The anterior tibial nerve (interosseous nerve), rather larger than the musculo-cuta neous, passes from beneath the extensor corn munis digitorum, having previously perforated the deep surface of the peronieus longus, to the interosseous membrane, which it crosses obliquely downwards, forwards, and inwards ; and a little below the middle of the leg is placed in front of the corresponding artery. It continues to accompany the vessel beneath the annular ligament, passing first to the in side of it, then to the outside, and again to its inside, while behind the annular ligament it divides into an internal and external terminal branch. The nerve in this course is placed first between the tibialis anticus and extensor communis digitorum ; then between the former and the extensor proprius pollicis, and lastly between the extensor pollicis and the extensor communis digitorum. In its course from the leg to the ankle the anterior tibial gives off branches to the different muscles between which it passes ; and also one or two delicate satellite filaments to the anterior tibial ves sels.
The terminal branches are both rather deeply seated. The internal deep branch, the continuation of the trunk in reference to di rection, but not to size, being smaller than the external, passes beneath the dorsal artery of the foot and the tendon of the extensor brevis destined for the great toe, gives fila ments to supply the inner part of this muscle, and reaches the first interosseous space, send ing a few twigs to the first interosseous muscle. At the anterior part of this space it communi cates with the musculo-cutaneous, and ter minates by dividing into two branches destined for the opposed sides of the first and second toes.
The external deep branch passes obliquely outwards beneath the exterior brevis, supplies this muscle, and gives off from its anterior part several delicate filaments, which running close to the tarsus reach the three outer interosseous spaces, and expand in the sub stance of the interosseous muscles.
The nerve (the external peromeal), commencing its course below and behind the anterior tibial, and running more superficial and external than it, is directed, first obliquely then vertically downwards in the substance of the peronmus longus ; it is then situated behind the fascia, and at a vari able distance from the ankle, generally at the lower third, perforates the fascia, between the extensor communis, and peronus tertius. Subcutaneous in the remainder of its extent, it follows die course of the extensor com munis, and after runnirn, for a greater or less distance parallel to it, divides into an internal and external branch which diverge consider ably from each other. 'Fhis bifurcation is sub ject to variation, taking place gometimes while the nerve is situated behind the fascia, at others over or very near the annular liga ment, and occasionally the two divisions re unite over the annular ligament, and form an irregular oval space between them. While passing deeply between the muscles of the leg this nerve sends two filaments to the peronmus longus, the inferior of which, given off about the upper fourth of the leg, can be traced running in the subtance of the muscle, to within two or three inches of the ankle. The upper part of the peronmus brevis
also receives a small branch. Shortly after perforating the fascia, the musculo-cutaneous sends off its nzalleolar branches directed down wards and outwards to the skin over the outer ankle, and anastomosing with cutaneous branches either from the external saphmnus, or the termination of the peronmal cutaneous.
The internal terminal branch, passing over the annular ligament giving a few branches to it, and some to communicate with the internal saphxnus and anterior tibial, is di rected along the inner border of the foot to the inside of the great toe as far as its ex tremity. The external branch, having passed over the annular ligament, divides into three branches which are dIrected along the three outer interosseous spaces, and near their an terior extremities, each branch again sub divides into two filaments supplying the op posed sides of the four outer toes, the most external filament anastomosing with the ex ternal saphmnus. Both terminal branches, in their course from the annular ligament to the toes, send otf numerous filaments to the skin on the dorsum of the foot. Such is the usual distribution of tbe musculo-cutaneous nerve ; but frequently the outer branch does not sup ply the inner side of the little toe, and occa Gionally gives filaments only to the opposed sides of the second and third toes. In these instances an extension of the external sa phmnus nerves compensates for the deficiency.
The tibial nerve (tibial-sciatic, internal po plitwal) much larger than the peronmal or external poplitmal, is in a direct line with the sciatic nerve. It passes through the centre of the poplitmal space, rather nearer the semi membranous than the biceps, then between the two heads of the gastrocnemius to the lower border of the poplitxus. It perforates the tendinous arch of the solmus, reaches the front of that muscle, and passes down the leg between it on the one hand and the deep seated muscles on the other. At the lower third of the leg it runs from beneath the inner border of the solmus, and continues its ter minal superficial course, anterior and internal to the tendo Acbillis, as far as the lower extra. mity of the tibia, and, on a level with the base of the external malleolus, divides into the internal and external plantar. In the upper part of the poplitmal span, the tibial nerve is superficial and external to the po plitwal vessels in the middle immediately be hind, and at the lower part is placed internal to them. This last relation the nerve holds as far as the lower third of the leg, when it crosses the posterior tibial artery again to its outer side. It continues very gradually to separate from the vessel ; so that in the in terval between the heel and malleolus the nerve is a quarter of an inch nearer the os calcis than the vessel. The branches given off' from the tibial are muscular, articular, and cutaneous.