 

 and without, downwards and inwards — ^not vertically, there- 

 fore, but very obliquely. Hence, even if the x^ivot fitted 

 its socket quite accurately, there would stiU be abundant 

 opportunity for flexion and extension, though the movement 

 of the pes would be obliqiiely inwards, as well as iipwards, in 

 the former case ; and obliquely outwards, as well as down- 

 wards, in the latter. But the socket fits the pivot loosely, 

 and hence, as experiment demonstrates, the movement 

 of the pes in flexion and extension is but very slightly 

 oblique. 



The true movement of abduction and adduction is so 

 much less extensive than the movement in flexion and 

 extension, because it is checked by the short and strong 

 internal and external lateral ligaments of the ankle-joint. 



With respect to the rotation of the foot on its own axis— 

 it is to be observed, in the first place, that the calcaneum, 

 cuboides, naviculare, the three cuneifortnia, the three com- 

 plete and the three rudimentary metatarsals, and the three 

 basal phalanges of digits ii.. Hi., and iv., are ankylosed to- 

 gether into one bony mass ; while, as in the manus, there 

 is hardly any motion between the basal and the middle 

 phalanges. Practically, in fact, the only bones of the pes 

 which are movable upon one another are: 1. The distal 

 phalanges, which have a movement of extension and flexion 

 through 180° upon the midclle phalanges. 2. The tarso- 

 phalangeal synostosis above described is freely movable on 

 the astragalus ; and the joint is disposed in such a manner 

 as to allow the sole of the foot to be rotated from the 



