2S6 A MANUAL OF ANATOMY 



The astragalus derives its blood-supply from branches of the 

 dorsalis pedis artery. 



Articulations. — Superiorly with the shaft, and infernally with the 

 internal malleolus, of the tibia ; externally with the external malle- 

 olus of the fibula ; inferiorly with the os calcis ; and anteriorly with 

 the navicular, and occasionally with the lower and inner angle of 

 the cuboid. 



Structure. — The astragalus, being a short bone, is composed of 

 cancellated tissue, surrounded by a thin shell of compact bone. 

 The lamellae of the cancellated tissue are arranged in a curved 

 manner, and in two sets. Some pass downwards and backwards 

 from the superior surface to the posterior calcaneal facet, whilst 

 others arch downwards and forwards from the superior surface to 

 the neck, these being the directions in which weight is transmitted. 



Varieties. — (i) The external tubercle on the posterior border may form a 

 separate ossicle, called the os trigonum. (2) There may be a pressure facet on 

 the upper surface of the neck at its outer part, due to prolonged contact with 

 the anterior margin of the lower end of the tibia. 



The Os Calcis. 



The OS calcis, or calcaneum, is the largest bone of the tarsus, and 

 is characterized by its elongation, lateral compression, and en- 

 largement posteriorly into a tuberosity. It is situated below the 

 astragalus, and behind the cuboid, where it lies with its long axis 

 directed forwards and outwards. It presents two extremities and 

 four surfaces. 



The posterior extremity, which is enlarged, forms the tuberosity 

 or tuber calcis, and constitutes the prominence of the heel. 

 Posteriorly it is divided into three zones — an upper, which is 

 smooth and separated from the tendo Achillis by a bursa ; a 

 middle, rough and vertically ridged, for the insertion of the tendo 

 Achillis ; and a lower, which is continuous with the tubercles on 

 the plantar aspect, and supports the fat of the heel. In front of 

 the tuber calcis there is a constriction, called the neck. 



The anterior extremity presents a large, somewhat triangular 

 facet, narrow towards the sole, which is concave from above down- 

 wards and outwards, and convex from side to side, for articulation 

 with the cuboid. 



The superior surface presents over its anterior part two facets for 

 the astragalus, separated by an oblique groove, and posteriorly a 

 non-articular surface. The antero-internal or sustentacular facet 

 surmounts the sustentaculum tali. It is concave and somewhat 

 elliptical, its long axis being directed forwards and outwards. It 

 is constricted in front of the centre, and is sometimes broken 

 up into two facets by a rough groove The postero-external facet 

 is large, oval, and convex from behind forwards and outwards. 

 The intervening groove, which is directed forwards and outwards, 

 becomes wide and shallow externally, and in front of the outer 



