It is in relation in front with the two ge tnelli and obturator internus, the quadratus fernoris and adductor magnus. Behind these muscles it passes successively from above downwards, is in dose contact with the su perior, and separated from the adductor magnus by a quantity of fat and cellular membrane. The branches given off' frotn the sciatic nerve are muscular and articular. The muscular branches come away above the middle of the thigh, with the occasional ex ception of that for the short head of' the biceps, which arises near the middle.
There are several branches for the long head of the biceps, some of vvhich ascend to be dis tributed to the muscle at its origin; others descend for some distance, and enter its an terior surface.
The nerve for the semi.tendinous is a long delicate filament, which usually passes down to the lower third of the thigh before it enters its surface.
The senzi-nzenzbranosus generally receives two or nzore branches : and from the lower is not unfrequently derived a branch for the ad ductor magnus, which also receives a branch from the main trunk.
The articular nerve is usually given about the middle ; but as this nerve, in the majority of instances, is derived from the peronmal, it will be described with that nerve.
The peronwal nerve (external poplitmal — external poplitmal-sciatic) is more superficial, and not much more than a third the size of the posterior tibial. It is directed down wards and outwards along the inner edge of the biceps muscle, behind the outer condyle of the femur, the outer head of the gas trocnemius, and the outer and back part of the head of the tibia, to below the head of the fibula, where it divides into four branches, the anterior tibial, and musculo-cutazzeous, the former being larger than the latter.
The peronmal nerve, during this course, gives off superficial cutaneous branches, and occasionally deep articular : the former being represented by the peronceal cutaneou,s and peroncal saphwnus, the latter by the superior and inferior external articular.
The peronwal cutaneous proceeds from the back part of the nerve, generally an inch or two after its commencement. Having passed superficially with the trunk as far as its ter mination, and having supplied the integu ments in its course, it gives branches on the one hand to the integuments immediately on the outside of the external saphmnus, and on the other over the upper part of the pero nmus longus, the middle terminal filaments extending below the middle of the leg, and communicating with cutaneous branches from the external saphmnus.
The peronwal saphenus (communicans fibu lm— communicating saphmnus) usually taking its origin above and to the inside of the pe ronmal cutaneous, is directed downwards and inwards beneath the skin, and communicates with a corresponding branch from the pos terior tibial to form the external saphmnus. This communication is very variable as to situ ation, nsually taking place below the middle of the leg, where it perforates the fascia, oc casionally, however in the lower part of the poplitmal space in 'front of the fascia. The nerve now and then runs quite distinct from its corresponding branch, which consequently in these instances entirely constitutes the external saphmtms. It is either very small, terminating about the middle of the leg, or divides opposite the lower part of the tendo Achillis into branches which pass over the lower part of the peronmus longus to the skin of the external malleolus, where they communicate with small descending branches from the musculo-cutaneous ; and into those which supply the skin at the lower and outer part of the heel, communicating in the in terval between the heel and malleolus with branches from the external saphmnus.
The deep articular branches are external and internal, the one arising above the other. They are thus described by Mr. Ellis*: — " The superior external articular nerve, arising either from the trunk of the sciatic or the external poplitmal in the case of a high divi sion of the sciatic, is a long slender nerve, which descends deeply into the poplitmal space, under cover of the biceps muscle, nearly as low as to the outer condy le, then passes from the space beneath the tendon of the biceps, reaches the superior articular artery, which it accompanies to the front of the joint, and supplies the synovial membrane of the articulation.
The inferior external articular, more fre quently a branch of the external poplitmal than of the sciatic, is also a long nerve close to the biceps, and has the same direction as the preceding ; but it extends lower down, passing beneath the tendon of the biceps, and below the condyle of the femur, to the artery of' the same name, and it divides on the outer side of the articulation into many branches that extend forwards, perforate the capsules, and supply the synovial membrane.