THE TARSUS: THE ASTRAGALUS. 203 



Achillis, Plantaris, Abductor hallucis, Abductor minimi digiti, Flexor brevis digi- 

 torum, Flexor accessorius, and Extensor brevis digitorum. 



The Astragalus (Fig. 140). 

 The Astragalus (dffTfjdyatoz, a die) is the largest of the tarsal bones, next to 



For 



navicular. Neck . Stlp . surface For inner 



malleolus. 



For navicular. 



For 

 malleolus. 



FIG. 140 The left astragalus. A. Superior and external view. B. Inferior and internal view. 



the os calcis. It occupies the middle and upper part of the tarsus, supporting the 

 tibia above, articulating with the malleoli on either side, resting below upon the 

 os calcis, and joined in front to the navicular. This bone may easily be recognized 

 by its large rounded head, by the broad articular facet on its upper convex surface, 

 or by the two articular facets separated by a deep groove on its under concave 

 surface. It presents six surfaces for examination. 



The superior surface presents, behind, a broad smooth trochlear surface for 

 articulation with the tibia. The trochlea is broader in front than behind, convex 

 from before backward, slightly concave from side to side ; in front of it is the upper 

 surface of the neck of the astragalus, rough for the attachment of ligaments. 

 The inferior surface presents two articular facets separated by a deep groove. 

 The groove runs obliquely forward and outward, becoming gradually broader 

 and deeper in front : it corresponds with a similar groove upon the upper surface 

 of the os calcis, and forms, when articulated with that bone, a canal, filled up in 

 the recent state by the interosseous calcaneo-astragaloid ligament. Of the two 

 articular facets, the posterior is the larger, of an oblong form and deeply concave 

 from side to side ; the anterior is shorter and narrower, of an elongated oval form, 

 convex longitudinally, and often subdivided into two by an elevated ridge; of 

 these, the posterior articulates with the lesser process of the os calcis ; the anterior, 

 with the upper surface of the inferior calcaneo-navicular ligament. The internal 

 surface presents at its upper part a pear-shaped articular facet for the inner malleo- 

 lus, continuous above with the trochlear surface; below the articular surface is a 

 rough depression, for the attachment of the deep portion of the internal lateral 

 ligament. The external surface presents a large triangular facet, concave from 

 above downward for articulation with the external malleolus ; it is continuous above 

 with the trochlear surface ; and in front of it is a rough depression for the attach- 

 ment of the anterior fasciculus of the external lateral ligament of the ankle-joint. 

 The anterior surface, convex and rounded, forms the head of the astragalus ; it is 

 smooth, of an oval form, and directed obliquely inward and downward ; it articu- 

 lates with the navicular. On its under and inner surface is a small facet, continu- 

 ous in front with the articular surface of the head, and behind with the smaller 

 facet for the os calcis. This rests on the inferior calcaneo-navicular ligament, being 

 separated from it by the synovia! membrane, which is prolonged from the anterior 

 calcaneo-astragaloid joint to the astragalo-navicular joint. The head is surrounded 



