THE ASTRAGALUS. 



423 



part of which is coated with cartilage. The external surface of the bone is deeply 

 notched. The internal surface is mostly rough, but presents at about the middle 

 an articular facet {qx the external cuneiform, broad above, narrow below, and not usu- 

 ally reaching the plantar surface. Commonly another smaller facet for the scaphoid 

 is found behind this one, from which it is separated sometimes completely, but more 

 often merely by a ridge, which makes no real interruption. The anterior surface, 

 articular for the bases of two metatarsals, has an inner, an upper, and a lower border, 

 the two latter meeting at a rounded angle externally. A faint vertical ridge, nearer 

 the inner than the outer border, usually divides this facet into an inner oblong and 

 an outer triangular part for the fourth and fifth bones. The curves of these articu- 

 lations vary greatly : sometimes both parts are concave from above downward ; 

 sometimes both are practically plane. The posterior surface, entirely articular, 

 is the complement of the front of the os calcis. The cuboid articulates with the 

 calcaneum, the external cuneiform, the fourth and fifth metatarsal bones, often with 

 the scaphoid, and at times with the astragalus. 



Development. — There is but one centre, appearing at about birth ; in our 

 experience, more often after than before. 



For Secondary Cuboid, see Scaphoid. 



THE ASTRAGALUS. 

 The astragalus, ' or talus, is a very irregular bone devoted almost wholly to 

 articular surfaces. It is enclosed above by the socket of the leg bones. Its main 

 part, or body, rests on the calcaneum, and presents in front a constricted neck 

 bearing a rounded head, projecting forward and inward into the hollow on the back of 

 the scaphoid. The upper surface presents a pulley-like articular facet covering the 

 greater part of the bone, convex from before backward, slightly concave transversely, 

 decidedly broader in front than behind. The cartilage covering it is continued 

 down on either side to meet the articular surfaces of the malleoli. The inner border 



Head 



For inter- 

 nal mal- 

 leolus 



Trochlear 

 surface 

 for tibia 



Fig. 435. 



Scaphoid 



Fig. 436. 



For exter- 

 nal mal- 

 leolus 



Triangular 

 surface 



Scaphoid 



For calcaneo- 

 scaphoid lig. 



Calcaneum 



Internal tubercle 



External 

 tubercle 

 Groove for tendon of flex. long. hall. 



Right astragalus from above. 



Calcaneum 



Internal 

 tubercle 



Groove'for flex, 

 long. hall. 



External tubercle 

 Right astragalus from below. 



of the upper articular surface is distinct, but generally not sharp ; the outer, which 

 reaches higher, is better defined in the region just anterior to its middle, but behind 

 on the dry bone it seems rounded. A very well-marked bone shows (what is very 

 striking in the freshly opened joint) that this blunted edge is really a narrow tri- 

 angular area belonging to the superior surface, broadest behind, made apparently 

 by the pressure of the posterior tibio-fibular ligament from the external malleolus to 



1 Talus. 



