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series. Each set comprises one of the two essential terminal parts of the sciatic— peroneal and tibial nerves-and numerous smaller collateral branches.
It has already been shown how the sciatic nerve is formed. It comprises the two main nerves of the sacral plexus, bound together by an investing sheath, which contains, in addition to the common peroneal and tibial nerves, a subordinate branch of each, the nerve to the hamstring muscles, from the tibial, and the nerve to the short head of the biceps femoris, from the peroneal nerve. A thick band about half an inch in breadth is formed, consisting, from medial to lateral side, of (1) nerves to the hamstring muscles, (2) tibial, (3) common peroneal, (4) nerve to the short head of the biceps muscle. The sciatic nerve extends through the buttock into the back of the thigh. Forming a continuation of the sacral plexus, it enters the buttock by passing through the greater sciatic foramen, in the interval between the piriformis and superior gemellus. Concealed by the gluteus maximus muscle, it passes distally to the thigh, accompanied by the inferior gluteal artery, and the arteria comitans nervi ischiadici. It lies in the hollow between the greater trochanter of the femur and the tuberosity of the ischium, and enters the thigh beneath the fold of the nates at the lower border of the gluteus maximus. At that spot it is comparatively superficial, lying in the angle between the edge of the gluteus maximus above and laterally, and the origins of the hamstring muscles medially. In the thigh it is placed upon the adductor magnus, anterior to the hamstring muscles, and it terminates at a variable point by dividing into the tibial and common peroneal nerves. As already stated, these two nerves may be separate from their origins, and their separation may occur at any point between the greater sciatic foramen and the proximal part of the popliteal fossa.
THE NERVES OF DISTRIBUTION FROM THE SACRAL PLEXUS.
These are divisible into two series-collateral and terminal branches. Each subdivision consists of a series of anterior and posterior trunks.
1. Collateral Branches.-The anterior branches are (a) muscular branches (to the quadratus femoris, gemelli, obturator internus, and hamstring muscles); and (b) articular branches (to the hip-joint). These nerves all arise from the anterior aspect of the sacral plexus.
The nerve to the quadratus femoris (and inferior gemellus) arises from the front of the fourth and fifth lumbar and first sacral nerves. It passes downwards over the back of the capsule of the hip-joint (to which it sends a fine branch) beneath the sacral plexus, gemelli, and obturator internus muscles. It supplies a nerve to the inferior gemellus, and terminates in the deep surface of the quadratus femoris. The nerve to the obturator internus (and superior gemellus) arises from the anterior aspect of the fifth lumbar and first two sacral nerves. In the buttock it lies medial to the sciatic nerve on the lateral side of the pudendal vessels; crossing the ischial spine, it enters the ischio-rectal fossa through the lesser sciatic foramen. The nerve supplies, in the buttock, a branch to the superior gemellus, and terminates by entering the pelvic surface of the obturator internus.
The nerve to the hamstring muscles forms the most medial part of the sciatic trunk in the lower part of the buttock. It arises from all the roots of the tibial nerve on their anterior aspect, viz., from the fourth and fifth lumbar and
the first three sacral nerves. These roots unite to form a cord which is closely associated with the tibial nerve and is placed in front of it and afterwards on its medial side. Extending into the thigh, the trunk is distributed to the hamstring muscles by means of two sets of branches. Just distal to the ischial tuberosity a proximal set of nerves enters the proximal part of the semitendinosus and the ischial head of the biceps. More distally in the thigh the remaining portion of the nerve separates off from the tibial part of the sciatic trunk and supplies branches to the semimembranosus, the distal part of the semitendinosus, and the adductor magnus.
Articular branches for the hip-joint arise from the nerve to the quadratus femoris, and often directly from the anterior surface of the tibial part of the sciatic nerve near its origin. They enter the back of the capsule of the joint in the region of the buttock.
The posterior branches are: (a) muscular branches-a nerve to the piriformis, the superior gluteal nerve, the inferior gluteal nerve, and a nerve to the short head of the biceps; (b) articular branches (to the knee-joint).
These nerves all arise from the posterior aspect of those roots of the sacral plexus, which are associated with the origin of the common peroneal nerve.
The nerve to the piriformis muscle may be double. It arises from the back of the second, or first and second sacral nerves, and at once enters the anterior surface of the muscle.
N. Glutæus Superior.-The superior gluteal nerve arises from the posterior surface of the fourth and fifth lumbar and first sacral nerves, and is directed backwards and laterally into the buttock, above the piriformis muscle, along with the superior gluteal artery. Under cover of the gluteus maximus and gluteus medius, it passes over the gluteus minimus, along with the inferior branch of the deep division of the superior gluteal artery, to the deep surface of the tensor fascia latæ, in which it ends. On its way it supplies branches to the gluteus medius and gluteus minimus muscles.
N. Gluteus Inferior. The inferior gluteal nerve arises from the posterior surface of the fifth lumbar and first two sacral nerves. It appears in the buttock at the lower border of the piriformis muscle, superficial to the sciatic nerve, and at once breaks up into a number of branches for the supply of the gluteus maximus. In its course in the buttock it is closely associated with the posterior cutaneous nerve of the thigh. Its origin is sometimes combined with that of the following
The nerve to the short head of the biceps springs from the lateral side of the common peroneal trunk in the proximal part of the thigh. When traced to its origin, it is found to arise (sometimes in combination with the inferior gluteal nerve) from the fifth lumbar and first two sacral nerves. In its course it is closely applied to the common peroneal nerve, from which it separates in the middle third of the thigh, usually in combination with the articular branches of that nerve for the knee-joint. In some cases it has an independent course in the thigh, and it may be associated in the buttock with the inferior gluteal nerve.
An articular branch for the lateral and anterior aspects of the knee-joint generally arises from the common peroneal nerve in common with the nerve to the short head of the biceps. When traced up to the plexus, it is found to arise from the posterior surface of the fourth and fifth lumbar and first sacral nerves. It passes through the proximal part of the popliteal fossa concealed by the biceps muscle, and separates into proximal and distal branches, which accompany the superior and inferior lateral articular arteries to the knee-joint.
Terminal Branches.-The common peroneal (O.T. external popliteal) and tibial (O.T. internal popliteal) nerves are the two main trunks resulting from the combination of the posterior and anterior cords of the sacral plexus respectively. The common peroneal nerve is homologous with the radial nerve of the upper limb; the tibial nerve represents a medio-ulnar trunk; and, as already stated, the two nerves, constituting the sciatic nerve, are enveloped in a common sheath for a variable distance before pursuing an independent course in the leg.
NERVUS PERONEUS COMMUNIS.
The common peroneal (O.T. external popliteal) nerve arises from the posterior part of the sacral plexus from the fourth and fifth lumbar and first two sacral nerves. Incorporated with the sciatic nerve in the buttock and proximal part of the thigh, it passes distally from the bifurcation of that nerve through the popliteal fossa, to its termination at a point about an inch distal to the head of the fibula. It is concealed at first by the biceps muscle. Following the tendon of that muscle, it passes obliquely through the proximal and lateral part of the popliteal fossa and over the lateral head of the gastrocnemius muscle to the posterior aspect of the head of the fibula. In the distal part of its course it is quite superficial, but at its termination it is covered by the peronæus longus muscle.
Collateral Branches.-These are divided into two sets: (a) Nerves arising from the roots or trunk of the nerve while it is in combination with the tibial nerve in the sciatic trunk. These have been already described, as a muscular branch to the short head of the biceps, and an articular branch to the knee-joint. (b) Nerves arising in the popliteal fossa. These are cutaneous branches, viz., the lateral sural nerve or lateral cutaneous nerve of the calf and the peroneal anastomotic ramus.
N. Cutaneus Suræ Lateralis.-The lateral sural branch is irregular in size and distribution, and may be represented by two or more branches (Fig. 628, p. 731), Arising from the common peroneal nerve in the popliteal fossa, often in common with the succeeding nerve, it pierces the deep fascia over the lateral head of the gastrocnemius, and is distributed to the skin on the lateral aspect of the back of the leg in the proximal two-thirds. The extent of its distribution varies with that of the posterior cutaneous nerve of the thigh and the nervus suralis.
Ramus Anastomoticus Peronæus.-The peroneal anastomotic nerve (O.T. communicans fibularis), arising in the popliteal fossa, passes over the lateral head of the gastrocnemius beneath the deep fascia to the middle third of the leg, where it assists in forming the nervus suralis by its union with an anastomotic (communicating) branch of the tibial nerve called the medial sural nerve or medial cutaneous nerve of the calf. In many cases the two branches do not unite. In such cases the peroneal anastomotic nerve may be limited in its distribution to the skin of the lateral side of the leg, heel, and ankle, or it may be distributed to the area usually supplied by the nervus suralis.
Terminal Branches. The terminal branches of the common peroneal nerve are three in number:-recurrent tibial, deep peroneal (O.T. anterior tibial), and superficial peroneal (O.T. musculo-cutaneous). They arise just distal to the head of the fibula, and are directed forwards, diverging in their course, beneath the peroneus longus muscle.
The recurrent tibial nerve is the smallest branch. Passing forwards under cover of the origin of the peronæus longus and the extensor digitorum longus muscles, it divides, distal to the lateral condyle of the tibia, into branches which supply the proximal fibres of the tibialis anterior muscle, the proximal tibio-fibular articula tion, and the knee-joint.
NERVUS PERONEUS PROFUNDUS.
The deep peroneal nerve (O.T. anterior tibial) passes obliquely distally under cover of the peronæus longus, extensor digitorum longus, and extensor hallucis longus muscles, to the front of the leg. In its course it is deeply placed upon the interosseous membrane and the distal part of the tibia, in company with the anterior tibial artery. At the ankle it lies under cover of the transverse ligament of the leg and the tendon of the extensor hallucis longus, and, crossing the anklejoint, it divides on the dorsum of the foot into its terminal branches.
1. Collateral Branches (in the leg).—These are given off to the muscles between which the deep peroneal nerve passes, namely: tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peronæus tertius. A fine articular branch supplies the ankle-joint.
2. Terminal Branches (on the foot).-The terminal branches are medial and lateral. The medial branch passes along the dorsum of the foot, on the lateral side of the dorsalis pedis artery, to the first interosseous space, where it divides into two dorsal digital branches for the supply of the skin of the lateral side of the great toe and the medial side of the second toe (nervi digitales dorsales, hallucis lateralis et digiti secundi medialis). Each of these branches communicates with branches of the superficial peroneal (O.T. musculo-cutaneous) nerve. gives off one or two dorsal interosseous branches, which supply the medial tarsometatarsal and metatarso-phalangeal articulations, and enter the first dorsal. interosseous muscle.
The lateral branch passes obliquely over the tarsus under cover of the extensor digitorum brevis, and ends in a gangliform enlargement (similar to the gangliform enlargement on the dorsal interosseous nerve of the forearm at the back of the wrist). From this enlargement muscular branches arise for the supply of the extensor digitorum brevis, along with branches for the tarsal, tarso-metatarsal, and metatarso-phalangeal articulations. Its dorsal interosseous branches may be as many as four in number. Of these the lateral two, extremely small, may only reach the tarso-metatarsal articulations. The medial two are fine branches, which, besides supplying the articulations, may give branches to the second and third dorsal interosseous muscles.
The branches from the nerve to the interosseous muscles are probably sensory, the motor supply of these muscles being certainly derived from the lateral plantar
NERVUS PERONEUS SUPERFICIALIS.
The superficial peroneal nerve (O.T. musculocutaneous), the last of the branches of the common peroneal nerve, passes distal to the head of the fibula and under cover of the proximal fibres of the peronæus longus muscle. Lying in a sheath in the intermuscular septum, between the peronæi and the extensor digitorum longus, it proceeds distally in front of the fibula to the distal third of the leg, where it pierces the deep fascia in two branches, medial and lateral.
Its branches are: (1) collateral muscular branches distributed to the peronæus longus and peronæus brevis, as the nerve lies in relation to these muscles; (2) terminal cutaneous branches, medial and lateral.
Nn. Cutanei Dorsales Medialis et Intermedius.The medial terminal branch (n. cutaneus dorsalis medialis) courses distally over the transverse ligament of the leg, and after supplying offsets to the distal third of the leg and to the dorsum of the foot, divides into three branches. (1) The most medial branch supplies the skin of the dorsum of the foot and the medial side of the great toe, and communicates with the saphenous nerve. (2) The intermediate branch 1.S, Saphenous nerve; M.C, passes to the interval between the great toe and the second, and divides into two branches which communicate with the medial branch of the deep peroneal nerve. (3) The lateral branch passes to the interval between the second and third toes, and divides into dorsal digital branches to supply the adjacent sides of these toes.
Superficial peroneal nerve; A.T, Deep peroneal nerve; E.S, Nervus suralis. The extremities of the toes are supplied by the medial and lateral plantar nerves (I.P, E.P).
The lateral terminal branch (n. cutaneus dorsalis intermedius) of the nerve passes over the transverse ligament of the leg, and after supplying branches to the distal part of the leg and to the dorsum of the foot, divides into two parts, which, passing to the intervals between the third and fourth, and fourth and fifth toes respectively, divide into dorsal digital branches for the adjacent sides of these toes. branches communicate with offsets of the nervus suralis (nerve of the calf).
The arrangement of the cutaneous branches of the superficial peroneal nerve is liable to considerable variation. The lateral division of the nerve may be increased in size, and may supply the nerve to the adjacent sides of the second and third toes; or it may be reduced in size, in which case the nervus suralis takes its place on the dorsum of the foot, often supply ing as many as two and a half toes on the lateral side.
The cutaneous nerves on the dorsum of the toes are much smaller than the corresponding plantar digital nerves. They are reinforced on the dorsum of the terminal phalanges by twigs from the plantar nerves, which supply the tips of the toes and the nails.
The tibial nerve (O.T. internal popliteal) arises from the anterior surface of the sacral plexus, usually from the fourth and fifth lumbar and first three sacral nerves (Fig. 631, p. 736). It is incorporated in the sciatic trunk in the buttock and proximal part of the thigh. At the bifurcation of the sciatic nerve it passes onwards through the popliteal fossa and the back of the leg. The part of the nerve from its origin from the plexus or the bifurcation of the sciatic nerve to the distal border of the popliteus muscle, was formerly called internal popliteal the part of the nerve in the back of the leg being then designated posterior tibial. The course of the nerve through the buttock and thigh has already been described (p. 728). In the popliteal fossa it is concealed at first by the semimembranosus and the other hamstring muscles. It passes to the medial side of the popliteal vessels, and is thereafter found upon the popliteus muscle, under cover of the gastrocnemius and plantaris. In the back of the leg, from the distal border of the popliteus muscle to the ankle, the tibial (O.T. posterior tibial) nerve lies on the tibialis posterior muscle and the tibia, and, along with the posterior tibial vessels, occupies a sheath in the intermuscular septum separating the superficial and deep muscles of the back of the leg. In the proximal part of the leg the nerve is medial to the vessels, but, crossing behind them, it lies on their lateral side in the distal portion of its course. It terminates under cover of the ligamentum laciniatum by dividing into the lateral and medial plantar nerves.
The collateral branches may be divided into three series, arising respectively in the region of the thigh, the popliteal fossa, and the back of the leg:
(a) Branches arising from the Roots or Trunk of the Nerve while it is incor porated with the Sciatic Nerve. These have been already described as muscular branches to the quadratus femoris, gemelli, obturator internus, and the hamstring muscles, and an articular branch to the hip-joint (Fig. 631, p. 736).
(b) Branches arising in the Popliteal Fossa proximal to the Knee-Joint.— These are in three sets-articular, muscular, cutaneous.
1. The articular branches are slender nerves, variable in number. There are usually two, an azygos branch which pierces the oblique ligament of the kneejoint, and a medial branch, a long fine nerve which, crossing the popliteal vessels, runs distally on the medial side of the fossa to accompany the distal medial articular artery to the knee-joint. In its course it gives off a branch, often absent, which accompanies the proximal medial articular artery.
2. The muscular branches are five in number. Nerves for the two heads of the gastrocnemius, and for the plantaris enter those muscles at the borders of the popliteal fossa. A nerve for the soleus enters the superficial surface of the muscle. A nerve for the popliteus muscle passes over the surface of that muscle. and after winding round its distal border, supplies it on its deep (anterior) surface. As this nerve passes below the popliteus it supplies branches to the tibials posterior, an interosseous branch for the interosseous membrane, which can be traced as far as the tibio-fibular syndesmosis, an articular branch for the proximal tibio-fibular joint, and a medullary branch for the shaft of the tibia.
3. N. Cutaneous Sure Medialis (O.T. N. Communicans Tibialis).The cutaneous branch is the medial cutaneous nerve of the leg. This nerve passe from the popliteal fossa in the groove between the two heads of the gastrocnemies muscle, and afterwards lies upon the tendo calcaneus. It pierces the deep fasc in the middle third of the back of the leg, and is joined immediately afterwards by the peroneal anastomotic ramus from the common peroneal nerve.