NERVUS PERONAUS 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 articulation, and the knee-joint.

NERVUS PERONEUS PROFUNDUS.

the

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 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.

It

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

nerve.

NERVUS PERONAUS 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.

LS

M.C.

E.P

FIG. 628.-DISTRIBUTION OF
CUTANEOUS NERVES ON THE
DORSUM OF THE FOOT.

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 I.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).

These

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.

NERVUS TIBIALIS.

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 incorporated 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-JointThese 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 passes from the popliteal fossa in the groove between the two heads of the gastrocnemius muscle, and afterwards lies upon the tendo calcaneus. It pierces the deep fascia 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. From

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.

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 toes. 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),

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