2. The subcutaneous cellular tissue.—The structure and properties of the subcutaneous cellular tissue are not the same throughout the whole region, but like the skin, which we have just considered, its characters vary in dif ferent situations. Thus, upon the instep, it is at the upper part loose and distensible, full of adipose cells, and similar in every respect to the subcutaneous tissue of the leg, of which it is a prolongation : as it descends, however, it bec6mes more dense and unyielding, and ad heres more closely to the skin which covers, and to the annular ligament which is placed beneath it. This anatomical fact at once ex plains why it is that when subcutaneous ab scess or infiltration occurs on the anterior part of the leg or foot, passage of the fluid either upwards or downwards is, for a time at least, impeded at the ankle-joint. It is like wise owing to the density of the subcutaneous tissue across the ankle, that its cells do not permit the accumulation of adipose substance here ; hence in very fat persons and also in children whose subcutaneous fat is usually abundant upon the leg and foot, the instep is as it were strangulated by a deep transverse furrow. Upon the malleoli the characters of the subcutaneous tissue present great differ ences : upon the inner one it is scanty and delicate, but of a compact structure, and con tains few if any adipose cells. Upon the outer one it is, on the contrary, much more copious, Of a loose and yielding texture, and usually contains a greater quantity of fat. These dif ferences of texture will explain why, after severe contusion, extravasations so frequently occur upon the outer part of the joint and so seldom upon the inner; why abscess is so much oftener met with in the one situation than in the other ; and why the transmission of pus and serum from any of the neighbour ing regions takes place so much more easily about the outer than about the inner ankle. At the posterior part of the region, the sub cutaneous tissue assumes again new characters : losing here its soft lamellated texture it be comes suddenly dense and filamentous, ad hering with great firmness to the integuments above, and to the fascia beneath: as we trace it down it becomes more dense and elastic ; the cells formed by the decussation of its filaments become loaded with a firm granular fat ; in a word, it already begins to put on the characters of the dense fibro-adipose cushion, which is found in the sole of the foot. Hence it is that wounds and abscesses of the part we are now considering, approach in character those of the plantar region : hence the slight swelling, the severe pain ; hence in both cases the necessity of a prompt and free evacuation of the matter.
Before leaving this subject we should ob serve that the subcutaneous tissue of the region we are now considering transmits certain ves sels and nerves. In front of the inner ankle we meet with the incipient branches of the great saphena vein and the ultimate filaments of the saphenus nerve : the venous branches are here of such a size that they have fre quently been selected by the phlebotomist as the seat of operation. Anteriorly we find the filaments of the musculo-cutaneous nerve, and externally the roots of the lesser saphena vein, and its accompanying nervous filaments.
3., The fascia or aponeurosis forths the next stratum we have to examine: it is placed be tween the subcutaneous tissue and the tendons. The fascia, like the two preceding layers, forms a general investment for the whole region.
Its structure and properties, like those of the preceding layers, vary considerably, according to the situation in which we view it. Upon the instep it becomes continuous, above with the aponeurosis of the leg, and inferiorly with the dorsal aponeurosis of the foot, but, for very obvious reasons, surpassing both of these in strength. This additional strength is owing to the accessory band of fibres which passes transversely across the instep, interlaced with the proper oblique fibres of the fascia, and to which is given the name of anterior annular ligament. Arising from the anterior edge of the inner ankle this annular ligament passes outwards and soon meets with the ten don of the tibialis anticus : at this point it splits into two layers ; the one passes before, the other behind the tendon, and they unite again at its outer edge. The same mechanism is repeated in the case of the extensor pollicis tendon which lies immediately external to the last-named tendon ; and lastly in those of the extensor digitorum longus and peroneus tertius. In contemplating the mechanism and uses of this ligament, the surgical anatomist cannot but perceive that certain inconveniences must result from its division : its use being obviously to bind down the tendons in this situation, and to form canals for their free and separate trans mission, it is clear that after its division in the living subject, when the individual attempts to flex the foot or extend the toes, these tendons will not only form an unseemly projection upon the instep, but also the accuracy and per fection of these motions will be much im paired. Upon the lateral parts of the region, the fascia is so intimately united to the peri osteum, that it is almost impossible to separate them from each other, and hence some have denied its existence here. Behind both mal leoli, it becomes however again very distinct, forming in both situations a band similar to that which we have just seen upon the instep. The internal annular ligament arising from the posterior edge of the inner malleolus passes backwards to the os calcis; it is thrown like a bridge across that deep gutter which divides the heel and ankle from each other, and it is destined like the anterior liga ment to form a covering to the tendons and other parts which pass through this m:ion. Like the anterior, the internal ligament also consists of two layers closely united to each other. To express more distinctly the me chanical disposition of these layers, we may say that the bridge formed by the internal annular ligament consists of two arches ; through the anterior arch are transmitted the tibialis posticus and the flexor digitorum longus tendons, wrapped each in its own synovial theca : the posterior arch is occupied with the posterior tibial vessels and nerves, and the tendon of the flexor longus pollicis muscle. Having thus safely conducted these important organs, the superficial layer of the ligament fixes itself into the os calcis, while the deep one passes backwards and upwards to become continuous with the deep fascia of the leg. Behind the malleolus, the fascia forms another but less remarkable liga ment, which Blandin calls the " external an nular :" this passes from the fibula to the astragalus, and forms with the posterior edge of the malleolus a deep osseo-fibrous canal for the transmission of the peroneus longus and brevis tendons.