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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.
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.
Ramus Profundus. -The deep branch of the lateral 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
BUTION OF THE PLANTAR
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:
1. Visceral branches.
2. Posterior cutaneous nerve of the thigh.
3. Perforating cutaneous nerve.
4. Pudendal nerve.
5. Muscular branches.
All the nerves, except the visceral branches, are distributed to the perineum.
Only two, the posterior cutaneous nerve of the thigh and the perforating cutaneous nerve, send branches to the lower limb.
Visceral Branches.-Like the other spinal nerves, the fourth and fifth sacral and coccygeal nerves are provided with fine gray rami communicantes from the sacral sympathetic trunk, which join them after a short course on the front of
the sacrum. The third (along with the second or fourth) sacral nerve, in addition, sends a considerable white ramus communicans or visceral branch direct to the pelvic plexus and viscera.
N. Cutaneus Femoris Posterior-Posterior Cutaneous Nerve of the Thigh (O.T. Small Sciatic). This nerve is complex both in origin and distribution (Fig. 631, p. 736). Springing from the junction of the sacral and pudendal plexuses, it is derived from the first three or second and third sacral nerves. is distributed to the lower limb and perineum, and is associated with other nerves belonging to both regions. It arises from the back of the roots of the sacral plexus in the pelvis. Its higher roots from the first and second sacral nerves are intimately associated with the origin of the inferior gluteal nerve; its lowest root from the third sacral nerve is associated with the origins of the perforating cutaneous or of the pudendal nerve. It enters the buttock through the greater sciatic notch, below the piriformis, along with the inferior gluteal artery and nerve. Proceeding distally, posterior to the sciatic nerve, it enters the thigh at the lower border of the gluteus maximus muscle, where it gives off considerable branches. Becoming gradually smaller as it courses distally over the hamstring muscles to the popliteal fossa, it finally pierces the popliteal fascia in one or more cutaneous branches, which supply the skin over the calf of the leg for a variable distance (Fig. 629, p. 733).
Branches.-The nerve is purely cutaneous. It supplies branches to the perineum, buttock, thigh, and leg.
Rami Perineales. -The perineal branch arises at the lower border of the gluteus maximus muscle (Fig. 631, p. 736). It sweeps in a medial direction to the perineum, lying on the origin of the hamstring muscles, distal to the ischial tuberosity; and becoming subcutaneous after passing over the pubic arch, its terminal branches supply the skin of the scrotum and root of the penis, or, in the female, the labium majus and clitoris, some of them being directed backwards towards the anus and central point of the perineum. They communicate with the inferior hæmorrhoidal and perineal branches of the pudendal nerve, and with the ilio-inguinal nerve. In its course to the perineum the nerve gives off collateral branches to the skin of the proximal and medial part of the thigh.
Nn. Clunium Inferiores.-The inferior gluteal branches are large and numerous (Fig. 631, p. 736). They arise from the nerve beneath the gluteus maximus, and become subcutaneous by piercing the fascia lata at different points along its lower border. They supply the skin of the lower half of the buttock. The most lateral branches, reaching to the back of the greater trochanter, overlap the terminal filaments of the gluteal branches of the lateral cutaneous nerve of the thigh, and the posterior rami of the first three lumbar nerves. The most medial branches, which may pierce the sacro-tuberous ligament, reach nearly to the coccyx, and are co-extensive in their distribution with the branches of the perforating cutaneous nerve, which they reinforce and not infrequently replace.
The femoral branches are divisible into two sets-medial and lateral. They pierce the fascia lata of the thigh at intervals, and supply the skin of the back of the thigh.
The sural branches are two or more slender nerves which pierce the fascia over the popliteal fossa, and are distributed for a variable extent to the skin of the back of the leg. They may stop short over the popliteal fossa, or may extend as far as the ankle. Usually they innervate the skin as far as the middle of the calf. They communicate with the nervus suralis.
In cases where the sciatic nerve is naturally divided at its origin into tibial and common peroneal nerves (e.g. by the piriformis muscle), the posterior cutaneous nerve also is separated into two parts: a posterior part, associated with the common peroneal nerve and arising in common with the lower roots of the inferior gluteal nerve (usually from the first and second sacral nerves), and comprising the gluteal and lateral femoral branches; and an anterior part, associated with the tibial nerve and arising usually from the second and third sacral nerves, along with the perforating cutaneous and pudendal nerves, and comprising the perineal and medial femoral branches.
Perforating Cutaneous Nerve Nerve (n. (n. perforans ligamenti tuberoso - sacri (Schwalbe), n. cutaneus clunium inferior medialis (Eisler)).—This nerve arises
from the back of the second and third sacral nerves (Fig. 631, p. 736). At its origin it is associated with the lower roots of the posterior cutaneous nerve of the thigh. Passing distally it pierces the sacro- tuberous ligament, along with the coccygeal branch of the inferior gluteal artery; and after winding round the lower border of the gluteus maximus muscle, or in some cases piercing its lower fibres, it becomes cutaneous a little distance from the соссух, and supplies the skin over the lower part of the buttock and the medial part of the fold of the nates.
The perforating cutaneous nerve is not always present. In a minority of cases it is associated at its origin with the pudendal nerve. When absent as a separate nerve, its place is taken by (1) gluteal branches of the posterior cutaneous nerve of the thigh, or (2) a branch from the pudendal nerve, or (3) a small nerve (n. perforans coccygeus major, Eisler), arising separately from the posterior part of the third and fourth sacral nerves.
Muscular Branches.-Between the third and fourth sacral nerves (occasionally reinforced by the second, Eisler) a plexiform loop is formed, from which muscular nerves are given off to the levator ani (supplying the muscle on its pelvic surface), coccygeus, and external sphincter. The nerve to the external sphincter (perineal branch of fourth sacral) pierces the sacro-tuberous ligament and the coccygeus muscle, to which it gives offsets, and appears in the ischio-rectal fossa between the gluteus maximus and the external sphincter. Besides supplying the posterior fibres of the external sphincter, it distributes cutaneous offsets to the skin of the ischio-rectal fossa and the fold of the nates behind the anus. This nerve, in some instances, replaces the perforating cutaneous nerve.
Nn. Anococcygei (Ano-coccygeal Nerve).-By the union of the remaining part of the fourth with the fifth sacral and coccygeal nerves, the so-called plexus coccygeus (coccygeal plexus) is formed. A fine descending branch of the fourth sacral nerve passes over or through the sacro-tuberous ligament, to join the fifth sacral nerve. This fifth sacral nerve, joined by the descending branch of the fourth, descends alongside the coccyx and is again joined by the coccygeal nerve, so that a plexiform cord, the ano-coccygeal nerve results, homologous with the inferior caudal trunk of tailed animals. Fine twigs arise from it, which pierce the sacrotuberous ligament and supply the skin in the neighbourhood of the coccyx, medial to the branches of the perforating cutaneous nerve and behind the anus.
The pudendal nerve (O.T. pudic) is the principal nerve for the supply of the perineum. It arises in the pelvis usually by three roots from the second, third, and fourth sacral nerves (Fig. 631, p. 736). (Frequently one of its branches, the inferior hæmorrhoidal nerve, arises independently from the third and fourth sacral nerves The nerve passes to the buttock through the greater sciatic foramen, below the sciatic nerve, and lies on the sacro-spinous ligament, or the spine of the ischium, medial to the internal pudendal artery. It enters the perineum along with the artery through the lesser sciatic foramen. In the perineum it is deeply placed in the lateral wall of the ischio-rectal fossa, enclosed in a special sheath derived from the parietal pelvic fascia covering the medial surface of the obturator internus muscle. At the anterior limit of the ischio-rectal fossa, the nerve approaches the surface and divides at the base of the urogenital diaphragm into its terminal branches, the perineal nerve and the dorsal nerve of the penis.
The branches of the nerve are essentially the same in the two sexes. As a rule no branches are given off till it enters the perineum, but sometimes the inferior hæmorrhoidal nerve has an independent origin from the plexus, merely accompanying the pudendal nerve in the first part of its course; and in exceptional cases the perforating cutaneous nerve of the buttock is a branch of the pudendal
Nn. Hæmorrhoidales Inferiores.-The inferior hæmorrhoidal nerve arises from the pudendal nerve under cover of the gluteus maximus, at the posterior part of the ischio-rectal fossa. In cases in which it has an independent origin from the plexus. it arises from the third and fourth sacral nerves. It crosses the ischio-rectal fossa
in company with the inferior hæmorrhoidal vessels, and separates into numerous branches-muscular, cutaneous, and communicating.
The muscular branches end in the external sphincter ani muscle. The cutaneous branches supply the skin around the anus. The communicating branches connect the inferior hæmorrhoidal with three other nerves-the perineal branches of the posterior cutaneous nerve of the thigh, pudendal, and fourth sacral nerves.
Nervus Perinei. The perineal nerve, one of the two terminal branches of the pudendal nerve, arises near the base of the urogenital diaphragm. It almost immediately divides into two parts, superficial and deep.
The superficial part is purely cutaneous and consists of two nerves, the posterior
or lateral and the anterior or medial superficial perineal nerves (nn. scrotales posteriores or nn. labiales posteriores), which pass, along with the superficial perineal vessels, to the anterior part of the perineum. The posterior or lateral superficial perineal nerve, at the anterior limit of the ischio-rectal fossa, usually passes over the base of the urogenital diaphragm and over the (superficial) transversus perinei muscle. The anterior or medial superficial perineal nerve, lying more deeply, pierces the base of the fascia inferior of the urogenital diaphragm and goes underneath or through the transversus perinei muscle. Becoming superficial in the anterior (urethral) triangle of the perineum, they are distributed to the skin of the scrotum (or labium majus), and communicate with the perineal rami of the posterior cutaneous nerve of the thigh and with the inferior hæmorrhoidal nerve.
The deep part of the perineal nerve is mainly but not entirely muscular