334 THE ANATOMY OF TEETEBRATED ANIMALS. 



plantigrade position in which it is perjjendicular 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, how- 

 ever, 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 miiscles, and have no 

 efficient antagonists ; so that their tonic contraction must 

 pull the navicular metatarsal tuberosity into which they are 

 inserted as far upwards as it will go, causing the tarso- 

 phalangeal synostosis to rotate upon the astragalus, and 

 thus obliging the sole of the foot to look inwards. 



In the two-toed Sloth, or Unau {Choloepiis), the general 

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

 the astragalus looks almost directly outwards, and the pivot 

 of the fibula is more nearly horizontal, when the leg is 

 vertical. The tibial facet of the astragalus looks directly 

 upwards. Hence, the movement of the pes is more ex- 

 clusively one of flexion and extension than in the Ai. No 

 ankylosis of the tarsal, metatarsal, and phalangeal bones 

 occurs, but the rotation of the distal moiety of the tarsus 

 upon the astragalus is much more complete and permanent 

 than in the Ai. The calcaneum is t^\asted roiind under the 

 astragalus, in such a manner that its proper external face 

 becomes inferior, while the articular siu-face for the cuboid 

 is not only below, but is partially internal to, the navicular 

 facet of the astragalus. As a result of this position of the 

 cuboid, the outer metatarsals, which it sajiports, are placed 

 directly beneath the inner ones, and the pes rests absolutely 

 upon its outer edge, the plane of the sole being vertical. 



The Sloths, it thus appears, are natui'aUy club-footed; 

 but neither in the Ai, nor in the Unau, does this depend in 

 any way on the structure of the ankle-joint. On the con- 

 trary, it results, in the Unau, from the manner in which 

 the calcaneum and naviculare articulate with the astragalus ; 

 and, in the Ai, from the action of the muscles on the tarso- 

 phalangeal synostosis. Neither in the Ai, nor the Unau, 

 is there anything to interfere with free flexion and exten- 

 sion of the pes. 



