The fibula, as it approaches the foot, becomes suddenly enlarged in size, applies itself firmly to the tibia, and then descends nearly an inch and a half below its point of union with that bone. The prominence formed by the fibula in this situation is named the malleolus externus ; it is much larger than the internal, and placed behind and somewhat below it. The external surface of this fibular malleolus is covered merely by the integuments ; the internal surface is tipped with cartilage, and convex in the ver tical direction, being received upon a corres ponding concavity on the outer side of the astragalus; upon the lower and back part of this inner surface may be seen a deep depres sion, where the posterior fibulo-tarsal ligament arises ; the anterior edge of the malleolus is sharp, and gives origin to the anterior fibulo tarsal ligament ; the posterior edge is marked by a deep groove, which transmits the tendons of the peronei muscles, longus and brevis. The apex of the malleolus is below, and gives origin to the middle fibulo-tarsal ligament.
The astragalus enters into the formation of the ankle-joint by its superior surface, and a portion of its two lateral surfaces. On the superior surface we observe, anteriorly, a well marked groove forming part of the neck of the astragalus ; into this groove the anterior tibio tarsal ligament is inserted. Immediately be hind the groove we meet with an articulating eminence of an oblong quadrilateral form, an inch and a half in its antero-posterior, and about an inch and a quarter in its transverse measure ment ; (this transverse measurement is, however, a little greater in front than behind ;) the emi nence is remarkably convex from before back wards, and concave from side to side; the outer edge elevated than the inner ; it is col pletely covered with cartilage, and cor responds to the articulating cavity upon the in ferior exremity of the tibia. Upon the inner side of the astragalus, we find a small articu lating surface of a triangular form, with the base above and below ; it is convex in the vertical direction, and is tipped with car tilage prolonged ftom the superior surface : upon thetriangular surface the internal malleolus plays ; the remaining portion of the inner side of the astragalus is rough, and occupied chiefly by the insertion of the internal tibio-tarsal ligament. The external side of the astragalus is also marked by an articulating surface of a much greater size for the reception of the ex ternal malleolus : it too is of a triangular form with the base above ; concave in the vertical, and slightly convex in the antero-posterior direction.
b. Ligaments.—We have already compared the mechanism of this joint to that of the tenon and mortise ; the mortise cavity, however, is not, as we have seen, cut out of a solid bone, but being formed in great part in the lower extremity of the tibia, is completed on the outer side by the fibula, which is firmly united with the tibia by strong ligaments, forming what is called the inferior tibia-fibular articulation. We shall not now describe the ligaments which here unite the tibia and fibula, referring to the article on the AlIFICULATION ; but we must observe that, however it may be advisable, in anatomical descriptions, to separate this last named articulation from the ankle-joint, they are perfectly inseparable in their functions, the integrity of the latter being essentially dependent on that of the former : indeed it may be said, that, by virtue of the great strength of the liga mentous connexion between the tibia and fibula in the former articulation, the mortise is as strong, nay, in some respects stronger, than if it had been formed out of solid bone.
The ligaments which connect the tenon and mortise together, or to speak more literally, which tie the tibia and fibula with the tarsus, are five in number, namely, two tibio-tarsal and three fibulo-tarsal ligaments.
1. The internal tibio-tarsal ligament is also called the internal lateral, and by Weitbrecht the deltoid ligament. There is, however, no reason why we should not apply to it likewise that principle of nomenclature which is so gene rally and with such advantage applied to other ligaments. It arises by a truncated apex from the point of the inner malleolus, and from the little fossa at its outer surface ; its fibres change as they proceed downwards and are fixed into the inner surface of the a.stragalus and os calcis, some proceeding as far forwards even as the scaphoid bone. The posterior fibres are strong
but short ; the anterior are much larger and not so thick. Its internal surface is lined by the synovial membrane of the joint; and on its internal surface it is covered by the tendon of the tibialis posticus, and it sends some of its fibres to the sheath of the flexor longus digitorum tendOn. In flexion of the leg the anterior fibres are relaxed, and the posterior are rendered tense : in extension the reverse of course takes place. 2. The anterior tibio tarsal ligament (lig. tibio-tarsal, Cloquet) con sists of a few loose fibres scattered over the synovial membrane, and in some instances so delicate and so separated by pellicles of fat as to be scarcely perceptible. They arise from the fore part of the inner malleolus and the adjacent anterior portion of the tibia, and de scend obliquely downwards and outwards to he inserted into the neck of the astragalus. This ligament is covered anteriorly by the ten dons of the tibialis anticus, extensor proprius pollicis, and extensor digitorum longus poste riorly it is in contact with the synovial mem brane. 3. The anterior fibulo-tarsal ligament (lig. fibula anterius, Weitb., anterior external lateral, Boyer) arises from the anterior edge of the outer inalleolus, a few lines from its ex tremity; it descends obliquely forwards and inwards, and is fixed into the astragalus imme diately in front of the articulating surface which receives the fibula : it is scarcely an inch in length, of an oblong quadrilateral form, and is frequently subdivided into two distinct parts. In extension of the foot it is rendered tense ; in flexion it is relaxed. 4. The middle fibulo tarsal ligament (lig. fibulae medium perpen diculare, Weitb., external lateral ligament, Clog.) is a round fasciculus of fibres having almost the appearance of a tendon which arises from the apex of the external malleolus, de scends obliquely backwards, and is attached to the outside of the os calcis. It does not appear to us that in any position of the joint this ligament takes a perpendicular course, although that epithet has been applied to it by Weitbrecht. It is related superficially to the peroneus longus tendon, and by its deep sur face to the synovial membrane, to the astra gains, and os calcis. I ti flexion of the foot this ligament is rendered tense ; hence it appears designed to limit motion in this direction : in extension it is of course relaxed. 5. The pos terior fibulo-tarsalligament(lig.fibule e posterius, Weitb., posterior external lateral, Boyer) arises from the little fossa upon the inner and back part of the outer malleolus ; it passes backwards and inwards almost horizontally, or at least de scends very slightly, and is inserted upon the back part of the astragalus into the outer edge of that groove which transmits the flexor longus pollicis tendon. This ligament is stronger than either of the two preceding, and is frequently divided into several distinct. fasciculi. From its superior an accessory hand sometimes passes upwards and inwards over the synovial capsule to be fixed into the tibia. Walther has described this band under the name of the oblique ligament, and it is well represented by Weitbrecht (fig. 65, tab. xxii.) The synoviul membrane of the ankle-joint is of very great extent : it lines not only the articular surface of each malleolus, the several ligaments we have just described, and the articulating cavity upon the lower portion of the tibia, but it is prolonged upwards between the tibia and fibula, forming in that situation a little cul-de-sac : this, however, is merely for the extent of a few lines, for its further progress up wards is interrupted by the inferior interosseous ligament, (jig. 61.) From the circumference of the tibio-fibular mortise the synovial mem brane passes downwards upon the astragalus, covers its superior articulating eminence, and sends prolongations upon its lateral articulating surfaces. It is remarkably loose upon the anterior and posterior parts of the joint, and is said to contain a greater quantity of synovia than any other synovial membrane in the body. Certainly its strength is much increased by those scattered fibres to which we have given the name of anterior tibio-tarsal ligament : posteriorly it is weakest, for here few if any ligamentous fibres can be detected, though Boyer and Weitbrecht speak confidently of such.