The anterior astragalo-calcanien articulation is formed by a slightly convex surface on the astragalus, which is received by a concavity on the upper surface of the sustentaculum of the os ealcis. This articulation is furnished with a synovial membrane, which Is only a prolongation from that of the joint between the astragalus and scaphoid.
The chief bond of union between the as tragalus and os calcis is by means of the intcrosscous ligament (apparatus ligamentosus cavitatis sinuosr, \Veitbr.): this ligament oc cupies the hollow which is manifest on the outside between the os calcis and the neck of the astragalus. It consists of a series of strong ligamentous fibres, which arise all along the inner part of the depression on the astragalus in a curved course, and descend vertically, or nearly so, to be inserted into the corresponding depression between the two articular surfaces on the os calcis. A con siderable quantity of fat occupies this space, and covers this ligament, and is intermixed with its fibres.
The bones forming the second row of the tarsus are articulated as follows :— The scaphoid or navicular bone is articulated with the three cuneiform, by means of the triple surface already described on the former bone ; to each division of which one cuneiform is adapted (runco-sraphoid articulation). A common synovial membrane lines the surface on the scaphoid, the surfaces of the cuneiform bones, and passes in between them to line the lateral articular facets on the latter bones. The three cuneiform bones are connected to the navicular by means of six ligaments, which pass from the former to the latter ; three un the dorsal surface and three on the plantar. The dorsal ligament of the internal cuneiform extends directly from behind forwards, those of the others proceed obliquely forwards and out wards. The internal cuneiform has likewise an internal ligament, which proceeds from its internal part directly backwards to the navi cular; it lies above the tendon of the tibialis posticus. As to the plantar ligaments, that of the internal cuneiform is the strongest : it is extended between the tubercle on the na vicular bone and that on the cuneiform, and is in part confounded with the tendon of the tibialis posticus, which sends a process out wards to the other cuneiform bones, and strengthens the ligamentous fibres which belong to them.
The cuneiform bones are articulated to each other by means of the lateral facets, which are lined by synovial membrane prolonged from that of the cuneo-scaphoid articulation.
Each joint is strengthened by a dorsal, a plantar, and an intcrosseous ligament. The two former are extended transversely from one cuneiform bone to the other, the dorsal being considerably the stronger. The principal bond of union, however, is by the interosseous liga ment, which is extended between the non articular parts of the lateral surfaces of each cuneiform bone.
The cuboid bone is articulated with the external cuneiform (euboido-eunecri articula tion) in a manner so similar to that by which the cuneiform bones are articulated with each other as to render a separate description super fluous. Its synovial membrane is continuous with that of the cuneo-scaphoid, and its liga ments are precisely similar to those of the cuneiform articulations.
The cuboid bone is united to the scaphoid by means of ligaments. The outer extremity of the latter bone is in contact with a small portion of the inner surface of the former, near its posterior superior angle, and sometimes a small articular facet indicates the point of each bone where contact is established. The liga ments which pass between these bones under all circumstances are a dorsal ligament, directed obliquely from without inwards, a plantar ligament, transverse and very thick, and an in tcrosseous ligament extended between the cor responding surfaces of the two bones, excepting where the facets are found, when they exist.
Articulation of the two rows of tarsal bones to each other.—This is effected by means of the astragalus and os calcis behind, and the scaphoid and cuboid in front.
Astrogalo-scaphoid artindation.—The head of the astragalus is received into a cavity which is in greatest part formed by the glenoid cavity of the scaphoid bone, and is completed infe riorly and internally by a ligament (the iqferior calcaueo-scaphoid), which extends from the sustentaculum of the ns calcis to the inner part of the inferior surface of the scaphoid. On the outer side and inferiorly the head of the astragalus is supported by a short ligament (the external caleaneo-scaphoid) which is at tached posteriorly to the inner part of the os calcis, and in front to the external extremity of the scaphoid. The extension of the recipient cavity for the head of the astragalus by means of the ligaments just named was rendered necessary by the considerable excess in the size of the head of the astragalus over the glenoid cavity of the scaphoid. By means of these ligaments, too, the os calcis is connected with the scaphoid, although there is no articulation between them.