Tarsus

The Tarsus is composed of seven bones-the astragalus, os calcis, scaphoid, cuboid, and three cuneiforms.

Astragalus

The Astragalus is the uppermost bone, and, articulating with the tibia and fibula, supports the weight of the body. It is of irregular cubical form, and on the upper, inner, and outer surfaces is one large confluent articular facet, fitting into the space enclosed by the under surface of the tibia, and the two malleoli. This facet is saddle-shaped, and is broader in front than behind. Anteriorly is a somewhat constricted neck supporting the rounded head, which articulates with the scaphoid bone. On the inferior surface is a large concave facet for articulation with the os calcis. A deep groove divides this facet into two parts, the anterior part articulating with the sustentaculum tali, while the posterior part rests on the body of the os calcis. A strong interosseous ligament is inserted in this groove. Posteriorly the bone is divided by a groove, oblique from above downwards and inwards, into two tubercles. In the groove runs the tendon of tibialis posticus.

Fig. 16. - Bones of Foot (Dorsal Surface).

Fig. 17. - Bones of Foot (Plantar Surface).

OS Calcis

The OS Calcis is the largest bone of the tarsus. It is roughly cubical, with a projection posteriorly, forming the heel. The upper surface has anteriorly two facets, divided by a groove to correspond with the facets on the under surface of the astragalus. On the outer side of the anterior facet is a rough surface for muscular origins. The anterior surface has a smooth, convex surface articulating with the cuboid bone. Posteriorly, the tuberosity, which is confluent with the body of the bone, has a surface divided into three parts. The upper part is smooth and covered by a bursa, the middle portion is roughened for the attachment of the tendo Achillis, and the lower part is rough and confluent with the inferior surface. On the inner surface of the body is the sustentaculum tali, a stout projection of bone supporting the astragalus. On the outer surface is the peroneal spine, a small tubercle separating the tendons of peroneus longus and brevis.

The plantar surface is slightly concave from before back-wards, and roughened for the attachment of muscles and ligaments. Posteriorly, there are two tubercles, of which the inner one is the larger.

Scaphoid

The Scaphoid is a flat, oval bone, compressed from before backwards, the long axis being horizontal. It articulates posteriorly with the head of the astragalus, and anteriorly with the three cuneiform bones. The inner surface of the bone projects beyond the inner border of the foot, and forms a rounded tubercle easily felt in the living subject.

Cuneiform Bones

The Cuneiform Bones are three in number-internal, middle, and external. They articulate posteriorly with the scaphoid, and anteriorly with the first, second, and third metatarsals. They are wedge-shaped in form, with the points downwards, the first being the largest, and not so pointed inferiorly as the others. This arrangement helps to keep the concave plantar surface presented by the bones of the foot.

Cuboid Bone

The Cuboid Bone is roughly cubical, and lying on the outer side of the foot articulates posteriorly with the anterior surface of the os calcis and anteriorly with the fourth and fifth metatarsals. On its inner side it articulates with the external cuneiform bone. On the plantar surface is an oblique ridge running from without inwards and forwards, and just in front of this a groove for the tendon of peroneus longus.

Tuber Calcis

Ossification

The astragalus and the os calcis are partially ossified at birth. The other bones commence early in life, and the tarsus is completely ossified by the twentieth year.