The astragalo-scaphoi d articulation is strength ened by but one proper ligament, and that is situated in the dorsal aspect; it is the superior astragalo-seaphoid ligament, and is attached posteriorly to the neck of the astragalus, and in front to the margin of the glenoid cavity; the transverse extent of this ligament is equal to that of the scaphoid bone on its dorsal surface; the direction of its fibres is forwards and out wards. It is a thin fibrous expansion, covered superiorly by the extensor brevis digitorum muscle, and on its inferior surface lined by the synovial membrane of the joint.
Calcanco-cuhoid artieulatian.—The articular surface on the os caleis is slightly concave in the direction from above downwards; that on the cuboid is convex in the same direction. The two surfaces are closely adapted to each other, and their union maintained by the fol lowing ligaments The superior or dorsal calcanco-ruboid ligament, which consists of but a few fibres extending from the superior and anterior part of the as calcis to the cuboid. 2. 'rile internal ealruneo-cuboid ligament, a short, strong, quadrilateral ligament from three to four lines in breadth, placed in great part over the superior aspect of the joint; the fibres pass with a slight obliquity inwards from the os calcis to the cuboid. 3. The plantar or inferior caleaneo-cuboid ligament, the strongest and largest of the foot ligaments, seems destined not alone for the articulation under considera tion, but also to strengthen the arch of the tarsus generally on its plantar surface. It is attached behind to the inferior surface of the os caleis, commencing from the angular depres sion between the two tubercles. After leaving the os caleis a distinction between its superficial and deep fibres becomes very manifest; the former proceed forwards and inwards, pass under the cuboid bone, forming an adhesion to the posterior lip of its groove, then pass under that groove and its contained tendon, and are ultimately inserted into the posterior extremities of the third and fourth metatarsal bones. The deep fibres diverge immediately after they have left the os calcis, and are inserted into the whole inferior surface of the cuboid posterior to the groove.
It will be observed that the two joints last described lie beside each other in the same line, a circumstance which favours the surgical operation of partial amputation of the foot in that line. Each joint, however, has its proper synovial membrane lining the cartilaginous incrustations of the bones and the articular surfaces of the ligaments ; that of the astragalo scaphoid is the more lax, and indicates the existence of a considerable range of motion in that joint.
Motions of the tarsal joints. --All these joints belong to the class Arthrodia, some of them being planiform. The motion in all is that of simple gliding, limited by the strength, number, and position of the ligaments. The close inspection of the bones of the meta tarsal row, and the firm ligamentous bands which pass between them, occasion a very limited mobility of the bones of that row. Between the astragalus and os caleis, on the other hand, the motions are much more mani fest; these are gliding motions in the direction from before backwards and vice versa, or from side to side. When the foot is turned inwards or outwards the latter motion is called into play, and the gliding in the antero-posterior direction takes place when the weight of the body presses on the foot, causing its elongation and the diminution of the curvature of its antero-posterior arch. Mien the weight presses, the astragalus glides forward upon the os calcis; when the weight is removed, the bone returns to its firmer condition by gliding backwards.
But the greatest mobility exists in the articu lation between the two rows of tarsal bones. There, indeed, the principal motions of the tarsus take place. The motions of the foot, which many have erroneously attributed to a supposed power of lateral motion in the ankle joint, really take place in this line of articula tion. When the foot is turned so that its sole is directed outwards, the scaphoid glides from above downwards on the head of the astragalus, the astragalus glides from within outwards on the os ealeis, in consequence of which the hollow space between the last-named bone and the neck of the astragalus is diminished, the interosseous ligament relaxed, the external lateral ligament of the ankle-juint likewise relaxed, and the internal lateral ligament ren dered tense. On the other hand, when the sole of the foot is turned inwards, which may be done much more completely than in the opposite direction, the scaphoid glides from below upwards upon the head of the astragalus, the inferior surface of the os caleis is turned inwards, the astragalus glides upon the last named bone from without inwards, enlarging the interosseous space, stretching the ligament which occupies that space, and also rendering tense the external lateral ligaments of the ankle joint. It is therefore natural to expect, as Bichat has remarked, that in those sprains which result from too great inversion or eversion of the foot, the ligaments of the articulations between the tarsal rows should suffer most.