ANKLE, JOINT OF THE.—(Normal ana tomy.) (Fr. articulation du coude-pied. Germ. _Fussgelenk. Ital. caviglia.) The ankle-joint, or tibio-tarsal articulation, results from the junc tion of the leg and foot. For reasons which will appear when we come to explain its mo tions, it is 'ranked in the excellent and com prehensive classifications of I3ichat and Cloquet as a perfect angular ginglymus. The security of the ankle-joint, more perhaps than of any other in the body, is owing to the peculiar form of its bones, and to their exact adap tation to each other ; in this respect it has aptly been compared to the tenon and mortise joint, so frequently used by mechanics, the strength of which, as is well known, is chiefly owing to the peculiar form and close fitting of its component parts. Upon the i.ipper part of the foot, we meet with, it is said, a true and well defined tenon, and upon the lower part of the leg a tolerably perfect mortise for the reception of the tenon. The comparison, though perhaps not strictly correct, will however assist us in understanding how much the security of this joint depends upon the form and fitting of its bones ; and will explain to the beginner why, in treating of the ankle-joint in particular with a view to demonstrate its use and mechanism, a brief account of its bones becomes part of our description no less essential than of its liga ments themselves. In our account, therefore, of this articulation, we shall, in the first place, describe its bones; next its ligaments ; and, lastly, shall offer some remarks upon its me chanism and uses.
a. The Bones.—Three bones contribute to the formation of the ankle-joint; the tibia and fibula form, by the union of their inferior por tions, a deep depression, into which the head of the astragalus is received. The tibia, as it ap proaches the joint, looses gradually its prismatic shape, and assumes a well-defined cubical or quadrangular form. On its lower extremity it presents a quadrilateral articulating cavity, covered in the recent state with cartilage ; this cavity is transversed from before backwards by an obtuse ridge which subdivides it into two smaller cavities. Of the four sides or margins
of this articulating cavity, the anterior is almost straight transversely, but convex or rounded off in the vertical direction, with the obvious design of permitting a greater flexion to the foot; the anterior tibio-tarsal ligament arises from this margin. The posterior margin is also straight transversely, but vertically convex, to permit an increased extension to the foot ; the posterior tibio-fibular ligament is connected here : a shallow oblique groove is met with upon the outer part of this surface, for the transmission of the flexor longus pollicis tendon. The ex ternal side presents a depression for the reception of the fibula; this articulating portion is pro longed upwards for nearly an inch, is of a triangular form with the base below ; the sides of the triangle give attachment to the anterior and posterior tibio-fibular ligaments ; and the area of the triangle is rendered rough, except at its lowest part, by the attachment of the inferior interosseous ligament,—another strong bond of union between these bones. The inner edge is prolonged downwards nearly an inch in length, forming the prominence known by the name of malleolus internus; this is placed upon a plane superior and anterior to the malleolus externus; it is somewhat flattened in shape, and has one surface looking inwards or towards the mesial line ; this in the living subject is covered only by the integuments ; the outer surface enters into the formation of the joint; hence it is tipped with cartilage to permit the astragalus to play upon it; the anterior edge is sharp and gives origin to the anterior tibio-tarsal ligament; the posterior edge is traversed by a broad and generally well-marked groove, which transmits the tendons of the tibialis posticus and flexor digitorum longus ; the apex of the malleolus is below, and gives attachment to the deltoid or internal tibio-tarsal ligament.