254 A MANUAL OF ANATOMY 



The Astragalus. 



The astragalus, or talus, is characterized by having a head, neck, 

 and body. It is situated between the tibia above and the os 

 calcis below, is grasped laterally by the tibial and fibular malleoli, 

 and has the navicular in front. It is the only tarsal bone which 

 receives directly the weight of the body, and it lies with its long 

 axis directed forwards and inwards. In point of size it comes 

 next to the os calcis. 



The head forms the anterior part of the bone, and presents an 

 extensive convex articular surface, which looks forwards and also 

 downwards. It is divided into three facets, called navicular, 

 sustentacular, and ' spring.' The navicular facet, which is placed 

 on the anterior surface, is pyriform, and its long axis is directed 

 downwards and inwards. The sustentacular facet, continuous with 

 the foregoing, is situated on the inferior surface. It is convex and 

 eDiptical, and its long axis is directed forwards and outwards. It 

 is often crossed by an elevated ridge a little anterior to the centre, 

 and it articulates with the sustentacular facet on the upper surface 

 of the OS calcis. The spring facet is situated on the inner aspect 

 of the inferior surface, and is in contact with the superior surface of 

 the inferior calcaneo-navicular or ' spring ' ligament. 



The neck is the constricted part behind the head. It is con- 

 spicuous superiorly, and passes inferiorly into the interosseous 

 groove. This groove is directed forwards and outwards, its inner 

 part being narrow and deep, and the outer wide and shallow. It 

 gives attachment to the strong interosseous ligament which binds 

 the astragalus to the os calcis. The neck is perforated all round 

 with numerous nutrient foramina for offsets of the dorsalis pedis 

 artery and its tarsal branch. 



The body is quadrilateral, and presents four surfaces and a pos- 

 terior border. The superior surface presents an extensive trochlear 

 facet, which is concave from side to side, and convex from before 

 backwards. Posteriorly it slopes downwards, and in this situation 

 it usually presents a transverse groove for the play of the transverse 

 ligament. The inner border is straight' and slightly depressed, 

 and as a rule it extends rather farther back than the outer, 

 which latter is somewhat sinuous. The surface is broader in 

 front than behind. The external surface is deep, and presents a 

 large triangular facet for the external malleolus, the apex being 

 downwards. It is concave from above downwards, and, immediately 

 in front of it, the anterior fasciculus of the external lateral ligament 

 of the ankle-joint takes attachment. The internal surface pre- 

 sents superiorly a falciform facet, broad in front and pointed 

 behind, for the internal malleolus. This facet in the foetus en- 

 croaches on the inner side of the neck, a condition which is associ- 

 ated with the inversion of the foot at that period of life. This 

 sometimes occurs in the adult, and, if it does so to any marked 

 extent, it usually accompanies the condition known as talipes varus. 



