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third of the thigh on the medial side. It supplies the skin of the distal half of the thigh, extending as far as the knee, where it joins in the formation of the patellar plexus.

The distal branch represents the termination of the nerve. It passes along the medial side of the thigh over the sartorius muscle, and communicates in the middle third of the thigh with the saphenous and obturator nerves to form the obturator plexus. Piercing the fascia lata on the medial side of the thigh in the distal third, it ramifies over the side of the knee, and assists in the formation of the patellar plexus.

The size of the medial cutaneous nerve varies with the size of the cutaneous part of the obturator, and of the saphenous nerve.

N. Saphenus.—The saphenous nerve may be regarded as the terminal branch of the femoral nerve. It is destined for the skin of the leg and foot. From its origin in the femoral triangle it extends distally alongside the femoral vessels to the adductor canal. In the canal it crosses obliquely over the femoral sheath from lateral to medial side. At the distal end of the canal, accompanied by the saphenous branch of the arteria genu suprema, it passes over the tendon of the adductor magnus, and opposite the medial side of the knee-joint becomes cutaneous by passing between the sartorius and gracilis muscles. The nerve then extends distally in the leg along with the great saphenous vein, and coursing over the front of the medial malleolus it terminates at the middle of the medial border of the foot.

Branches.—1. A communicating branch arises in the adductor canal, and, passing medially behind the sartorius, joins with branches of the obturator nerve in forming the obturator plexus.

2. Ramus Infrapatellaris.—The infra-patellar branch arises at the distal end of the adductor canal, and piercing the sartorius muscle is directed distally and forwards below the patella, and over the medial condyle of the tibia to the front of the knee and proximal part of the leg. It enters into the formation of the patellar plexus.

3. An articular branch sometimes arises from the nerve at the medial side of the knee.

4. Rami Cutanei Cruris Mediales.—The terminal branches of the saphenous nerve are distributed to the skin of the front and medial side of the leg and the posterior half of the dorsum and medial side of the foot.

Plexus Patellaris. — The patellar plexus consists of fine communications beneath the skin in front of the knee, between the branches of the cutaneous nerves supplying that region. The nerves which enter into its formation are the infra-patellar branch of the saphenous, medial and intermediate cutaneous nerves, and sometimes the lateral cutaneous nerve of the thigh.

The accessory obturator nerve (n. obturatorius accessorius) is only occasionally present (29 per cent., Eisler). It arises from the third, or third and fourth lumbar nerves, between the roots of the obturator and femoral nerves. Associating itself with the obturator, from which, however, it is quite separable, it appears in the abdomen at the medial side of the psoas muscle, and coursing over the pelvic brim behind the external iliac vessels, it leaves the obturator nerve, and enters the thigh in front of the os pubis.

In the thigh, behind the femoral vessels, it usually ends in three branches : a nerve which replaces the branch from the femoral nerve to the pectineus, a nerve to the hip-joint, and a nerve which communicates with the superficial part of the obturator nerve. In some cases it only supplies the nerve to the pectineus; more rarely it is of considerable size, and reinforces the obtiirator nerve in the innervation of the adductor muscles.

The accessory obturator nerve was first described by Winslow as the n. accessorius anterioris cruralis. Schmidt later described it in great detail, and gave it the name it now bears. It is more closely associated with the femoral than with the obturator. Its origin is behind the roots of the obturator: it is separated, like the femoral, from the obturator by the pubic bone, and its chief branch, to the pectineus muscle, replaces the normal branch from the femoral nerve. On the other hand, for a part of its course it accompanies the obturator, and in rare cases it may replace branches of that nerve.

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PLEXUS SACRALIS.

The sacral portion of the lumbo-sacral plexus is destined almost entirely for the lower limb. It is usually formed by the anterior rami of a part of the fourth lumbar nerve (n. furcalis), the fifth lumbar, the first, and parts of the second, and third sacral nerves (n. bigeminus).

Communications with the Sympathetic.-- Each of the nerves named is connected to the lumbar or pelvic sympathetic by gray rami communicantes, as already described ; and white rami communicantes pass from the third and usually also from the second or fourth sacral nerves to join the pelvic plexus of the sympathetic.

Position and Constitution. The plexus is placed on the posterior wall of the pelvis between the parietal pelvic fascia and the piriformis muscle. In front of it are the pelvic colon, the hypogastric vessels, and the ureter.

The plexus is constituted by the convergence of the nerves concerned towards the inferior part of the greater sciatic foramen, and their union to form a broad triangular band, the apex of which is continued through the greater sciatic foramen below the piriformis muscle into the buttock, as the sciatic nerve. From the anterior and posterior surfaces of this triangular band numerous small branches arise, which are distributed to the parts in the neighbourhood of the origin of the nerve.

The sciatic nerve ends in the thigh by dividing into two large nerves, the tibial (0.T. internal popliteal), and common peroneal (O.T. external popliteal). In many cases these two nerves are distinct at their origin, and are separated sometimes by fibres of the piriformis muscle. In all cases, on removal of the sheath investing the sciatic nerve, the tibial and peroneal nerves can be traced up to the plexus, from which they invariably take origin by distinct and separate roots.

Formation.—The descending branch of the fourth lumbar nerve (n. furcalis) after emerging from the border of the psoas major muscle, medial to the obturator nerve, divides behind the iliac vessels into anterior and posterior (ventral and dorsal) parts, each of which joins a corresponding part of the fifth lumbar nerve. The anterior ramus of the fifth lumbar nerve descends over the ala of the sacrum, and divides into anterior and posterior parts, which are joined by the corresponding parts of the fourth lumbar nerve. The two resulting trunks are sometimes called the truncus lumbosacralis or lumbo-sacral trunk. The first and second sacral nerves pass almost horizontally laterally from the anterior sacral foramina, and divide in front of the piriformis into similar anterior and posterior parts. The third sacral nerve (n. bigeminus) divides into superior and inferior parts. The inferior part is concerned in forming the pudendal plexus. The superior part is directed laterally, and slightly upwards, towards the second nerve, and does not separate into two parts, but remains undivided.

These trunks combine to form the sacral plexus, and its main subdivisions, in the following way. Lying in apposition, and converging to the lower part of the greater sciatic foramen, the posterior (dorsal) trunks of the fourth and fifth lumbar nerves (lumbo-sacral trunk), and of the first and second sacral nerves, combine to form the common peroneal nerve and the subordinate nerves which arise from the posterior aspect of the plexus. The anterior (ventral) trunks of the fourth and fifth lumbar nerves (lumbo-sacral trunk), and of the first and second sacral nerves, together with that part of the third sacral nerve which is contributed to the plexus, unite to form the tibial nerve and the subordinate nerves arising from the front of the plexus.

Of these nerves the fifth lumbar and first sacral are the largest; the others diminishing in size as they are traced upwards and downwards. There is no distinct demarcation between the sacral and pudendal plexuses. The second and third sacral nerves (and in some cases the first sacral also) are concerned in the formation of both plexuses.

Branches. The nerves of distribution derived from the sacral plexus are divided according to their origin into an anterior (ventral) and a posterior (dorsal) series. Each set comprises one of the two essential terminal parts of the sciaticperoneal and tibial nerves—and numerous smaller collateral branches. Anterior (Ventral) Branches.

Posterior (Dorsal) Branches. Tibial nerve

Common peroneal nerve
Muscular branches-

Muscular branches-
Nerves to hamstring muscles

Nerves to short head of biceps quadratus femoris

piriformis gemelli

Superior gluteal nerve obturator internus

Inferior gluteal nerve Articular branches (to hip-joint)

Articular branches (to knee-joint)

· NERVUS ISCHIADICUS. 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 glutæus 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 glutæus maximus. At that spot it is comparatively superficial, lying in the angle between the edge of the glutæus 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 glutæus 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 glutæus medius and gluteus minimus muscles.

N. Glutæus 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 glutæus 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 nerve.

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 (0.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 PERONÆUS 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 ibularis), 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 articulation, and the knee-joint.

NERVUS PERONÆUS PROFUNDUS.

The deep peroneal nerve (0.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.

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