Pelvis Lab 6A

Lab 6A: Pelvic Vasculature, Pelvic Floor, and pelvic nerves

Lab Overview

Study the osteology of the pelvis (see Lab 7A). Osteology is critical for keeping oriented in the 3D complexity of the pelvic region.
Dissect out and identify the branches of the internal iliac artery (remove veins for a clearer view).
Dissect out and identify the muscles of the pelvic diaphragm and the piriformis muscle.
Dissect out and identify branches of the ventral rami of spinal nerves L4-S3 in the pelvis.

Short List of Structures - 6A

Be sure to study the osteology of the pelvis from lab 6B

General Pelvic Vasculature

NOTE: In female cadavers we want to keep the broad ligament intact on one side, so keep the peritoneum on one side.

ACTION:

On one side of the pelvis, use forceps to bluntly dissect and peel the peritoneum and extraperitoneal fat covering the iliac vessels and the pelvic nerves. Remove branches of the internal iliac vv. to more clearly demonstrate the arterial distribution. Identify the following:

- Common iliac a. — divides into two divisions at the level of the sacroiliac joint:
  - Internal iliac a. — anterior and posterior divisions that supply the pelvis and perineum
  - External iliac a. — courses deep to the inguinal ligament, after which it becomes the femoral a.

HELPFUL HINTS

Throughout your dissection of the pelvic vasculature, remove veins to clarify the dissection field.

Clinical Correlation: Hemorrhoids

Internal hemorrhoids (piles) are prolapses of rectal mucosa containing the normally dilated veins of the internal rectal venous plexus. Internal hemorrhoids are thought to result from a breakdown of the muscularis mucosae, a smooth muscle layer deep to the mucosa (see figure). Internal hemorrhoids that prolapse through the anal canal are often compressed by the contracted sphincters, impeding blood flow. As a result, they tend to strangulate, ulcerate, and bleed.
External hemorrhoids are thromboses (blood clots) in the veins of the external rectal venous plexus and are covered by skin. Predisposing factors for hemorrhoids include pregnancy, chronic constipation, and any disorder that impedes venous return.
Both the internal and external rectal plexus are drained by the superior, middle, and inferior rectal veins. The superior rectal veins drain into the inferior mesenteric vein (part of the portal system of veins), the middle rectal veins drain into the internal iliac veins (systemic system of veins), and the inferior rectal veins drain into the internal pudendal vein, which also drains into the internal iliac vein.
Any abnormal increase in pressure in the valveless portal system of veins (such as with portal hypertension) may cause enlargement of the superior rectal veins, resulting in an increase in blood flow through the rectal venous plexus and into the systemic venous system. Thus a portocaval anastomosis will form between the portal system and the systemic venous system.

Pelvic Vasculature - Male

Use forceps to peel away the peritoneum and extraperitoneal fat covering the iliac vessels and nerves, and attempt to find the following branches.
Know that you cannot positively identify these vessels without seeing where they are going; the branching pattern is variable. So to definitely identify the vesical artery you would need to see it entering into the substance of the bladder. Likewise, you need to see the middle rectal artery entering the substance of the rectum. On a practical exam, you will either need to see where the artery is going, or we will tell you where it is going as part of the question.

- Anterior division of the internal iliac a.
- Umbilical a. — continuation of the anterior division to the anterior abdominal wall; becomes the medial umbilical ligament (obliterated umbilical a.); may give off three to four superior vesical aa.
- Obturator a. — courses through the obturator canal
- Inferior vesical a. — trace to the posteroinferior part of the bladder, prostate gland, and seminal vesicles (not present in females)
- Middle rectal a. — to the rectum
- Internal pudendal a. — exits the pelvis through the greater sciatic foramen to enter the gluteal region, inferior to the piriformis m.; enters the perineum by traveling through the lesser sciatic foramen. You’ll be able to better see this artery after reflection of the gluteal muscles.
- Inferior gluteal a. — exits the pelvis, between S2 and S3 nn. usually; courses through the greater sciatic foramen to enter the gluteal region, inferior to the piriformis m.

Posterior division of the internal iliac a.

Superior gluteal a. — exits the pelvis between the lumbosacral trunk (L4-L5) and S1 superiorly through the greater sciatic foramen and enters the gluteal region above the piriformis m.
Iliolumbar a. — located lateral to the vertebrae near the lumbosacral trunk; ascends along the posterior pelvic wall
Lateral sacral aa. — located along the lateral border of the sacrum, with branches exiting through the sacral foramina
Internal pudendal a. – exits the pelvic cavity through the greater sciatic foramen

HELPFUL HINTS

There is significant variation in the branching pattern of the pelvic vasculature. The most conclusive way to identify the branches is to trace the arteries out to the organs they supply blood to.

You’ll need to spend some time carefully cleaning the adipose and connective tissues out of this region, and exposing arteries, before you start putting names to structures.

Pelvic Vasculature (female)

Use forceps to peel away the peritoneum and extraperitoneal fat covering the iliac vessels and nerves. Use forceps to reveal and identify the following branches

Anterior division of the internal iliac a.

Internal Iliac Artery (Anterior Division)

Umbilical a. — continues to the anterior abdominal wall, becomes the medial umbilical ligament (obliterated umbilical a.); may give off three to four superior vesical aa.
Obturator a. — courses through the obturator canal
Uterine a. — trace it to the isthmus of the uterus; located within the cardinal ligament; divides into a large superior branch to the body and fundus of the uterus and a small branch to the cervix, vagina, and inferior bladder; anastomoses with the ovarian and vaginal aa.; crosses superior to the ureter, near the lateral fornix of the vagina
Vaginal a. — often is a branch of the uterine a.; trace to the vagina and posteroinferior surface of the urinary bladder.
Middle rectal a. — to the lateral wall of the rectum
Internal pudendal a. — exits the pelvis through the greater sciatic foramen, inferior to the piriformis m.
Inferior gluteal a. — exits the pelvis between S2 and S3 nn.; courses through the inferior part of the greater sciatic foramen to enter the gluteal region inferior to the piriformis.

Use forceps to reveal and identify the following branches:

Posterior division of the internal iliac a.

Superior gluteal a. — exits the pelvis between the lumbosacral trunk (L4-L5) and S1 superiorly through the greater sciatic foramen and enters the gluteal region above the piriformis m.
Iliolumbar a. — located lateral to the vertebrae near the lumbosacral trunk; ascends along the posterior pelvic wall
Lateral sacral aa. — located along the lateral border of the sacrum, with branches exiting through the sacral foramina
Internal pudendal a. – exits the pelvic cavity through the greater sciatic foramen

Clinical Correlation: Round Ligament Pain

The round ligament is the partial remnant of the gubernaculum (the other part became the ligament of the ovary). The gubernaculum would have pulled the gonads through the fetal body wall and inguinal canal to sit within the scrotum (the labia majora in a female) if the hormones related to a male phenotype had turned on in the womb. The ligament does not degenerate in people with a female phenotype. Instead it becomes two of the suspensory ligaments of the uterus (the round ligament and the ligament of the ovary, as stated above).

The pain may be experienced by the pregnant person in the abdominal area or down in the groin, and may be dull or sharp. The only way to relieve round ligament pain is to rest with the hips flexed and to avoid sudden movements. Round ligament pain is not dangerous, but it is always best for the patient to consult their doctor or a specialist.

Figure 2.24 Right Sagittal Oblique view

Figure 2.25 Right Sagittal view

Clinical Correlation: Caesarean Section

A Caesarian section (C-section) is the use of surgery to deliver a baby. It is the most common surgical procedure performed in the U.S., used in a third of all births. It is used to deliver babies that would be risky to deliver vaginally for reasons such as multiple births, breech birth, and hypertension in the mother.

Before the incision is made, a catheter is used to drain the bladder so that it shrinks back behind the public bone and is less prone to injury (see image). The bladder is nevertheless the most frequently injured organ in C-section. An incision is then made through the uterus and overlying tissue, and the baby is manually delivered through the incision.

Figure 2.26 Right Sagittal view

Clinical Correlation: Inferior Vena Cava Syndrome

In pregnancy, the abdominal organs are generally displaced superiorly and posteriorly as the uterus expands to occupy the anterior abdomen. In late pregnancy, women can develop inferior vena cava (IVC) syndrome by lying supine. In this position, the uterus can compress the IVC, reducing venous return to the heart. IVC syndrome in late pregnancy is characterized by nausea and vomiting, pallor, hypotension, and pedal edema. It is treated by lying the patient on their left side, which can relieve compression of the IVC by the uterus.

Pelvic Diaphragm Musculature

To visualize the deep and thin musculature of the pelvic floor, you will need to mobilize and remove some of the pelvic viscera and soft tissue structures. Please keep one side of your pelvis intact.

The openings of the bony pelvis are closed by muscles bilaterally: identify the following:

Lateral: obturator internus m. — covers the obturator foramen and is pierced by the obturator n., a., and v., which pass through the obturator canal
Center: pelvic diaphragm See details below
Posterior: piriformis m.

Identify the following structures:

Tendinous arch — inferior to the opening of the obturator canal, the obturator internus fascia thickens and forms a tendinous arch between the ischial spine and pubic bone; the tendinous arch serves as the lateral attachment for the iliococcygeus part of the levator ani m.
Obturator internus m. — covers the obturator foramen and is pierced by the obturator n., a., and v., which pass through the obturator canal
Pelvic diaphragm — a funnel-shaped muscle that forms the floor of the pelvic outlet; the pelvic organs are anchored to the middle of the pelvic diaphragm; composed of two paired muscles (levator ani m. and coccygeus m.) that fuse at the midline:
- Levator ani m. — consists of three contiguous parts that are named by their attachments (it will be difficult to isolate individual components):
  - Puborectalis m. — medial part
  - Pubococcygeus m. — intermediate part
  - Iliococcygeus m. — lateral part (arises from the tendinous arch of the obturator internus fascia)
- Coccygeus (ischiococcygeus) m.— arises from the ischial spines and from the posterior part of the pelvic diaphragm; attaches to the coccyx
- Piriformis m. — closes the pelvic outlet posterior to the pelvic diaphragm
  - Arises from the sacrum and attaches to the femur
  - The gap between the piriformis m. and pelvic diaphragm is the primary passageway for nerves and vessels from the pelvis to the gluteal region and perineum

HELPFUL HINTS

The individual muscles of the levator ani are very thin and difficult to demonstrate. While you should know the relative positions of the individual muscles you need not attempt to dissect them out.

Pelvic Innervation

Use forceps to move the dissected arteries to gain access to the nerves of the pelvic wall, which you will dissect with forceps to identify:

Lumbosacral trunk (joined L4, L5 spinal cord segments) — contributes to the sacral plexus; located at the lateral side of the sacral promontory
S1, S2, S3 nn. — anterior rami from the spinal cord levels of S1-S3 that emerge from the sacral foramina, inferior to the lumbosacral trunk, with which they merge to form the sciatic n.
Sciatic n. (L4-S3 spinal cord segments) — exits the pelvis through the greater sciatic foramen and enters the gluteal region anterior to the piriformis m.
Pudendal n. (S2, S3, S4 spinal cord segments) — arises from the inferior surface of S4 n.; courses deep to the coccygeus m. to enter the perineum
Superior gluteal n. — courses with the superior gluteal a.; exits the pelvis between the lumbosacral trunk and S1 n. usually
Inferior gluteal n. — courses with the inferior gluteal a.; exits the pelvis between S1 and S2 nn. usually
Obturator n. — courses with the obturator a. and v. to the obturator canal

Sympathetic trunk— the sympathetic chain and its ganglia are medial to the sacral foramina and under the peritoneum

Figure 3.21 Left Sagittal View

Clinical Correlation: Innervation of pelvic viscera, nerve blocks

A spinal block, in which the anesthetic agent is introduced with a needle into the spinal subarachnoid space at the L3–L4 vertebral level produces complete anesthesia inferior to approximately the waist level. The perineum, pelvic floor, and birth canal are anesthetized, and motor and sensory functions of the entire lower limbs, as well as sensation of uterine contractions, are temporarily eliminated.
With the caudal epidural block, the anesthetic agent is administered using an in-dwelling catheter in the sacral canal. The entire birth canal, pelvic floor, and most of the perineum are anesthetized, but the lower limbs are not usually affected. The mother is aware of her uterine contractions.
A pudendal nerve block is a peripheral nerve block that provides local anesthesia over the S2–S4 dermatomes (most of the perineum) and the inferior quarter of the vagina. It does not block pain from the superior birth canal (uterine cervix and superior vagina), so the mother is able to feel uterine contractions.

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