THE ANKLE-JOINT OF THE SLOTHS. 285 



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 re- 

 ceived a correspondingly conical process of the distal end of 

 the fibula, which is directed from above and without, down- 

 ward and inward — not verticallj-, therefore, but very oblique- 

 ly. Hence, even if the pivot fitted its socket quite accurately, 

 there would still be abundant opportunity for flexion and ex- 

 tension, though the movement of the pes would be obliquely 

 inward, as well as upward, in the former case ; and obliquely 

 outward, as well as downward, 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 ex- 

 ternal 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, cu- 

 boides, naviculare, the three cuneiformia, the three complete 

 and the three rudimentary metatarsals, and the three basal 

 phalanges of digits ii., Hi., and «"«., are anchylosed together 

 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 mov- 

 able upon one another are : 1. The distal phalanges, which 

 have a movement of extension and flexion through 180° upon 

 the middle phalanges. 3. 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 plantigrade position in which it is 

 perpendicular to the axis of the leg, to the scansorial position, 

 in which it lies parallel with the axis of the leg. It may be 

 doubted, however, whether the former position can be given 

 to the sole by the living animal. The tibialis anticus and the 

 extensor hallucis longus are extremely strong muscles, and 

 have no efiicient antagonists ; so that their tonic contraction 

 must pull the navicular metatarsal tuberosity, into which they 

 are inserted, as far upward as it will go, causing the tarso-pha- 

 langeal synostosis to rotate upon the astragalus, and thus 

 obliging the sole of the foot to look inward. 



In the two-toed Sloth, or Unau ( Cholmpus), the general 

 structure of the ankle-joint is the same, but the fossa of the 



