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 flexion the astragalus rolls from before back wards in the tibio-fibular mortise; it may be 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 the same. It is no less interesting than instruc tive to contrast these two articulations with each other, for in doing so we find that the modifications of structure here, as well as in all other instances, are referable to the peculiar function which each part is destined to perform. The hand in the human subject is exclusively an organ of prehension ; the foot is one merely of support :—now this simple fact at once fur nishes us with a clue to all difficulties. The great strength and sudden expansion of the tibia and fibula at the ankle, are evidently a provision to sustain the weight of the body and to increase the basis of its support; in the radius and ulna such size and strength would have been to no purpose, and hence these bones at the wrist are comparatively thin and delicate. At the ankle we should naturally have expected frequent dislocations, owing to the great weight from above, and to the great mobility which for the purposes of progression must at the same time necessarily exist here ; these are two most formidable causes of displacement ; but, as if in compensation, we find two strong buttresses (the malleoli) projecting one upon either side of the joint, and rendering such displacement, under ordinary circumstances, almost impossible. At the wrist, where there is no weight to be sup ported, such lateral splints would have been superfluous : hence the imperfect and almost rudimental malleoli of the radius and ulna; hence the shallow and imperfect cavity ; hence, in a word, the anatomical confor mation which constitutes the ankle-joint a ginglymus, and the wrist an arthrodia. In the motions of the ankle and wrist-joints we observe likewise a striking difference: in the former, lateral motion would have been super flous in reference to the function of the foot ; at the wrist, on the contrary, a free lateral mo tion is indispensable to increase the sphere of action of the hand.