At the back part of this region, the fascia is also found covering the great tondo Acltillis ; this tendon also, like the smaller ones we have just spoken of, is not merely covered super ficially, but is contained within a sheath, formed by the splitting of the fascia into two layers : the posterior layer we may regard as the continued fascia itself; the deep one passes in front of the tendon, and if we trace this up wards, we shall find it becoming ultimately continuous with the deep fascia of the leg. An acquaintance with the disposition and structure of the fascia we have thus described, will en able the surgical anatomist, in almost every in stance, to explain the time, situation, and pro gress of abscesses occurring in this region: he will at once comprehend that three sorts of abscess may form here :—one in the subcutaneous tissue, and which being super ficial to the fascia can hardly penetrate deeply toward the joint ; another, occurring between the two layers of that membrane, in those situations where it splits to include the ten dons ; such an abscess will have little tendency to point in front, being bound down by the superficial layer of the fascia, or to penetrate deeply for a similar reason ; but to its free passage upwards or downwards in the course of the tendons, little or no obstacle is presented. Lastly, matter may accumulate under both layers of the fascia, where its deep position and close confinement render it alike dangerous, and of difficult detection.
4. The next stratum is perhaps less entitled to that name than those we have hitherto described. Instead of forming, like them, a general investment for the whole region, it consists of several distinct and independent or gans scattered irregularly about the joint : we shall enumerate them in the order in which we propose to treat of them, viz., tendons, mus cles, arteries, veins, lymphatics, and nerves.
a. Tendons. Upon the instep we find no fewer than seven tendons passing towards the foot : the internal is the largest of all, it is that of the tibialis anticus running obliquely for wards and inwards to the inner cuneiform bone. Close upon its outer side is the tendon of the extensor pollicis ; still more outwards we meet with the four tendons of the extensor digitorum longus, and most externally of all, or nearest to the outer ankle, that of the peroneus tertius. We need not revert to the subject of the fibrous sheaths furnished to these tendons by the fascia or annular ligament ; but we should here care fully observe, that both sheaths and tendons are completely lined by a synovial apparatus. He who is at all acquainted with the general pathd logy of synovial membrane will understand why it is that effusions so frequently form about the instep ; why adhesion of the opposite walls of these synovial sheaths will almost destroy the power of extending the toes and of flexing the foot.; and, lastly, he cannot hut draw the im portant practical deduction, that in operations about the instep we should avoid, if possible, cutting into these synovial sacs.
Behind the inner malleolus we meet with three tendons, — that of the tibialis posticus most anterior, and in close connexion with the posterior surface of the malleolus internus; that of the flexor digitorum longus a little further back; and still more posterior, and at a little distance from the others, the tendon of the flexor pollicis longus. These are included, as we have already explained, in fibrous sheaths formed by the internal annular ligament, each sheath and tendon having its own synovial lining. We may here observe a good anatomical
reason, why inflammation affecting the sheath of the flexor digitorum will, e(eteris paribus, be more likely to prove dangerous than that of the tibialis posticus : for, as the synovial sheaths of the former extend along the whole sole of the foot, little or no obstacle is presented to the disease extending itself into that region : whereas the tendon of the tibialis being inserted, not upon the sole, but rather upon the inner edge of the foot, its synovial membrane forms here a cul-de-sac, no doubt presenting some obsta cle to the inflammation extending beyond this point. Behind the outer malleolus there exists a deep groove, in which two important tendons are contained, those, namely, of the peroneus longus and brevis. They are lodged in a canal which we have already described as formed by the bone and the external annular ligament, and this canal is lined by a distinct synovial membrane reflected upon it from the tendons. Having passed over the ligaments of the outer ankle, the peronei tendons are next applied upon the surface of the os calcis ; and here, though previously in close apposition, and in deed contained within the same synovial sheath, they become separated by a ridge projecting from the bone. The peroneus longus tendon plays behind it as upon a pulley, and instances have occurred, where, owing to the fracture of this little osseous septum, the peroneus longus has been dislocated forwards upon that of the brevis. It has also happened that both peronei tendons have been dislocated forwards from their groove behind the malleolus, and thrown in front of that eminence. Were such an acci dent left without surgical interference, it is inte resting to reflect how completely altered would be the action of these two muscles, if that action were not completely suspended by the inflam mation and obliteration of the synovial sheath consequent on the accident ; instead of extend ing the foot and pointing the toe, as they do in their natural state, they would become con verted into flexors and abductors of the foot. At the posterior part of the region, the tendo Achillis forms a remarkable projection. ,In our account of the fascia, we have described the sheath within which this tendon is contained. We may further observe that this tendon is separated from the joint, and also from the deep vessels and nerves of the leg, by a consi derable interval, so that it has frequently cut across without injury to the articulatio0or wound of any other important part. Its mode of insertion into the os calcis is also worthy attention ; instead of being fixed into the whole posterior surface of that bone, it occupies by its insertion merely the lower half of it ; supe riorly the bone and tendon are not even in con tact, for here a distinct synovial bursa is inter posed between them. The liability of this large bursa to inflammation and effusion should be carefully borne in mind by the surgeon : and he who is aware of its office, placed as a friction roller between the tendon and bone, will duly estimate how much disease of this bursa will impede the motions of progression. Owing to the interposition of the bursa, rupture of the tendo Achillis has occurred even below the upper edge of the os calcis ; and if, having cut across the tendon, we forcibly extend the foot so as to elevate the heel, we shall at once com prehend how indispensably necessary it is to maintain the extended position in our treatment of this important accident.