The Lower Extremity and Pelvis 177 



The head has articular surfaces in front, below, and on the inner side. It rests 

 below on the sustentaculum tali and shows a definite facet for this, which is continuous 

 in front and externally with another inferior facet which shows where the head rests 

 on the front part of the body of the os calcis : occasionally these two facets are more 

 or less discontinuous. In the articulated foot it can be seen that an interval exists 

 between the sustentaculum and scaphoid, and in this interval a part of the lower 

 surface of the head is visible : the interval is bridged by the inferior calcaneo-navicular 

 ligament, on which this part of the head rests, and it therefore presents another facet, 

 between and continuous with the others, for this ligament. The inner aspect of the 

 head lies above the level of the sustentaculum and ligament : it is covered, as shown 

 in Fig. 143, by intermediate fibres of the internal lateral ligament, running down 

 to decussate with those of the lower ligament, and over these lies the expansion 

 from Tibialis posticus to the cuboid, which often presents a nbro-cartilaginous 

 thickening. 



Observe the surfaces by which the astragalus articulates with the os calcis. That 

 under the body is concave, while the under surface of the head is convex, and their 

 axes are practically parallel. This disposition hinders any rocking or lateral move- 

 ment of the bones on one another, but the astragalus can be moved forwards and 

 inwards on the os calcis, the two facets sliding over their opposed surfaces, and this is 

 what occurs when weight is thrown on the astragalus : it slightly opens out the inner 

 arch and the astragalus moves a little forward, carrying the scaphoid forward, and 

 pressing its head on the strong inferior calcaneo-navicular ligament and the underlying 

 expansion of Tibialis posticus. 



The somewhat inward direction of the neck and head is more apparent in the 

 foetal and child's foot. In these the natural position of the foot is one of inversion, 

 and the subsequent comparative straightening of the bone is associated with the change 

 of shape of the foot that results from putting the sole on the ground. 



The neck is also comparatively longer and narrower in the new-born foot and the 

 trochlear cartilage is prolonged on to it for some distance. The angle made by the 

 axis of the neck with that of the body in such a foot is about 148, while in the adult 

 astragalus it is about 152. 



The astragalus is the only bone in the tarsus that has no muscular or tendinous 

 attachments. It is partly ossified at birth. 



Os CALCIS. 



The upper aspect shows that the astragalus is carried on the anterior half of the 

 bone : behind this the upper surface is covered by the fatty tissue that lies between 

 the tendo Achillis and the ankle. 



The astragalus facet on the body is convex, while the anterior one is concave. 

 This last carries the head of the astragalus, and is situated on the sustentaculum and 

 the front part of the body of the bone : it is frequently divided, more or less completely, 

 into two, that on the sustentaculum being termed the middle (astragalar) facet and the 

 remainder being the anterior facet. These are separated from the posterior facet by 

 the interosseous sulcus, which widens out externally. The astragalar facets have 

 their long axes put obliquely across the os calcis, so that the astragalus is directed 

 inwards and forwards with reference to this bone and can only move in that direction, 

 sliding on the calcaneum. It follows from this that the upper and front part of the os 

 calcis must be exposed externally, and it is from this part, to the outer side of the 

 astragalus, that the Extensor brevis digitorum takes origin : behind it, and fused with 

 its origin, the lower part of the annular ligament is attached to the os calcis. 



F.A. 12 



