212 ASTRAGALUS. 



into a large smooth head, which is slightly hollowed in the 

 middle ; and therefore resembles a superficial pulley, by 

 which it is fitted to the lower end of the tibia. The internal 

 side of this head is flat and smooth, to play on the inter- 

 nal malleolus. The external side has also such a surface, 

 but larger, for its articulation with the external malleolus. 

 Round the base of this head there is a rough fossa ; and 

 immediately before the head, as also below its internal smooth 

 surface, we find a considerable rough cavity. 



The lower surface of the astragalus is divided by an irregular 

 deep rough fossa, which, at its internal end, is narrow, but 

 gradually widens as it stretches obliquely outwards and 

 forwards. The smooth surface, covered with cartilage, behind 

 this fossa, is large, oblong, extended in the same oblique situa- 

 tion with the fossa, and concave for its conjunction with the 

 os calcis. The posterior edge of this cavity is formed by two 

 sharp-pointed rough processes, between which is a depression 

 made by the tendon of the flexor pollicis longus. The lower 

 surface before the fossa is convex, and composed of three 

 distinct smooth planes. The long one behind, and the exterior 

 or shortest, are articulated with the heel bone ; while the 

 internal, which is the most convex of the three, rests and moves 

 upon a cartilaginous ligament, that is continued from the os 

 calcis to the os naviculare, without which ligament the astra- 

 galus could not be sustained, but would be pressed out of its 

 place by the great weight it supports; and the other bones of 

 the tarsus would be separated. Nor would a bone be fit here, 

 because it must have been thicker than could conveniently be 

 allowed ; otherwise it would break, and would not prove such 

 an easy bending base, to lessen the shock which is given to the 

 body, in leaping, running, &c. 



The forepart of this bone is formed into a convex oblong 

 smooth head, which is received by the os naviculare, and is 

 placed obliquely ; its longest axis inclining downwards and 

 inwards. Round the root of this head, especially on the upper 

 surface, a rough fossa may be remarked. 



The astragalus is articulated above to the tibia and fibula, 

 which together form one cavity. In this articulation, flexion 



