THE OS C A LOIS, OR HEEL-BONE. 143 



the os calcis, which has an additional one, by which the 

 posterior portion is completed. The bones remain carti- 

 laginous for some time after birth; in fact, complete 

 ossification does not occur in some of them before the 

 fifteenth year. 



THE ASTRAGALUS. 



The astragalus is the uppermost bone of the tarsus. 

 It articulates above with the tibia, and laterally with the 

 malleoli. It presents for examination a head, neck, and 

 superior, inferior, .external, internal, and posterior sur- 

 faces. The head is in front ; it is rounded, and articulates 

 with the scaphoid; behind the head the bone is con- 

 stricted, and forms the neck. The superior surface pre- 

 sents a quadrilateral articular surface for the inferior 

 extremity of the tibia; it is convex from before back- 

 ward, and concave from side to side. The internal sur- 

 face presents an articular surface for the internal malle- 

 olus. It is continuous with the articular surface on the 

 superior portion of the bone. The external surface pre- 

 sents an articular facet for the external malleolus continu- 

 ous with the superior articular surface. This arrange- 

 ment of the superior and lateral articular surfaces on 

 the astragalus makes a saddle of articular surfaces on 

 the bone. Inferiorly the astragalus presents a large 

 articular surface, divided into a larger posterior and a 

 smaller anterior portion by a deep groove, for the attach- 

 ment of interosseous ligaments. The anterior articular 

 facet rests on the inferior calcaneo-scaphoid ligament, the 

 posterior on the upper surface of the os calcis. Posteriorly, 

 the astragalus presents a groove for the long flexor of the 

 great toe. 



THE OS CALCIS, OR HEEL-BONE. 



The os calcis, or heel-bone, is the largest bone of the 

 tarsus. It presents six surfaces, -anterior, posterior, 



