The amount of movement that takes place between the bones of the foot is not as great as would be expected from their number. It is only in the subastragaloid joint that any considerable motion takes place, while a less amount occurs at the midtarsal joint. The contiguous tarsal bones are joined by numerous band-like, capsular, and interosseous ligaments which allow a limited amount of movement between them. In the aggregate these movements are considerable and make the foot as a whole quite flexible.

Fig. 582. - The subastragaloid joint; lower surface. The astragalus has been removed.

The Subastragaloid Joint (Articulatio Talo-Calcaneo-Navicularis)

This is a horizontal joint formed by the astragalus above and the os calcis and navicular (scaphoid) below and in front. It runs obliquely forward and inward. The astragalus is not wedged in between the os calcis and scaphoid like the keystone of an arch, but the foot moves freely beneath it. It has an inward motion of adduction around an anteroposterior or longitudinal axis with internal rotation around a vertical axis, and an outward motion of abduction with external rotation. The abduction and adduction movements cannot occur independently of rotation, they are combined. The astragalus is joined to the os calcis below and scaphoid below and in front by short fibrous bands which help to form the capsule. The under surface of the sub-astragaloid joint is formed first by the surface of the scaphoid, next by the inferior calca-neoscaphoid ligament, then by the upper surface of the sustentaculum tali, then by the interosseous ligament, and finally by the posterior surface of the os calcis. The inferior calcaneoscaphoid ligament is the most important one in maintaining- the integrity of the arch (Fig. 582). In addition, in order to provide against luxation, which is favored by the superincumbent body weight, the joint is strengthened by three ligaments, viz.:

Fig. 583. - View of inner side of ankle-joint, showing the internal lateral ligament.

1. The interosseotts astragalo-calcaneal ligament, which runs obliquely forward and outward between the os calcis and astragalus and divides the subastragaloid joint into an anterior and posterior portion. It is very strong (Fig. 582).

2. The internal lateral {deltoid) ligament of the ankle, which sends fibres by its deep part from the tibia above to the side of the astragalus below and likewise to the scaphoid in front, and by its superficial part to the sustentaculum tali (Fig. 583). 3. The external lateral ligament of the ankle, the anterior and posterior fasciculi of which are both attached to the astragalus and the middle fasciculus of which goes to the os calcis below (Fig. 584).

Fig. 584. - Ligaments of the outer side of the ankle.

When the weight of the body is transmitted to the foot it tends to flatten the anteroposterior arch. If the arch descends it can only do so either by pushing the astragalus up - luxating it - or by the ligaments of the arch stretching or rupturing and allowing the two pillars of the arch to separate. In disease the ligaments elongate and by violence they may be ruptured, the arch in each case falls. If the ligaments supporting the astragalus remain intact then excessive lateral movement ruptures those on the side and a sprain of the subastragaloid joint is produced which is often called a sprain of the ankle.