Use your probe to define an imaginary line on the bisected specimen, dividing areas with exclusively autonomic innervation from areas with somatic innervation. This line represents the division between visceral and somatic structures, including nerves, blood vessels, and epithelium.
The anus, like the urethra, is closed by an involuntary internal anal sphincter of smooth muscle, as well as a voluntary external anal sphincter derived from the body-wall musculature. The smooth muscle sphincters are hard to visualize.
Carefully reflect the ventral wall of the rectum dorsally and expose the reproductive viscera.
Note the autonomic nerve fibers (pelvic splanchnic nerves) passing forward on either side of the rectum to the bladder and pelvic genitalia. These fibers pass ventrally from the sacral region to reach the target organs.
Contrast this with the vessels traveling medially from the side wall of the pelvis --the blood supply of the rectum will be studied in more detail below.
3. URETERS and REPRODUCTIVE DUCTS
Trace the ureters to the lateral corners of the bladder.
Note that the reproductive ducts in both sexes come together in the midline, and that the ureters pass lateral to them.
4. RECTOVESICAL POUCH (MALE)
Find and explore the rectovesical pouch located between the bladder and the rectum.
Dissect the seminal vesicles and the ductus deferens out of the rectovesical fascia.
5. RECTOUTERINE AND VESICOUTERINE POUCHES (FEMALE)
In the female, identify the transverse cervical ligament along the base of the broad ligament to its attachment to the side of the uterus and the uterine artery contained within it.
HINT– the transverse cervical ligaments are a meshwork of tissue rather than a defined ligament. In the act of dissecting to see the uterine artery and ureter, much of this ligament will be lost. Dissect carefully with awareness of where this structure is in order to appreciate its structure.
Examine the cervix and os of the uterus.
Probe the opening of the uterine tube and visualize the course of an ovum from ovary to uterus.
Find the rectouterine pouch, which is the fold of peritoneum between the rectum and uterus.
Note that the posterior fornix of the vagina-- the vaginal recess dorsal to the cervix of the uterus--is in close association to the peritoneum of the rectouterine pouch.
Identify the two small openings of the ureters and the internal urethral orifice.
Look for the trigone, the triangular region defined by the two ureteral orifices and the internal urethral orifice.
Insert the tip of a probe into the orifice of the ureter.
Note the diagonal course of the ureters through the wall of the bladder.
In the male, locate the sphincter urethrae muscle below the prostate gland, but above the perineal membrane.
HINT - Note, that in the female, the presence of the vagina typically reduces the sphincter urethrae to a U-shaped slip of muscle tissue running over the top of the urethral orifice.
Compare the male and female urethral sphincters.
The loose connective tissue in the retropubic space permits mobility of the expanding bladder and cushions it from the pubic bones.
The diagonal course of the ureters through the wall of the bladder ensures that their distal ends are pressed shut when the bladder is distended. This prevents backflow of urine into the ureters. The three openings of the bladder form a triangle: the trigone, the triangular area of smooth mucosa bounded by the ureteric openings and the internal urethral orifice.
7. PROSTATE (MALE)
Identify the three parts of the urethra: prostatic urethra, membranous urethra, and spongy urethra.
Examine the interior of the prostatic urethra.
On the dorsal wall of the urethra, look for a small opening in the midline of the longitudinal urethral crest. This is the prostatic utricle.
NOTE - the utricle is a midline structure, and the two ejaculatory ducts (mentioned below) are just lateral to the utricle. Depending on the manner in which these structures were sectioned when the pelvis was hemisected, they may have been obliterated, or they may be visible on just one side of your cadaver.
On either side of the utricle, find the two pinpoint openings of the ejaculatory ducts.
Using a dissecting needle, attempt to probe the connection between the ejaculatory duct and urethra.
The ejaculatory duct conveys sperm and seminal fluid from the vas deferens and seminal vesicle into the urethra. The prostate adds its secretions via twenty or thirty small ducts visible as small openings along the length of the prostatic urethra.
8a. INTERNAL ILIAC ARTERY- ANTERIOR DIVISION
FUNCTIONAL ANATOMY: The pelvic portion of the gut, unlike the abdominal gut, is supplied by paired arteries that arise from the internal iliac arteries.
Using blunt dissection, follow the internal iliac artery into the pelvis.
Umbilical artery – find its obliterated remnant in the medial umbilical ligament and the superior vesical arteries, which branch from the patent portion of the umbilical artery. Branches from the superior vesical artery supply the terminus of the vas deferens and seminal vesicle in the male and the part of the uterus and vagina in the female.
Obturator artery – passes through the obturator canal
Internal pudendal artery - passes around the superior edge of the sacrospinous ligament to exit the pelvis. Passes through the greater sciatic foramen inferior to the piriformis muscle, and accompanied by the pudendal nerve.
Middle rectal artery – identify by tracing it to the rectum
Inferior gluteal artery – exits the pelvis by passing through the greater sciatic foramen inferior to the piriformis muscle.
NOTE – the branching pattern of the internal iliac artery can be quite variable, not only between individuals, but sometimes even within a single individual (the right side might differ from the left). Be sure to keep this in mind as you do your dissection and find the arteries by tracing them to their target.
8b. INTERNAL ILIAC ARTERY- POSTERIOR DIVISION
Find and identify the branches of the posterior division of the internal iliac artery:
Iliolumbar artery - ascends between the obturator nerve and the ventral rami of spinal nerves L4 and L5
Lateral sacral artery - running across Sl-S3
Superior gluteal artery - between the lumbosacral trunk and S1 before passing through the greater sciatic foramen, superior to the piriformis muscle.
9. SACRAL PLEXUS
Use your fingers to dissect the rectum from the anterior surface of the sacrum and coccyx.
FUNCTIONAL ANATOMY: The pelvic diaphragm is body wall muscle and hence is innervated by ventral rami (S2-S4, pudendal nerve).
Retract the rectum, bladder, prostate and seminal vesicles or uterus medially.
Use blunt dissection to remove any remaining fat and connective tissue from the superior surface of the pelvic diaphragm.
Identify the three muscles that form the levator ani muscle:
Puborectalis muscle – its proximal attachment is the body of the pubis.
Pubococcygeus muscle – its proximal attachment is the body of the pubis. Its fibers run generally parallel to the puborectalis but more lateral. It attaches to the coccyx and the anococcygeal raphe.
Iliococcygeus muscle – its proximal attachment is the tendinous arch of the levator ani (also known as the fascia of the obturator internus). It attaches distally to the coccyx and the anococcygeal raphe.
NOTE – when you identify the muscles of the levator ani, make note also of the obturator internus muscle, part of which is visible above and below the tendinous arch from which the levator ani takes it origin. This is a helpful landmark for orienting yourself to the pelvis and perineum.
The puborectalis forms the lateral boundary of the urogenital hiatus. It also forms a “puborectal sling” around the rectum that when contracted pulls the rectum and perineal body forward thus causing the anorectal flexure. During defecation, the puborectalis muscles relax, the anorectal flexure straightens, and the elimination of fecal matter is facilitated.
Identify the coccygeus muscle, which completes the pelvic diaphragm posteriorly. Its proximal attachment is the ischial spine and distally it attaches to the lateral border of the coccyx and the lowest part of the sacrum.
HINT– the coccygeus muscle parallels the sacrospinous ligament, one of two pelvic ligaments that form the boundaries for the greater and lesser sciatic foramina.
Identify the obturator internus muscle, which forms part of the lateral wall of the pelvic cavity.
HINT - The proximal attachment of the obturator internus is the margin of the obturator foramen and the inner surface of the obturator membrane. Superior to the tendinous arch of the levator ani muscle, the obturator internus forms the lateral wall of the pelvic cavity. Inferior to the tendinous arch, the obturator internus forms the lateral wall of the ischioanal fossa in the perineum.
11. PUDENDAL NERVE
Identify the pudendal nerve coming from the lumbosacral plexus.
Follow the pudendal nerve as it courses anterior to the piriformis muscle and leaves the pelvis through the greater sciatic foramen, just posterior to coccygeus muscle.
Continue to trace the pudendal nerve as it curves back ventrally and enters the ischioanal fossa through the lesser sciatic foramen.
Find the two nerves that are often confused with the pudendal nerve: the genitofemoral nerve (anterior to the inguinal canal) and the obturator nerve (passing through the obturator foramen)
Because the prostate lies inferior to the bladder and because the urthera passes through the prostate, enlargement of the prostate due to benign swelling (benign prostatic hypertrophy) or cancer can have profound effects on urination in men. When the prostate swells it puts pressure on the bladder leading to the frequent need to urinate. Men with BPH or other prostatic changes may produce smaller amounts of urine more frequently. In addition, because the swollen prostate can compress the urethra it is often harder for men with BPH or other prostatic changes to produce a steady flow of urine or to initiate urination. Given what you have learned about pouches in the pelvis and the order in which organs are found, what would be the best way to digitally examine a patient for prostatic swelling?
To examine and collect abnormal fluid in the pelvis of female patients doctors perform culdocentesis. In this procedure generally a needle is passed through the posterior fornix of the vagina. Fluid is withdrawn through that needle. See the figure below. Should the patiemnt be prone or supine during this procedure?
Iliac artery occlusion The internal iliac artery supplies blood to pelvic organs and to gluteal muscles that run from the pelvis to the femur and move the hip joint. If the internal iliac artery is occluded, obstructed, or narrowed, blood flow to the gluteals can be reduced. This vascular insufficiency can lead to pain. Cramping, and discomfort. This is often referred to as buttock claudication and can be misinterpreted as neurological in origin. If the obturator artery also experienced reduced blood flow from the occlusion, what other regions will experience vascular insufficiency?