OS CALCIS. 213 



and extension are the most considerable motions ; the other 

 motions being restrained by the malleoli, and by the strong 

 ligaments which go out from the points of these processes, to 

 the astragalus and os calcis. When the root is bent, as it 

 commonly is when we stand, no lateral or rotary motion is 

 allowed in this joint ; for then the head of the astragalus is sunk 

 deep between the malleoli, and the ligaments are tense: but 

 when the foot is extended, the astragalus can move a little to 

 either side, and with a small rotation. By this contrivance, the 

 foot is firm, when the weight of the body is to be supported on 

 it ; and, when a foot is raised, we are at liberty to direct it 

 more exactly to the place we intend next to step upon. 



The astragalus is joined below to the os calcis ; and before 

 to the OS naviculare, in the manner to be explained when these 

 bones are described. 



The OS calcis is the largest bone of the seven. Behind, it is 

 formed into a large knob, commonly called the heel, the poste- 

 rior surface of which is rough below for the insertion of what 

 is called the tendo-achillis, and oblique above to allow the heel 

 to be depressed without pressing against the tendon. On the 

 upper surface of the os calcis, there is an irregular oblong 

 smooth convexity, adapted to the concavity at the back part of 

 the astragalus ; and beyond this a narrow fossa is seen, which 

 divides it from two small concave smooth surfaces, that are 

 joined to the forepart of the astragalus. The posterior of these 

 smooth surfaces, which is the largest, is the upper surface of a 

 process which projects inwards : and under it is a small 

 sinuosity for the tendon of the flexor digitorum longus. 



The external side of this bone is flat, with a superficial fossa 

 running horizontally, in which the tendon of the musculus 

 peroneus longus is lodged. The internal side of the heel bone 

 is hollowed, for lodging the origin of the massa carnea, and for 

 the safe passage of tendons, nerves, and arteries. Under the 

 side of the internal smooth concavity, a particular groove is 

 made by the tendon of the flexor pollicis longus ; and from the 

 thin protuberance of this internal side a cartilaginous ligament 

 that supports the astragalus, goes out to the os naviculare; on 



