JOINT Ol' THE ANKLE. 



153 



ligament is rendered tense ; hence it appears 

 designed to limit motion in this direction : in 

 extension it is of course relaxed. 5. The pos- 

 teriorfibulo-tarsal ligament^ lig. fibula poster ius, 

 Weitb., poster ior 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. Erom 

 its superior edge an accessory band 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 tynavitH 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, (fig. 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. 



c. Mechanism and function of the ankle-joint. 



-To understand properly the mechanism and 



function of the ankle-joint, we must carefully 



contemplate it in the opposite conditions of rest 



and motion. 



1. Viewing it, then, in the first place, as the 

 individual stands at rest, we observe that the 

 leg and foot meet each other in the ankle-joint 

 at a right angle, and we are particularly struck 

 with this fact upon finding that this disposition 

 occurs in scarcely any other animal than man. 

 This interesting fact in comparative anatomy is 

 by no means an accidental arrangement; its 

 design is obviously in reference to the proper 

 position of the body in each animal. It has, 

 for instance, frequently been alluded to as one 

 of the many anatomical proofs that the erect 

 position is natural to the human subject : had 

 the leg and foot been articulated at any other 

 than a right angle the upright position of the 

 body could not be maintained, at least without 

 great and incessant muscular exertion. Another 



point worthy of our attention is that when the 

 ankle is at rest and the body in the upright 

 position, the fibula plays no part in the func- 

 tion performed by the joint: it is the tibia 

 alone which receives the weight of the body, 

 and transmits it to the astragalus. This fact 

 should be carefully borne in mind, for it has 

 considerable influence upon the accidents so 

 frequently occurring here. The astragalus, from 

 the way in which it supports the body, has often 

 been compared to the key-stone of an arch, the 

 arch being represented by the foot. That the 

 foot presents an arched concavity at its lower 

 part cannot be doubted ; but it is by no means 

 so certain that this is designed upon the prin- 

 ciple of the architectural arch to support the 

 weight of the body : in fact, the astragalus, 

 which receives the entire weight, does not cor- 

 respond to the centre of this arch. The true 

 design of the vaulted form of the foot is to 

 permit its accommodating itself to the several 

 irregularities of surface which, both in standing 

 and progression, it must encounter. 



The motions of flexion and extension are the 

 only ones permitted at the ankle-joint. In 

 Jlexion the astragalus rolls from before back- 

 wards in the tibio-fibular mortise; it maybe 

 continued until the foot and leg form with 

 each other an angle of about sixty degrees ; 

 at this point further flexion is prevented, partly 

 by the tension of the middle fibulo-tarsal liga- 

 ment, and still more effectually by the neck of 

 the astragalus coming into contact with the 

 lower edge of the tibia. In flexion the anterior 

 tibio-tarsal and fibulo-tarsal ligaments are both 

 relaxed ; the posterior and middle fibulo-tarsal 

 are rendered tense ; the internal tibio-tarsal 

 ligament has its posterior fibres stretched and 

 its anterior ones loosened. 2. In extension 

 the foot not only returns to its rectangular posi- 

 tion with the leg, but may even be carried 

 beyond this, so as to form with the tibia an 

 obtuse angle of about one hundred and fifty 

 degrees.* Further extension is at this point 

 prevented by the tension of the ligaments which 

 lie in front, and also by the astragalus behind 

 coming into contact with the lower edge of the 

 tibia. During extension the astragalus rotates 

 forwards in the tibio-fibular mortise ; the pos- 

 terior ligaments are relaxed, the anterior are put 

 upon the stretch, the state of each individual 

 ligament is, in short, reversed from what we 

 have just described as its condition in the 

 opposite motion of the joint. 3. A slight degree 

 of lateral motion of the ankle is perceptible in 

 the dead subject, but during life it cannot be 

 said to exist : hence, in the classification of 

 Cloquet and Bichat, the joint is properly 

 ranked under that variety of ginglymus to which 

 we apply the term "perfect." 



The ankle is the analogue of the wrist-joint 

 in the superior extremity, and accordingly, 

 though there are certain points of difference 

 between them, the general character of both is 



* According to Hildebrandt the angle of flexion 

 is 45, and the angle of extension according to 

 Rosenthal (Handb. der Chir. Anat.) is 175.- ED. 



