Perineum

LabLink

Note: Concepts of sex and gender merit more than what is discussed as part of this laboratory manual. While as concepts, neither sex nor gender may be divisible binarily, this manual only provides foundational anatomical descriptions and dissection instruction for males and females.

Understand the basic organization of the perineum.

Find these structures:

- Perineum
  - - Urogenital triangle
    - Anal triangle
    - Perineal body/central tendinous point
    - Perineal membrane

1.) Note the boundaries of the perineum. You may want to return the hemisected portion to anatomical position to better visualize the entirety of this area.

Note: The perineum is the region of the pelvis superficial to the pelvic floor, and may be best conceptualized as a diamond-shaped structure consisting of two non-coplanar triangles: the urogenital triangle and the anal triangle. The anterior point of the diamond rests superficial to the pubic symphysis, the posterior point of the diamond rests superficial to the coccyx, and the lateral points of the diamond rest superficial to the ischial tuberosities.

2.) Locate the inter-ischial line, and differentiate between the urogenital triangle and anal triangle.

Note: The inter-ischial line is the plane between ischial tuberosities and the boundary between the urogenital triangle and the anal triangle.

Note: The urogenital triangle of the perineum consist of the following layers (superficial to deep):

- Skin
- Subcutaneous tissue
- Superficial perineal space (pouch)
- Perineal membrane
- Deep perineal space (pouch)
- Levator ani m. & perineal body

Note: The anal triangle contains the anal canal and anal sphincters, and the ischio-anal (ischiorectal) fossae with pudendal canals.

Photo 1. Perineal triangles (with boundaries)

3. Remove the skin from the perineum.

Dissect the perineum, and locate pudendal neurovasculature.

Find these structures:

- Anal triangle
  - - Ischio-anal fossa
- Pudendal n.
  - - Inferior rectal n.
    - Perineal n.
      - Posterior labial/scrotal nn.
        Deep perineal n.
    - Dorsal nn. of clitoris/penis
- Internal pudendal a. & v.
  - - Inferior rectal a.
    - Perineal a.
      - Posterior labial/scrotal aa.
        Artery of vestibule/bulb of penis
    - Deep aa. of clitoris/penis
    - Dorsal aa. of clitoris/penis

4. Conceptualize the boundaries of the ischio-anal fossa.

Note: The ischio-anal fossae compose the majority of the superficial anal triangle, and are located lateral to the anal canal and sphincters. The main contents of these fossae are adipose tissue, in addition to pudendal neurovasculature.

Boundaries of the ischio-anal fossa:

- Anterior: pubic bodies
- Posterior: gluteus maximus m. and sacrotuberous ligament
- Medial: external anal sphincter and portions of levator ani m.
- Lateral: lateral pelvic wall, obturator fascia, and obturator internus m. This includes the pudendal (Alcock’s) canal.

5. With blunt dissection, remove adipose tissue in the ischio-anal fossae to locate pudendal neurovasculature. There is substantial adipose tissue in this area, so this step may take a while.

Note: The pudendal n. arises from VPR of S2-4 and follows the internal pudendal a. out the greater sciatic foramen, back through the lesser sciatic foramen (where there is a close relationship with the ischial spine), and through the pudendal (Alcock’s) canal to serve as the primary nerve of the perineum.

Note: The internal pudendal a. is one of the terminal branches of the anterior division of the internal iliac a. It exits the pelvis through the greater sciatic foramen, and then enters the ischio-anal fossa via the lesser sciatic foramen and pudendal canal. The internal pudendal a. is the primary source of blood to the skin, muscles, and erectile bodies of the perineum.

Photo 3. Pudendal neurovasculature in the ischio-anal fossae

6. Locate the inferior rectal neurovasculature in the ischio-anal fossa.

Note: The inferior rectal n. and a. branches in the proximal portion of the pudendal (Alcock) canal, and supplies the external anal sphincter. This is typically the most obvious nerve in the ischio-anal fossa.

7. Locate the perineal body and perineal membrane.

Note: The perineal membrane is a fascial layer that separates the superficial and deep perineal spaces. It presents as a robust, shiny white layer.

Note: The perineal body, a fibromuscular mass, sits on the midline between the urogenital and anal triangles, at the level of the muscles within the superficial perineal space and external anal sphincter. It may be palpated through the skin, but it may only be seen upon dissection into the superficial space of the urogenital & anal triangles.

Photo 4. Perineal body

8. Follow the pudendal n. and internal pudendal a. into the urogenital triangle, and locate the following branches: perineal n. & a., posterior labial/scrotal n. & a., deep perineal n., artery of vestibule/bulb of penis, deep artery of clitoris/penis, dorsal n. & a. of clitoris/penis.

Note: The perineal n. divides into 2 branches: deep perineal n. & posterior labial/scrotal n.

The deep perineal n. runs deep to the perineal membrane to provide efferent innervation to mm. of the perineum (ischiocavernosus m., bulbospongiosus m., etc.). Sometimes this nerve is a direct branch of the pudendal n.

The posterior labial/scrotal n. and a. runs superficial to the perineal membrane to provide afferent innervation from and supply to the skin of the posterior labia majora/scrotum and inferior vagina.

Note: The artery of vestibule/bulb of penis supplies the corpus spongiosum (bulb of vestibule and penis; greater vestibular gland/bulbo-urethral gland.

Note: The deep aa. of clitoris/penis is a terminal branch of the internal pudendal a. (with dorsal a.) and travels through the perineal membrane and tunica albuginea. This artery supplies the corpora cavernosa through helicine and straight aa. branches.

Note: The dorsal n. and a. of the clitoris/penis are easiest to locate on the dorsal side of the clitoris/penis, and provides blood supply and afferent innervation from the skin of the shaft of the clitoris/penis.

Photo 5. Pudendal n. branches

Photo 6. Internal pudendal a. branches

Examine female erectile bodies and associated musculature.

Female erectile bodies

- Corpus cavernosum of clitoris
  - - Crus of clitoris (pl. = crura of clitoris)
- Ischiocavernosus m.
- Corpus spongiosum
  - - Vestibular bulbs
    - Glans of clitoris
- Bulbospongiosus m.

9. Locate the ischiocavernosus & bulbospongiosus mm.

Note: Superficial to each crus of clitoris is the ischiocavernosus m.

Ischiocavernosus m.:

M1 S12 Ischiocavernosus m.

Note: Superficial to each vestibular bulb is a bulbospongiosus m.

Bulbospongiosus m.:

M1 S12 Bulbospongiosus m. (2)

Photo 7. Ischiocavernosus & bulbospongiosus mm.

10. Unilaterally, reflect/remove the ischiocavernosus and bulbospongiosus mm. to identify the erectile bodies.

Note: Most of the clitoris consists of paired elements of erectile tissue called corpora cavernosa (sing. = corpus cavernosum). The corpora cavernosa of clitoris consist of the crura of clitoris (which run adjacent to the inferior margins of the ischiopubic rami) and the body of clitoris (which consists of the paired, yet separated termini of the corpora cavernosa). The body of clitoris may be palpated through the skin, and the glans of clitoris sits atop the inferior portion of the body of clitoris. The body of clitoris is often referred to as the shaft of the clitoris.

Photo 8. Clitoris

Note: Deep to the labia minora and flanking the vestibule, the bulbs of the vestibule each consist of corpus spongiosum, spongy erectile tissues which unite anteriorly (but are separate compartments) to cover the body of clitoris. The portion of erectile tissue that covers the body of clitoris is the glans clitoris.

Photo 9. Clitoris and vestibular bulb

Examine male erectile bodies and associated musculature.

Male erectile bodies

- Corpus cavernosum of penis
  - - Crus of penis (pl. = crura of penis)
- Ischiocavernosus m.
- Corpus spongiosum
  - - Bulb of penis
    - Glans penis
- Bulbospongiosus m.

11. Locate the ischiocavernosus & bulbospongiosus mm.

Note: The corpus cavernosum in the crus of the penis is covered by the ischiocavernosus m.

M1 S12 Ischiocavernosus m.

Note: The bulbospongiosus m. surrounds the corpus spongiosum at the bulb of the penis.

Bulbospongiosus m.:

M1 S12 Bulbospongiosus m.

12. Unilaterally, reflect/remove the ischiocavernosus and bulbospongiosus mm. to identify the erectile bodies.

Note: In males, as in females, the proximal portions of the corpora cavernosa follow and are closely associated with the ischiopubic rami. These portions of the corpora cavernosa are known as the crura of the penis. As the corpora cavernosa travel through the fundiform ligament of the penis, they become an integral portion of the body of the penis. Surrounded by and supplying the corpora cavernosa are the deep (cavernous) aa. of the penis. The deep aa. of the penis are the terminal brs. of the internal pudendal aa.

Note: The corpus spongiosum is an tubular mass of erectile tissue of the penis, but it has significantly less erectile tissue than the corpora cavernosa, and the tunica albuginea that surrounds the corpus spongiosum is thinner than the layers that surround the corpus cavernosa.

Note: Homologous to the vestibular bulbs in females, the bulb of the penis is a proximal dilation of the corpus spongiosum. The bulb sits just inferior to the perineal membrane where it covers the bulbo-urethral glands and accepts the urethra. At this point, the urethra is the spongy (penile) urethra.

Note: The corpus spongiosum sits ventral to the paired corpora cavernosa in the body of the penis. Towards the distal penis, the corpus spongiosum dilates to form the glans penis, which contains the navicular fossa and external urethral meatus of the urethra.

Photo 10. Corpora cavernosa & ischiocavernosus m.

Photo 11. Corpus spongiosum, bulb of penis, and bulbospongiosus m.

Examine the pelvic diaphragm.

Find these structures:

- Pelvic diaphragm
  - - Levator ani m.
      - Puborectalis m.
        Pubococcygeus m.
        Iliococcygeus m.
    - (Ischio)coccygeus m.
    - Urogenital hiatus

13.) Using blunt dissection, uncover the muscles of the pelvic diaphragm. View the pelvic diaphragm from external (inferior) and internal (superior) to the pelvis. Remember that there is ample fascia in this area occluding the view of muscles, coupled with the fact that many of these muscles are mostly tendinous in presentation. Pubococcygeus m. is only visible from an inferior view.

Note: The levator ani m. forms the bulk of the pelvic diaphragm - the muscular, supportive floor of the pelvis. The levator ani m. consists of several muscles: puborectalis m., pubococcygeus m., and iliococcygeus m. These muscles largely originate on the tendinous arch of levator ani, a tendinous band formed from obturator fascia, connecting the posterior aspect of the body of the pubic bone to the ischial spine. When tense, the levator ani m. elevates the pelvic floor. In turn, this elevates pelvic viscera and aids in the voluntary retention of urine and feces.

Note: Puborectalis m. originates on the posterior aspect of the pubic bone, and wraps around (meets counterpart) the rectum to form a sling. It is often categorized as a component of the pubococcygeus m.

Note: The urogenital hiatus is the anterior gap between pubococcygeus mm. and the pubic symphysis through which the urogenital tracts are conducted.

Note: Pubococcygeus m. originates on the posterior aspect of the pubic bone and pubic ramus (lateral to puborectalis m.)., and attaches to the coccyx.

Note: Iliococcygeus m. originates on the tendinous arch of levator ani (a condensation of the muscular fascia of the obturator internus m.), and attaches to coccyx and anococcygeal raphe. This muscle often appears more aponeurotic than muscular.

Note: (Ischio)coccygeus m. originates on the ischial spines, and insert on the coccyx and inferior portions of the sacrum. Coccygeus mm. lie superficial to the sacrospinous ligaments, and are supplied by the inferior gluteal a., and innervated by direct sacral nn. (brs. of S3 and/or S4).

Photo 12. Pelvic diaphragm

Examine the structures of the rectum

Find these structures:

- Rectum
  - - Longitudinal layer of musculature
    - Rectal ampulla
    - Transverse folds of rectum

14.) Remove peritoneum on the anterior surface of the rectum. Identify the longitudinal layer of musculature that invests the entire length of the rectum.

Photo 13. Rectum

Note: The outer longitudinal muscle layer of the rectum is continuous, which differs from the distinct, separated taeniae coli of the large intestine.

Note: The distal portion of the rectum is dilated and referred to as the rectal ampulla. The more proximal portions are similar in diameter to the sigmoid colon.

Note: The internal rectum typically has three transverse (semilunar, horizontal) folds. These folds are permanent, but are most distinguishable in rectal distension.

Examine the anal canal

Find these structures:

- Anal canal
  - - Internal anal sphincter
    - Longitudinal musculature
    - External anal sphincter
    - Anal columns
    - Pectinate line

15.) Locate the anal canal, and identify its features.

Note: The anal canal begins at the anorectal junction and ends at the anus. The puborectalis m. (part of levator ani m.) forms a sling around the anorectal junction that imparts a flexure between the anus and rectum. As a result, the anal canal is oriented posteriorly.

Note: The mucosal lining of the proximal two-thirds of the anal canal contains numerous infoldings called anal columns. The anal columns terminate at the pectinate line.

Note: The pectinate line demarcates the proximal two-thirds from the distal one-third of the anal canal. The pectinate line is useful to distinguish regions of the anal canal that vary with respect to: embryological origin, neurovascular service, lymphatic drainage, type of epithelial tissue, and classification of hemorrhoid (see table below).

M1 S12 Anal Canal features

Photo 15. Anal canal mucosa

Note: The proximal two-thirds of the anal canal is surrounded by three layers of muscle: the internal anal sphincter (nearest to the alimentary canal), the longitudinal musculature, and the external anal sphincter (surrounding the other two layers).

Note: The internal anal sphincter is comprised of smooth muscle innervated by S4 parasympathetic fibers. At ‘rest,’ the internal anal sphincter is constricted.

Note: The longitudinal musculature is a continuation of the longitudinal smooth musculature of the rectum. The longitudinal musculature atrophies with age, and is replaced by connective tissues.

Note: The external anal sphincter is comprised of skeletal muscle tissue innervated by the inferior rectal nn. (from pudendal nn.). The external anal sphincter is served by the inferior rectal aa.

Photo 16. Anal canal musculature

Identify the divisions of the male urethra.

Find these structures:

- Urethra
  - - Prostatic urethra
    - Membranous urethra
    - Spongy (penile) urethra

16.) Observe the divisions of the male urethra: prostatic, membranous, and spongy (penile).

Note: The male urethra (~18-20 cm in length) is considerably longer than the urethra of a female (~4 cm in length). There are three parts to the male urethra: prostatic, membranous, and spongy.

Note: The prostatic urethra (~3-4 cm in length) is surrounded by the anterior portion of the prostate gland. Prostatic secretions open into the prostatic sinuses, which are located on either side of the urethral crest - a midline elevation of the posterior wall of the prostatic urethra. A more pronounced elevation is located midway of the urethral crest length, the seminal colliculus. The seminal colliculus has a slit-like opening, the prostatic utricle. The ejaculatory ducts open either within or just lateral to the prostatic utricle.

Note: The membranous urethra (~2-2.5 cm in length) is the shortest portion of the male urethra and passes through the perineal membrane. Bulbo-urethral glands drain into this region of the urethra during sexual arousal. The spongy urethra is the distal-most and longest (~15 cm in length, in a flaccid penis) portion of the male urethra, and is surrounded by corpus spongiosum.

Photo 17. Sagittally sectioned male pelvis

Identify the arterial branches of the internal pelvis.

Find these structures:

- Ovarian a. & v.
- Testicular a. & v.
- Common iliac a. & v.
  - - External iliac a. & v.
      - Inferior epigastric a. & v.
    - Internal iliac a. & v.
      - Posterior division branches
        Superior gluteal a.
        Anterior division branches
        Umbilical a.
        Superior vesical aa.
        Medial umbilical ligament (obliterated umbilical a.)
        Obturator a.
        Uterine a.
        Vaginal a.
        Inferior vesical a.
        Prostatic brs. (Male)
        Internal pudendal a.
        Inferior gluteal a.

17.) Identify the ovarian/testicular a. & v. as they enter the pelvic cavity. It may be easiest to locate this gonadal vasculature in the abdomen, and then follow into the pelvis.

Note: Most pelvic viscera receive arterial supply from branches of the internal iliac aa. The gonads are an important exception, deriving blood supply from the ovarian and testicular aa. These arteries are branches of the abdominal aorta (at approximately L2), branching just distal to the renal aa. It is important to be mindful of the long, retroperitoneal course of the gonadal aa. and the ureters from the abdominal cavity into the pelvic cavity in cases of arterial ligation.

Note: The ovarian aa. travel within the suspensory ligament of ovary (infundibulopelvic [IP] ligament) to reach the ovaries. While mostly supplying the ovaries, the ovarian a. also sends branches to supply the uterine tubes and anastomoses with the uterine aa.

Note: The testicular aa. will enter the deep inguinal ring to travel within the spermatic cord to reach and supply the testes.

Photo 18. Gonadal a.

18.) Locate the common iliac a. & v., and identify the bifurcation into the external and internal iliac a. & v. Locate the inferior epigastric a. as it branches from the external iliac a.

Note: The common iliac a. bifurcates into its terminal branches, external & internal iliac aa., at approximately the level of the pelvic brim or the L5/S1 intervertebral disc.

The external iliac aa. are larger than the internal iliac aa., and supply the lower limb predominantly. The internal iliac aa. supply structures associated with pelvic and gluteal regions.

Photo 19. Common iliac a. & v. bifurcation

Photo 20. Inferior epigastric a. branching from external iliac a.

19.) Identify the two divisions of the internal iliac a.: posterior and anterior. The division occurs at approximately the superior portion of the greater sciatic foramen. If a uterus, uterine tubes, and/or ovaries are present in the donor, these viscera will need to be moved (not removed) laterally in order to dissect this vasculature.

Note: The posterior division of internal iliac a. primarily serves musculature of the back and hip. The anterior division of internal iliac a.supplies pelvic viscera & musculature.

Photo 21. Anterior and posterior divisions of internal iliac a.

20.) Locate the superior gluteal a. as it branches from the posterior division of internal iliac a.

Note: The posterior division of internal iliac a. has three main branches: iliolumbar, lateral sacral, and superior gluteal. The superior gluteal a. is the continuation of the posterior division, and is the largest branch of the internal iliac a. It supplies multiple muscles of the pelvis: gluteus maximus, gluteus medius, gluteus minimus, piriformis, and obturator internus mm.

Photo 22. Posterior division of internal iliac a. and superior gluteal a.

21.) Locate branches of the anterior division of internal iliac a.: umbilical a., superior vesical a., obturator a., uterine a., vaginal a., inferior vesical a. and prostatic brs., internal pudendal a., and inferior gluteal a. Follow the patent umbilical a. as it continues onto the anterior abdominal wall as the medial umbilical ligament (obliterated umbilical a.).

Note: The umbilical a. is typically the first branch of the anterior division of internal iliac a. The umbilical a. splits into the superior vesical aa., which supply the superior bladder, distal ends of ureters, and proximal ends of ductus deferens and seminal vesicles, and the medial umbilical ligament (obliterated umbilical a.).

Note: The obturator a. runs medially on the obturator fascia to exit the pelvis via the obturator canal. It supplies branches to the medial compartment of the thigh (adductors of hip) and some pelvic mm.

Note: The inferior vesical a. is variable in origin with possible branching points including directly from anterior division, umbilical a., vaginal a., uterine a. and middle rectal a. This artery supplies the inferior portion of the bladder. In males, the inferior vesical a. has distinct prostatic brs. that supply the prostate, seminal vesicles, and ductus deferens.

Note: The vaginal a. is variable in origin, with the following possibilities: branch of uterine a., independent origin from the internal iliac a., and branch of middle rectal a. This artery will often have branches that supply the inferior bladder (inferior vesical brs.). The vaginal a. will often anastomose with the uterine a.

Note: The uterine a. will anastomose with the ovarian & vaginal aa. At the internal os of the uterus (inferior boundary of the broad ligament), the ureter crosses inferior (‘under’) the uterine a. You may hear this relationship referred to as ‘water [ureter] under the bridge [uterine a.].’ This relationship is particularly important in uterine surgeries when the uterine a. is ligated.

Photo 23. Ureter with uterine a.

Note: The internal pudendal a. exits through the greater sciatic foramen, and then enters the ischio-anal fossa via the lesser sciatic foramen and pudendal canal. It is the primary source of blood to the perineum.

Note: The inferior gluteal a. may be the terminal branch of the anterior division. It exits the pelvis via the greater sciatic foramen inferior to piriformis m. The inferior gluteal a. supplies portions of the pelvic diaphragm, muscles of the hip, and some portions of the hamstring mm.

Photo 24. Anterior division of internal iliac a. and brs. - FEMALE

Photo 25. Anterior division of internal iliac a. and brs. - MALE

Locate the sacral plexus

Find these structures:

- Sacral plexus
  - - Lumbosacral trunk
    - Ventral rami (S1-S4)

22.) On the lateral wall, locate the piriformis m. Moving medially, locate the lumbosacral trunk and ventral rami S1-S4 that form the sacral plexus.

Note: The lumbosacral trunk is formed by a portion of the ventral rami of L4, L5, and S1. It is visible medial to psoas major m., and the lumbar portion joins S1 anterior to the sacro-iliac (SI) joint.

Note: The sacral plexus is composed of the lumbosacral trunk, ventral rami of S1-S3, and a portion of the ventral ramus of S4. The ventral rami of the sacral spinal nerves enter the pelvic cavity through the anterior sacral foramina. The sacral plexus has numerous branches, including superior & inferior gluteal nn., sciatic n., and pudendal n.

Photo 26. Sacral plexus

Dissect and conceptualize the autonomic nerves & plexuses of the pelvic cavity

Find these structures:

- Sympathetics
  - - Sacral sympathetic trunk
      - Sacral splanchnic nn.
- Parasympathetics
  - - Pelvic splanchnic nn.
- Peripheral autonomic plexuses
  - - Superior hypogastric plexus
      - Hypogastric nn.
    - Inferior hypogastric plexus

23.) Locate the sacral sympathetic trunk on the donor, medial to the anterior sacral foramina and sacral plexus. Locate the sacral splanchnic nn., as they branch medially from the sympathetic trunk.

Note: Sacral splanchnic nn. are typically derived from the 1st and 2nd sympathetic sacral ganglia, and extend into the inferior hypogastric plexus.

Photo 27. Sacral sympathetic trunk and sacral splanchnic nn.

24.) Locate the pelvic splanchnic nn., branching from the ventral rami of the sacral nn.

Note: Pelvic splanchnic nn. consist of parasympathetic fibers derived from the ventral rami of S2-S4. The main destination of pelvic splanchnic nn. is the inferior hypogastric plexus, but some fibers travel to the hypogastric nn. or directly to the descending and sigmoid colon.

Photo 28. Pelvic splanchnic nn.

25.) Locate the autonomic plexuses: superior hypogastric plexus, hypogastric nn. and inferior hypogastric plexuses.

Note: The superior hypogastric plexus lies within extroperitoneal connective tissue in the midline, anterior to the abdominal aortic bifurcation. It is derived from three sources: lumbar splanchnic nn. (sympathetic), pelvic splanchnic nn. (parasympathetic), and the aortic plexus (sympathetic and parasympathetic). The organization of this plexus is widely variable, with possible presentations ranging from a web-like appearance to distinct nerve trunks.

Note: The typically paired hypogastric nn. are located between the superior & inferior hypogastric plexuses, medial to internal iliac vasculature and lateral to the anterior sacral foramina. These nerves consist of both sympathetic and parasympathetic fibers.

Note: The inferior hypogastric plexuses are located on either side of the rectum. The main sources of autonomics for this plexus are derived from the pelvic splanchnic nn. (parasympathetic) and sacral splanchnic nn. (sympathetic). Sacral splanchnic nn. are typically derived from the 1st and 2nd sympathetic sacral ganglia. Pelvic splanchnic nn. consist of parasympathetic fibers derived from the ventral rami of S2-S4. The main destination of pelvic splanchnic nn. is the inferior hypogastric plexus, but some fibers travel to the hypogastric nn. or directly to the descending and sigmoid colon.

It is important to note that the inferior hypogastric plexus sends sub-plexuses to most internal pelvic viscera, including uterus, vagina, urinary bladder, seminal vesicles, and prostate.

Photo 29. Peripheral autonomic plexuses

Report abuse