The Ankle-Joint, between the upper, inner, and outer surfaces of the astragalus and the lower ends of the tibia and fibula.

It is a hinge joint, capable of movement round one axis only :

Transverse-flexion and extension.

Owing to the shape of the articular surface of the astragalus, which is wider in front than behind, a slight side to side rocking movement can be produced during extension, as in this position the narrower portion of the astragalus is in the tibio-fibular socket. During flexion the wider part of the astragalus is forced into the socket, and the joint is, so to speak, "locked," and inversion and eversion of the foot can only be performed by outside force. This movement takes place in the joint between the cuboid and os calcis, and between the astragalus and scaphoid, not at all in the ankle-joint itself.

A capsule completely surrounds the joint, and has various strengthened parts.

The Anterior ligament is a thin ligament attached above to the anterior edge of the tibia, and below to the upper border of the head of the astragalus. A pad of fat fills the hollow between.

Fig. 22. - Ankle-Joint.

The Posterior ligament is attached to the edges of the tibia and astragalus, and receives some fibres from the external malleolus. The transverse ligament of the inferior tibio-fibular joint strengthens this aspect of the ankle-joint. The ligament on the external aspect of the joint is divided into three parts :

1. Anterior Part of the ligament from the anterior border of the external malleolus to the astragalus in front of the articular surface.

2. Middle Part of the ligament from the lower border of the external malleolus to the external surface of the os calcis just above the peroneal spine; this is a strong rounded cord.

3. Posterior Part of the ligament from the fossa on the inner surface of the external malleolus to the external tubercle on the posterior surface of the astragalus; this is the strongest of the three parts.

The Deltoid ligament is the one on the inner aspect of the joint; it is triangular in shape and attached by its apex to the lower part of the internal malleolus, and by its base to the inner surfaces of the scaphoid, astragalus, and os calcis, in one continuous line.

The synovial membrane lines the joint and is continued up into the inferior tibio-fibular joint. Pads of fat are found in front and behind the joint, and in the socket where the three bones meet.

Intertarsal Joints

The Intertarsal Joints are all gliding joints. They are surrounded by capsules which are divided into definite bands of fibres, as in the intercarpal joints. There are, however, three ligaments of special importance, as they are mainly responsible for supporting the longitudinal arch of the foot.

The Inferior Calcaneo-Navicular or Spring ligament is an extremely strong band of fibro-cartilage. It is attached by one end to the sustentaculum tali of the os calcis, and by the other to the plantar surface of the scaphoid. Some of its fibres radiate upwards to join the deltoid ligament.

The Inferior Calcaneo-Cuboid ligaments are two in number; the superficial or long plantar ligament is attached by one end to the plantar surface of the os calcis in front of the tuberosities, and by the other to the ridge of the cuboid and passing over the groove to the bases of the third, fourth and fifth metatarsals.

The deep or short plantar ligament is attached to the front of the plantar surface of the os calcis, and to the plantar surface of the cuboid just behind the ridge; this is a short band of great strength.

The tarso-metatarsal joints, intermetatarsal joints, metatarsophalangeal joints, and interphalangeal joints all resemble the corresponding joints in the hand; but the movement is very limited, as the foot is for the purpose of supporting the weight of the body, and strength rather than flexibility has to be considered. The toes can be spread to a certain extent, constituting abduction and adduction at the metatarso-phalangeal joints. This movement takes place about a line drawn through the second toe, not the middle one. Inversion - the movement of raising the inner border of the foot-and eversion-raising the outer border of the foot - has already been described. This can be done by outside force when the ankle is flexed, or voluntarily when the foot is on the ground.