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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. It 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 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.
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. These 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 supplying 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 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-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 tibialis 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 Suræ 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.
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 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),
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.
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.
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)