Last's Anatomy: Regional and Applied

Part seventeen. Pelvic vessels and nerves

Pelvic vessels

The pelvic walls and viscera are supplied by branches of the internal iliac artery and drain into tributaries of the internal iliac veins. Arteries and veins lie within the parietal pelvic fascia and only their branches that pass out of the pelvis (except the obturator vessels) need to pierce this fascia.

Internal iliac artery

The common iliac artery bifurcates at the pelvic brim opposite the sacroiliac joint (Fig. 5.65). From this point the internal iliac artery passes downwards and soon divides into a short posterior and a longer anterior division. The posterior division breaks up into three branches, all of which are parietal: iliolumbar, lateral sacral, and superior gluteal. The anterior division usually has nine branches, three associated with the bladder (superior vesical, obliterated umbilical and inferior vesical), three other visceral branches (middle rectal, uterine and vaginal), and three parietal branches (obturator, internal pudendal and inferior gluteal). The internal pudendal and inferior gluteal vessels are considered to be the terminal branches of the anterior division. The obliterated umbilical artery is a continuation of the superior vesical, which is usually the first (highest) branch to arise from this division. The remaining branches arise at variable levels and some may have common stems.

Branches of the posterior division

The iliolumbar artery (Fig. 5.56) passes upwards out of the pelvis in front of the lumbosacral trunk and behind the obturator nerve, running laterally deep to the psoas muscle. Its lumbar branch is really the fifth lumbar segmental artery. It passes laterally to supply psoas and quadratus lumborum and, by its posterior branch, erector spinae. This vessel gives a spinal branch into the foramen between L5 vertebra and the sacrum.

The iliac branch supplies the iliac fossa, i.e. the iliacus muscle and the iliac bone. It extends to the anastomosis around the anterior superior iliac spine (deep and superficial circumflex iliac arteries, ascending branch of the lateral circumflex femoral artery and upper branch of deep division of superior gluteal artery).

The lateral sacral artery (frequently double) runs down lateral to the anterior sacral foramina, i.e. in front of the roots of the sacral plexus (Fig. 5.56). In the pelvis it supplies the roots and piriformis. Spinal branches enter the anterior sacral foramina, supply the spinal meninges and the roots of the spinal nerves and pass through the posterior sacral foramina to reach the muscles over the back of the sacrum. The artery takes over the segmental supply from the lumbar arteries; usually a superior sacral artery supplies the first two sacral segments and an inferior sacral artery supplies the remaining segments (Fig. 5.65).

The superior gluteal artery, the largest of all the branches of the internal iliac (Fig. 5.65), passes backwards by piercing the pelvic fascia usually between the lumbosacral trunk and S1 nerve, and leaves the pelvis through the greater sciatic foramen above the upper border of piriformis. Its course and distribution in the buttock are considered on page 126.

Branches of the anterior division

The superior vesical artery is the persistent patent proxi-mal part of the fetal umbilical artery. The distal part becomes obliterated to form the medial umbilical ligament (see p. 234) which thus appears as the direct continuation of the vesical vessel. The superior vesical artery runs first along the side wall of the pelvis (Fig. 5.65) and then turns medially to reach the upper part of the bladder. It also supplies the adjacent ureter and vas deferens.

The inferior vesical artery arises much lower than the superior and runs medially across the pelvic floor to supply the trigone and lower part of the bladder, the ureter, vas deferens, seminal vesicle and prostate.

The middle rectal artery as a source of blood supply to the muscle of the rectum is frequently absent, and when present small. It may be repalced by a small branch from an artery that supplies other pelvic viscera, such as the prostate and seminal vesicles in the male and the vagina in the female.

The uterine artery crosses the pelvis in the base of the broad ligament, passing above the ureter. At the cervix it turns upwards closely applied to the muscle thereof and runs alongside the uterus in the broad ligament. At the entrance of the uterine tube it turns laterally to supply the tube and anastomose with the tubal branch of the ovarian artery.

The vaginal artery supplies the upper part of the vagina and corresponds to the inferior vesical artery in the male. It may be a branch of the uterine artery.

The obturator artery passes along the side wall of the pelvis below the nerve (Fig. 5.65) to enter the obturator foramen with the nerve and the vein and pass into the thigh. The artery gives off a small branch to the periosteum of the back of the pubis, and this vessel anastomoses with the pubic branch of the inferior epigastric artery. In about 30% of cases this anastomotic connection opens up to become the accessory or abnormal obturator artery, replacing the normal branch from the internal iliac in the latter instance. Such an artery in its passage from the inferior epigastric to the obturator foramen usually passes on the lateral side of the femoral ring, i.e. adjacent to the external iliac vein (Fig. 5.8). When it lies at the medial side of the ring, alongside the edge of the lacunar ligament, it is vulnerable to injury or division if the ligament has to be incised to release a strangulated femoral hernia.

The inferior gluteal artery runs backwards through the parietal pelvic fascia, passes below S1 nerve root (Fig. 5.65) (or sometimes S2) and leaves the pelvis through the greater sciatic foramen below piriformis, to continue its course in the buttock (see p. 126).

The internal pudendal artery lies in front of the inferior gluteal (Fig. 5.65), pierces the parietal pelvic fascia and passes out of the pelvis through the greater sciatic foramen below piriformis. It is distributed in the perineum to the anal region and the external genitalia (see p. 321).

Internal iliac vein

The internal iliac vein, a wide vessel about 3cm long, begins above the greater sciatic notch by the confluence of gluteal veins with others that accompany branches of the internal iliac arteries. It passes upwards posteromedial to its artery to join the external iliac vein on the medial surface of psoas major and form the common iliac. Apart from tributaries that correspond to arteries, the internal iliac vein receives tributaries from the rectal, vesical, prostatic, uterine and vaginal venous plexuses in the appropriate sex. The presence of these venous plexuses and large draining veins below the pelvic peritoneum accounts for the severe retroperitoneal haemorrhage that may result from fracture of the pelvic bones. By the lateral sacral veins the internal iliac vein communicates with the vertebral venous plexuses. There are no valves in pelvic veins. Sudden increase in abdominal pressure (as in coughing) may be momentarily more than the inferior vena cava can accommodate, and this drives blood backwards up the internal vertebral plexus, into posterior intercostal veins and by azygos veins into the superior vena cava, bypassing the diaphragm. Emboli from disease of the pelvic viscera can thus find their way by reflux blood flow into the vertebrae. In this way secondary carcinomatous deposits may appear in the vertebrae from primary growths in any of the pelvic viscera.

Pelvic nerves

The obturator nerve is a branch of the lumbar plexus formed within the substance of psoas major from the anterior divisions of the second, third and fourth lumbar nerves (anterior rami). It is the nerve of the adductor compartment of the thigh, which it reaches by piercing the medial border of psoas and passing straight along the side wall of the pelvis to the obturator foramen. It crosses the pelvic brim medial to the sacroiliac joint (i.e. on the ala of the sacrum) and runs forward between the internal iliac vessels and the fascia on the obturator internus muscle. In front of the internal iliac vessels it is separated from the normally situated ovary only by the parietal peritoneum lining the pelvic wall. Pain from the ovary may be referred along the nerve to the skin on the medial side of the thigh. This may be less an irritation of the main nerve trunk than irritation or inflammation of the parietal peritoneum, which is here supplied by the obturator nerve.

The obturator artery and vein converge to the obturator foramen, in which the nerve lies highest, against the pubic bone (Fig. 5.65) with the artery and vein beneath it in that order. The nerve divides while in the foramen into anteriorand posterior divisions; the former passes anterior to the upper border of obturator externus, while the posterior division pierces the obturator externus, after giving off a branch to supply the muscle. The distribution in the thigh is considered on page 123.

The accessory obturator nerve, which is occasionally present, also emerges from the medial border of psoas. But like the femoral nerve it passes over the superior pubic ramus to the thigh, where it supplies pectineus.

Sacral plexus

Not all the lumbar nerves are used up in the formation of the lumbar plexus. A part of L4 and all of L5 anterior rami enter the sacral plexus. After L4 has given off its branches to the lumbar plexus it emerges from the medial border of psoas and joins the anterior ramus of L5 to form the lumbosacral trunk. This large nerve passes over the ala of the sacrum and crosses the pelvic brim medial to the obturator nerve from which it is separated by the iliolumbar artery and veins. It descends to join the anterior rami of the upper four sacral nerves in the formation of the sacral plexus (Figs 5.56 and 5.76).

The sacral plexus is a broad triangular structure formed by the junction of the nerves lateral to the anterior sacral foramina (Fig. 5.56). It rests upon piriformis and is covered anteriorly by the parietal pelvic fascia which invests that muscle. Anterior to the fascia the lateral sacral arteries and veins lie in front of the sacral nerves. At a higher level the common iliac vessels lie over the lumbosacral trunk. The superior and inferior gluteal arteries usually pass backwards above and below S1 respectively; they may instead pass above and below S2. The ureter, lying in front of the internal iliac vessels, is well anterior to the upper part of the plexus and in front of all are the parietal pelvic peritoneum and pelvic viscera. The sacral nerves receive grey rami communicantes from the sacral sympathetic ganglia.

The sacral nerves give off certain branches (see below) and then divide, as does the lumbosacral trunk, into anterior and posterior divisions which thereupon branch and reunite to form nerves for supply of flexor and extensor compartments of the lower limb.

The piriformis is supplied by separate twigs from the posterior divisions of S1 and 2.

The perforating cutaneous nerve arises from the posterior divisions of S2 and 3. It pierces the sacrotuberous ligament and curves round the lower border of gluteus maximus to supply the skin of the buttock.

The posterior femoral cutaneous nerve is formed by branches from the posterior divisions of S1 and S2 and the anterior divisions of S2 and 3. It passes backwards below piriformis behind the sciatic nerve, which separates it from the ischium. It thus enters the gluteal region (see p. 127).

The superior gluteal nerve is formed from the posterior divisions of L4, 5 and S1. It passes out of the pelvis above the piriformis muscle (see p. 126).

The inferior gluteal nerve is formed from the posterior divisions of L5 and S1 and 2. It passes out of the pelvis below the lower border of piriformis (see p. 126).

The coccygeal plexus consists of a minor mingling of a branch from S4 and S5 and the coccygeal nerve. Branches supply the postanal skin over the coccyx.

The tibial part of the sciatic nerve is a big branch formed by union of branches from all five anterior divisions (L4, 5, S1–3). The common peroneal (common fibular) part of the sciatic nerve is formed by union of branches from the posterior divisions of L4, 5, S1, 2. They usually join in the pelvis, and the sciatic nerve so formed leaves the pelvis below the lower border of piriformis lying on the ischium, lateral to the ischial spine (see Fig. 3.13, p. 124). Its course in the gluteal region is considered on page 127. If the two components of the sciatic nerve do not join in the pelvis, the common peroneal part pierces the lower part of piriformis as it leaves the pelvis.

The nerve to obturator internus (anterior divisions of L5, S1, 2) also supplies the superior gemellus. It leaves the pelvis, lateral to the pudendal vessels, below the piriformis (see p. 126).

The nerve to quadratus femoris (anterior divisions of L4, 5, S1) also supplies the inferior gemellus and the hip joint. It leaves the pelvis in front of the sciatic nerve, which holds it down on the ischium (see p. 127).

The pudendal nerve arises from the anterior divisions of S2, 3 and 4 nerves. The nerve passes back between piriformis and coccygeus (Fig. 5.56), medial to the pudendal vessels. In the buttock (see Fig. 3.13, p. 124) it appears between piriformis and the sacrospinous ligament, and curls around the latter to run forward into the ischioanal fossa (see p. 316).

Muscular branches of S3 and S4 supply the pubococcygeus and iliococcygeus components of levator ani and coccygeus on their upper (pelvic) surfaces. The perineal branch of S4 passes between coccygeus and levator ani to enter the ischioanal fossa and supply pubo rectalis, pubourethralis, pubovaginalis and perianal skin.

The parasympathetic pelvic splanchnic nerves (nervi erigentes) arise by several rootlets from the anterior surfaces of S2 and 3 and often 4; the contribution from S3 is usually the largest. They pass forward into the inferior hypogastric plexuses where they mix with the sympathetic nerves and are distributed to pelvic viscera and the distal colon (see below). The old term nervi erigentes is correct but incomplete; the nerves cause erection but much more (see below).

Sacral sympathetic trunks

The sympathetic trunks cross the pelvic brim behind the common iliac vessels and run down in the concavity of the sacrum along the medial margins of the anterior sacral foramina (Fig. 5.56). Each has usually four ganglia. The trunks converge at the front of the coccyx to unite at a small swelling, the ganglion impar.

Somatic branches are given off to all the sacral nerves (lower limb and perineum), and smaller vascular filaments to the lateral and median sacral vessels. Visceral branches join the inferior hypogastric plexuses.

Inferior hypogastric plexuses

The inferior hypogastric plexus is an autonomic plexus on the side wall of the pelvis on each side. In the male it is lateral to the rectum and posterolateral to the seminal vesicle, prostate and posterior part of bladder; the middle of the plexus is level with and just behind the top of the vesicle. In the female the plexus is lateral to the rectum, cervix, vaginal fornix and posterolateral to the bladder. The plexus is a rectangular, fenestrated plaque of nerves and ganglia, measuring nearly 5cm anteroposteriorly and 2cm vertically (Fig. 5.66).

Its sympathetic components are derived from the superior hypogastric plexus (see p. 281), via the hypogastric nerve (Fig. 5.47) and from the sacral sympathetic ganglia. Preganglionic parasympathetic fibres join the plexus from S2, 3 and 4 nerves; these are the pelvic splanchnic nerves. About half the fibres in the hypogastric nerves are myelinated (preganglionic) and they relay in the ganglia of the inferior hypogastric plexus. The remaining sympathetic fibres and all the parasympathetic fibres pass through without relay. The parasympathetic motor and secretomotor fibres relay in the walls of the viscera.

Visceral branches of the inferior hypogastric plexus accompany visceral branches and tributaries of the internal iliac artery and vein as neurovascular bundles. In general it appears that the muscles of the bladder (detrusor muscle) and rectum are innervated by parasympathetic nerves from the pelvic splanchnics, the smooth muscle of the bladder neck, prostate, seminal vesicle and vas deferens by sympathetic nerves from the superior hypogastric plexus, and the smooth muscle of the internal sphincter of the anal canal by branches from the sacral sympathetic ganglia; all these nerves emerge from the inferior hypogastric plexus. Normal sensations of distension of bladder and rectum probably pass through the pelvic splanchnic nerves; pain fibres are carried by both parasympathetic and sympathetic nerves.

As well as the pelvic viscera the pelvic splanchnic nerves supply the colon from the splenic flexure distally. These branches run up from the inferior hypogastric plexuses to the superior hypogastric plexus, or more often to its left, and then ascend with branches of the inferior mesenteric artery or as independent retroperitoneal nerves.

Thus the pelvic parasympathetics are motor to the emptying muscle of the bladder, and of the gut from splenic flexure to rectum. They are also secretomotor to the gut and vasodilator to the erectile tissue in the perineum. The sympathetics are motor to the visceral muscle of the bladder neck and internal anal sphincter. They are motor, too, to the vas deferens, seminal vesicles and prostatic muscle. The sympathetics also have a facilitating function in relation to uterine muscle.