The distal end is expanded to form the =lateral malleolus= (_i_). This bears a facet on the proximal portion of its medial surface near its dorsal margin, for the tibia, and distad of this is a second facet for the astragalus. The ventral and lateral surfaces are grooved for tendons.
=Tarsus= (Fig. 58).--The tarsus consists of seven bones. The longest of these, lying on the lateral side of the foot and forming the support of the heel, is the =calcaneus= or =os calcis= (_a_). It articulates distad with a bone, the =cuboid= (_c_), which bears the fourth and fifth metatarsals. Lying between the calcaneus and the tibia is the =astragalus= or =talus= (_b_), the distal end of which articulates with the boat-shaped =navicular= or =scaphoid= (_d_). The scaphoid bears on its distal surface the three cuneiform bones, lateral (_e_), medial (_g_), and intermediate, bearing the rudiment of the first metatarsal and the second and third metatarsals.
_Astragalus._ (_Talus BNA_) (Fig. 58, _b_).--The astragalus may be divided into body, neck, and head. The body is marked on its proximal surface by a deep pulley-like groove for the articulation with the distal end of the tibia, and on its lateral and medial surfaces by curved facets for articulation with the malleoli of the tibia and fibula. This entire surface for articulation with the bones of the leg is known as the trochlea. The lower surface is marked by two facets separated by a groove; these are for articulation with corresponding facets on the calcaneus. Distally the bone contracts to form the =neck= and enlarges at the end, forming the =head=, which is smooth on its distal surface for articulation with the navicular or scaphoid.
_Calcaneus_ (Fig. 58, _a_, and Fig. 59).--The calcaneus (os calcis) is the largest bone of the foot and forms the heel. It is two or three times as long as broad and has six surfaces: dorsal, ventral, medial, lateral, proximal, and distal. The proximal one-half of the dorsal surface (Fig. 59) is smooth, while the distal half is broadened and bears two facets which are separated by a groove. These articulate with the corresponding facets on the astragalus. The medial facet is borne on a projection of the bone, the sustentaculum tali (_c_). Distad of the facets the surface is rough. The ventral surface is smooth. The proximal end (_b_) is grooved for the tendon of Achilles. The lateral surface is smooth and marked by a grooved tubercle, the =peroneal tubercle= (_d_), near the distal end. The medial surface is marked by part of the articular facet for the astragalus, and also by the grooved sustentaculum tali. The distal end (_a_) articulates with the cuboid.