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their union the nervus suralis results, which reaches the foot, winding round the back of the lateral malleolus, along with the small saphenous vein. The nervus suralis supplies cutaneous branches to the lateral side and back of the distal third of the leg, the ankle and heel, and the side of the foot and little toe, as well as articular branches to the ankle and tarsal joints.

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FIG. 629.-DISTRIBUTION OF CUTANEOUS NERVES ON THE BACK OF THE LOWER LIMB.

In A the distribution of the several nerves is represented, their names being given.

In B a schematic representation is given of the areas supplied by the above nerves, the figures indicating the spinal origin of the branches of distribution to each centre.

The nervus suralis communicates on the foot with the superficial peroneal nerve, and its size varies with the size of that nerve. It may extend on to the dorsum of the foot for a considerable distance, and may either reinforce or replace the branches of the abovenamed nerve to the intervals between the fourth and fifth and the third and fourth The mode of formation of the nervus suralis is very variable. The usual arrangement is that described. Frequently the peroneal anastomotic nerve and the medial sural nerve (medial cutaneous nerve of the leg) do not unite, and in such cases the more usual arrangement is for the tibial trunk alone to form the nervus suralis (nerve of the calf),

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the peroneal anastomotic ramus extending only to the ankle and heel. It is less usual for the peroneal anastomotic ramus alone to form the nervus suralis, the medial sural nerve in these cases ending at the heel.

(c) Branches arising in the Back of the Leg distal to the Knee-Joint.-These branches are mainly muscular and cutaneous.

The muscular branches are four in number, comprising nerves to the soleus (entering its deep surface) and tibialis posterior, often arising by a common trunk, and nerves to the flexor digitorum longus and flexor hallucis longus, the latter generally accompanying the peroneal artery for some distance.

Rami Calcanei Mediales.-The cutaneous branches are the medial calcanean rami, which pierce the ligamentum laciniatum, and is distributed to the skin of the heel and posterior part of the sole of the foot.

In addition, a medullary nerve to the fibula, and a small articular branch to the ankle-joint, are supplied by the tibial nerve.

The terminal branches of the tibial nerve are the medial and lateral plantar nerves.

NERVUS PLANTARIS MEDIALIS.

The medial plantar nerve is homologous with the median nerve in the hand (Fig. 629, p. 733). It is rather larger than the lateral plantar. It courses forwards in the sole of the foot, under cover of the ligamentum lanciniatum and abductor hallucis, to the interval between that muscle and the flexor digitorum brevis, in company with the medial plantar artery.

The collateral branches are muscular, cutaneous, and articular. The muscular branches supply the abductor hallucis and the flexor digitorum brevis. The plantar cutaneous branches are small twigs which pierce the plantar aponeurosis in the interval between these muscles to supply the medial part of the sole of the foot. The articular branches are minute twigs which supply the tarsal and tarsometatarsal articulations.

Nn. Digitales Plantares Communes. The terminal branches are four in number, the common plantar digital nerves, and may be designated first, second, third, and fourth, from medial to lateral side.

The first (most medial) branch separates from the nerve before the others, and pierces the plantar aponeurosis behind the ball of the great toe. It supplies a muscular branch to the flexor hallucis brevis, and cutaneous branches to the medial side of the foot and ball of the great toe. It terminates as the plantar digital nerve for the medial side of the great toe.

The second branch arises along with the third and fourth; after supplying a branch to the first lumbrical muscle, it becomes superficial in the interval between the first and second toes, and terminates by dividing into two proper digital nerves for the supply of the adjacent sides of these toes.

The third and fourth branches are entirely cutaneous in their distribution. They become superficial in the intervals between the second and third and the third and fourth toes, respectively, and there divide into proper digital branches for the supply of the adjacent sides of these toes.

Nn. Digitales Plantares Proprii.—The plantar proper digital nerves supply the whole length of the toes on the plantar aspect, and, in relation to the terminal phalanges, furnish minute dorsal offsets for the supply of the nails and tips of the toes on their dorsal surface.

The medial plantar nerve.thus supplies the skin of the three and a half medial toes in the sole of the foot; and four muscles: the abductor hallucis and flexor digitorum brevis, the flexor hallucis brevis, and the first lumbrical muscle.

NERVUS PLANTARIS LATERALIS.

The lateral plantar nerve is homologous with the ulnar nerve in the hand. From its origin, under cover of the ligamentum laciniatum, it extends forwards

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and laterally in the sole, in company with the lateral plantar artery, between the flexor digitorum brevis and the quadratus plantæ, towards the base of the fifth metatarsal bone. There it terminates by dividing into superficial and deep branches.

Collateral Branches.—Muscular branches are given off from the undivided nerve to the quadratus plantæ and abductor digiti quinti muscles. Cutaneous branches pierce the plantar fascia at intervals along the line of the intermuscular septum, between the flexor digitorum brevis and abductor digiti quinti.

Terminal Branches-Ramus Superficialis.-The superficial branch is mainly cutaneous. Passing forwards between the flexor digitorum brevis and abductor digiti quinti, it divides into lateral and medial parts.

The lateral branch, after supplying the flexor quinti digiti brevis muscle, and sometimes one or both interossei of the fourth space, becomes superficial behind the ball of the little toe, and supplies cutaneous twigs to the sole of the foot and ball of the toe. It terminates as the proper digital branch for the lateral side of the little toe.

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PLEXUS PUDENDUS.

The medial branch passes forwards to the interval between the fourth and fifth toes, where it becomes cutaneous, and divides into two proper digital branches for the supply of the adjacent sides of these toes. It communicates with the fourth terminal branch of the FIG. 630.-SCHEME OF DISTRImedial plantar nerve.

BUTION OF THE PLANTAR
NERVES.

Ramus Profundus. The deep branch of the lateral In brown, medial plantar nerve, plantar nerve, passing deeply along with the lateral plantar artery, extends medially towards the great toe, under cover of (i.e. dorsal to) the quadratus plantæ and oblique head of the adductor hallucis. It gives off articular branches to the tarsal and tarso-metatarsal joints, and muscular branches to the interossei of each space (except in some cases the muscles of the fourth space): to the adductor hallucis, and the lateral three lumbrical muscles. These nerves enter the deep surface of the muscles, that to the second lumbrical reaching its muscle after passing forwards dorsal to the transverse head of the

and its cutaneous and muscular branches; F.B.D, Flexor digitorum brevis; A.H, Abductor hallucis; F. B. H, Flexor hallucis brevis; L.I, First lumbrical. In green, lateral plantar nerve, and its cutaneous and muscular branches; Quad. P, Quadratus plantæ; A.D.Q, Abductor digiti quinti; F.B.D.Q, Flexor brevis digiti quinti.

adductor hallucis.

The pudendal plexus constitutes the third and last subdivision of the lumbosacral plexus. It is composed, for the most part, of the spinal nerves below those which form the sacral plexus; but, as already stated, there is no distinct point of separation between the two plexuses. On the contrary, there is considerable overlapping, so that two and sometimes three of the principal nerves derived from the pudendal plexus have their origin in common with nerves of the sacral plexus.

The plexus is formed by fibres from the anterior rami of the first three sacral nerves, and by the whole of the anterior rami of the fourth and fifth sacral and coccygeal nerves. The size of the nerves diminishes rapidly from the first sacral to the coccygeal, which is extremely slender.

Position and Constitution. The plexus is formed on the posterior wall of the pelvis. Of the nerves forming it, the upper ones emerge from the anterior sacral foramina; the fifth sacral nerve appears between the last sacral and first coccygeal vertebra; and the coccygeal nerve appears below the transverse process of that vertebra. The nerves of distribution derived from the plexus are the following:

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 obturator 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.

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 (O.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.

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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)

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