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7. Pelvis, perineum

 

 

 

< Pelvic inlet and outlet >

 

 

This book does not emphasize identification of the individual bones (ilium, ischium, and pubis) in a hip bone, as if that in a skull is not emphasized.

 

Fig. 7-1.BMP

Fig. 7-1. Greater and lesser pelvises enclosed by pelvic skeleton.

 

The right and left hip bones, sacrum, and coccyx are assembled to make up the pelvic skeleton. The pelvic skeleton contains greater and lesser pelvises. The greater pelvis is part of the abdominal cavity. (So its other name is false pelvis.) In other words, the greater pelvis contains abdominal organs such as the cecum and the descending colon (Fig. 6-27), not pelvic organs.

The lesser pelvis, which is also known as the true pelvis, is divided into the pelvic cavity and the perineum by the pelvic diaphragm. Numerous structures are simultaneously situated in the pelvic cavity and the perineum (Fig. 7-10). Therefore, the two are usually regarded as a single region, like the title of this chapter (pelvis, perineum).

The pelvis consists of the pelvic cavity and the pelvic wall that are composed of the pelvic skeleton, ligaments, and muscles (Figs. 7-5,6,10). The pelvic diaphragm can be called the muscular pelvic floor.

The superior and inferior apertures of the lesser pelvis are the pelvic inlet and outlet, respectively. During delivery, a baby enters the lesser pelvis through the pelvic inlet, and then exits through the pelvic outlet.

 

Fig. 7-2.jpg

Fig. 7-2. Pelvic inlet surrounded by pelvic brim.

 

The bony outline of the pelvic inlet is called the pelvic brim. (We ignore the term “linea terminalis” which is a portion of the pelvic brim.) The pelvic brim includes two kinds of joints: cartilaginous joint (Fig. 1-6) (pubic symphysis) and synovial joints (Fig. 2-57) (sacroiliac joints). These joints shift during a delivery, or due to a trauma.

 

Fig. 7-3.jpg

Fig. 7-3. Lesser pelvis in anatomical position.

 

 

One should stand up and touch one’s anterior superior iliac spine and pubic crest, which are situated on a coronal plane. The pelvic outlet consists of the urogenital triangle that is almost horizontal and the anal triangle that is nearly coronal in the anatomical position.

 

Fig. 7-4.BMP

Fig. 7-4. Pelvic outlet.

 

This two-dimensional diagram (Fig. 7-4) and coronal plane (Fig. 7-1) may be misleading; some may think that the two triangles are located on the same plane. However, it is crucial to remember that the two triangles lie on different planes (Fig. 7-3).

While the urogenital triangle is bordered by bone, the anal triangle is bordered by the sacrotuberous ligament. The boundary between the two triangles is an imaginary line between bilateral ischial tuberosities.

 

Fig. 7-5.BMP

Fig. 7-5. Sacral ligaments and foramina.

 

The sacrotuberous and sacrospinous ligaments cover the lesser and greater sciatic notches, and thus turn those notches into foramina.

 

 

The sacrotuberous and sacrospinous ligaments bind not only to the sacrum but also to the coccyx.

The ischial spine is located between the greater and lesser sciatic foramina. The pelvic diaphragm which is attached on the ischial spine divides off the pelvic cavity and the perineum (Fig. 7-1). Consequently, the greater and lesser sciatic foramina can be thought as the gates of the pelvic cavity and the perineum, respectively. The pelvic cavity is greater and the perineum is lesser in size (Fig. 7-10).


< Muscles >

 

The piriformis, obturator internus, and pelvic diaphragm must be studied in order to understand the architecture of the pelvis and the perineum.

 

Fig. 7-6.BMP

Fig. 7-6. Piriformis, obturator internus (lateral view).

 

Fig. 7-7.BMP

Fig. 7-7. Piriformis, obturator internus, obturator externus (horizontal plane).

 

In reality, Fig. 7-7 is not a horizontal plane. The greater and lesser sciatic foramina cannot coincide in a plane (Fig. 7-5).

The piriformis starts from the sacrum and travels anterolaterally. The obturator internus starts from the internal surface of the obturator membrane and travels backward first, then forward (back and forth muscle). Respectively, the two muscles pass the greater and lesser sciatic foramina to end at the greater trochanter (Fig. 8-17).

Both the piriformis and the obturator internus perform a single action, the lateral rotation of the femur. The obturator externus from the external surface of the obturator membrane also has a similar insertion and thus similar action (Fig. 8-8). Recall that in most cases, the insertion, rather than the origin, of a muscle decides its action.

 

Fig. 7-8.BMP

Fig. 7-8. Pelvic inlet, outlet, and wall (anterosuperior view).

 

In the anterosuperior view of the pelvic skeleton, both the pelvic inlet and outlet are visible because the pelvic inlet (encircled by pelvic brim) is wider than the pelvic outlet (composed of anal and urogenital triangles) (Fig. 7-1).

In succession, the components of the pelvic wall (sacrum, coccyx, greater and lesser sciatic foramina) can also be observed in this anterosuperior view. Note that the greater sciatic foramen is closer to the pelvic inlet, while the lesser sciatic foramen is closer to the pelvic outlet. The border between the greater and lesser foramina is the ischial spine as previously mentioned (Fig. 7-5).

 

Fig. 7-9.BMP

Fig. 7-9. Pelvic diaphragm, piriformis, obturator internus (anterosuperior view).

 

The levator ani and coccygeus make up the pelvic diaphragm which shields the pelvic outlet from above (Fig. 7-1). In the same anterosuperior view, we can recognize the piriformis and obturator internus that are parts of the pelvic wall (Fig. 7-6).

Repeatedly, the piriformis starting from the sacrum travels anterolaterally through greater sciatic foramen (superior to ischial spine). The obturator internus first travels backward, passes through the lesser sciatic foramen (inferior to ischial spine), then travels forward (Figs. 7-6,7). The lower part of the obturator internus is hidden by the levator ani. A coronal plane below further demonstrates this spatial relationship.

 

Fig. 7-10.BMP

Fig. 7-10. Pelvis, perineum (coronal plane of anal triangle).

 

Strictly, a coronal plane does not show the whole rectum and anal canal simultaneously (Fig. 7-12). When dealing with the anal canal, we suppose that the anal triangle is horizontal to make it simple (Fig. 7-13,38)

Whereas the upper part of the obturator internus belongs to the pelvic wall, its lower part belongs to the perineum. The lower part cannot be seen from the above due to the levator ani. Peculiarly, the origin of the levator ani is a thickened fascia of the obturator internus, called the tendinous arch (Fig. 7-9).

 

 

The broad levator ani starts with an aponeurosis. However, its origin is known as the tendinous arch, rather than the aponeurotic arch. Such is the same with the central tendon (Fig. 5-47), the common tendinous ring (Fig. 4-46), and the tendinous intersection (Fig. 6-4).

Also, the tendinous arch is an arch that extends from the pubis to the ischial spine (Fig. 7-15). Speaking repeatedly, the tendinous arch serves as the origin of the levator ani which makes up the majority of the pelvic diaphragm (Fig. 7-9). Recall that the ischial spine is on the border between the pelvis and the perineum (Figs. 7-1,5).

The levator ani is in contact with the boundary of the rectum and anal canal as an insertion (Fig. 7-10). Because this muscle lies obliquely, its contraction induces an “elevation” of the muscle itself and the “anal” canal, hence the name is “levator ani.”

 

 

The level of the umbilicus is an ideal spot for observing both the diaphragm (Figs. 5-47,48) and the pelvic diaphragm of a cadaver.

In the coronal plane of the anal triangle (Fig. 7-10), one can identify a space between the obturator internus (attached to “ischium”) (Fig. 7-6) and “anal” canal. This space is the “ischioanal” fossa that contains subcutaneous tissue fat (Fig. 7-13).

During dissection, students should remove the fat in the ischioanal fossa to touch the inferior surface of levator ani. Simultaneously, they should touch its superior surface with another hand to feel the thickness of the levator ani. In other words, the two hands are placed in the perineum and pelvic cavity, separately (Fig. 7-10).

The other noteworthy structure in the coronal plane is the pudendal canal, formed by the fascia of the obturator internus (Figs. 7-10,15).

 

Fig. 7-11.jpg

Fig. 7-11. Pelvic diaphragm, passing structures.

 

Three parts of the levator ani are iliococcygeus, pubococcygeus, and puborectalis which are hardly distinguishable during the dissection.

 

Fig. 7-12.BMP

Fig. 7-12. Puborectalis.

 

The puborectalis makes flexure between the rectum and anal canal. Therefore, the muscle needs to be relaxed during defecation.

The puborectalis also encircles the urethra, prostate (male) (Fig. 7-23), and vagina (female) (Fig. 7-32). The prostate is the organ intervening between the pelvic cavity and the perineum.

Coccygeus, a small part of the pelvic diaphragm, is a very thin muscle that is located just above the sacrospinous ligament (Figs. 7-5,8). This negligible muscle doe not participate in the elevation of the anal canal, unlike the levator ani.

Students are recommended to imagine these three muscles of the pelvis and perineum (piriformis, obturator internus, pelvic diaphragm) by using a dry pelvic skeleton.

 

 

Nerves to all three muscles from sacral plexus (Fig. 7-15) are too short to have specific names. Since these short nerves are hardly disconnected, they are not important in clinics; this is another reason why they exist without specific names.

 

Fig. 7-13.BMP

Fig. 7-13. Perineum (sagittal plane).

 

The perineum is drawn as if the anal triangle were placed on a horizontal plane. One should note that this schematic figure that shows the ischioanal fossa is not a midsagittal plane (Fig. 7-10).

The urogenital triangle contains the urogenital diaphragm and the root of penis/clitoris. The inferior fascia of the urogenital diaphragm is the perineal membrane which is thick and easily discernible during the dissection. The perineal membrane is a clear border between the deep perineal pouch (comprising the urogenital diaphragm) (Fig. 7-14) and the superficial perineal pouch (comprising the root of penis/clitoris) (Figs. 7-29,37). The posterior median thickening of the perineal membrane is the perineal body (Figs. 7-14,29,37).

 

Fig. 7-14.BMP

Fig. 7-14. Urogenital diaphragm, external anal sphincter.

 

The urogenital diaphragm consists of the external urethral sphincter and the deep transverse perineal muscle. The main muscle, external urethral sphincter, constricts urethra and vagina in female (Figs. 7-23,25,30,32,33).

The accessory muscle, deep transverse perineal muscle, fixes the perineal body. The perineal body is fixed by the external anal sphincter (Fig. 7-39) as well. The fixation of the perineal body is beneficial for supporting the pelvic diaphragm and pelvic organs (Fig. 7-13).

The skeletal muscles in the root of penis/clitoris will be discussed in the urogenital system (Figs. 7-29,37). Innervation of perineal muscles (by perineal nerve) will be soon described (Fig. 7-16).

Between the oblique pelvic diaphragm and the horizontal urogenital diaphragm, there is another connective tissue (represented by asterisks (*) in Figs. 7-13,23,30,32). However, we will ignore it, as it has little clinical importance.


< Nerves >

 

Fig. 7-15.BMP

Fig. 7-15. Lumbosacral plexus (medial view).

 

The brachial plexus innervates the upper limb including the pectoral region, scapular region, and even the superficial back (Figs. 2-14,15). Likewise, the lumbosacral plexus (from the anterior rami of LN2–SN3) innervates the pelvis, perineum, and the lower limb (Fig. 1-17).

We have chosen to only emphasize the motor nerves of both plexuses. In the figure above, branches of the lumbosacral plexus are depicted separately like the branches of the cervical plexus (Figs. 3-14,15,16).

From LN2–LN4, the femoral nerve and the obturator nerve (Fig. 6-55) arise to control thigh muscles (Fig. 8-2). The obturator nerve passes through the obturator membrane to descend down to thigh muscles that it innervates. It implies that the obturator membrane does not cover the obturator foramen completely (Fig. 7-7).

Both the superior gluteal nerve (LN4–SN1) and inferior gluteal nerve (LN5–SN2) pass through the greater sciatic foramen. As their names imply, the superior and inferior gluteal nerves proceed superior and inferior to the piriformis, respectively (Figs. 7-6, 8-18).

LN4 and LN5 constitute the lumbosacral trunk (Fig. 6-55) that joins the sacral nerves (Fig. 7-18). The lumbosacral “trunk” is equivalent to the superior, middle, inferior “trunks” of the brachial plexus (Fig. 2-13).

The sciatic nerve, the thickest nerve in the body, is composed of five spinal nerves (LN4–SN3); it is noteworthy considering that the whole brachial plexus is composed of the five spinal nerves (Fig. 2-13). The sciatic nerve is distributed to vast regions of the lower limb (posterior thigh, whole leg, and whole foot) (Figs. 8-2,27,43). Accompanying the inferior gluteal nerve, the sciatic nerve advances inferior to the piriformis (Figs. 8-18,19).

 

 

SN4 belongs to the pudendal nerve, but not to the lumbosacral plexus. This irony may be explained by the fact that SN4 is too thin to contribute a significant amount to the lumbosacral plexus.

The pudendal nerve (SN2–SN4) passes the pelvic cavity, greater sciatic foramen (inferior to the piriformis), gluteal region, lesser sciatic foramen, and finally the perineum. The pudendal nerve getting out and in reminds us of the spinal root of XI getting in and out of the cranial cavity (Fig. 3-20). After the pudendal nerve reaches the perineum, it moves forward in the pudendal canal (Fig. 7-10).

 

Fig. 7-16.BMP

Fig. 7-16. Branches of pudendal nerve.

 

The 1st branch of the pudendal nerve is the inferior rectal nerve which innervates the external anal sphincter (Figs. 7-14,39). The 2nd branch is the perineal nerve which governs the urogenital diaphragm (deep perineal pouch) (Fig. 7-14) and the root of penis/clitoris (superficial perineal pouch) (Figs. 7-29,37). The last branch is the dorsal nerve of penis/clitoris which is a sensory nerve (Fig. 7-28).

Following the meaning of the Latin word “pudendus” (embarrassing), the nerve is related to the three “embarrassing” activities: defecation, urination, and sexual intercourse.

 

Fig. 7-17.BMP

Fig. 7-17. Sympathetic and parasympathetic nerves of pelvic cavity, perineum.

 

Let us finish the long story of autonomic nerve here. The autonomic innervation for the pelvic cavity and perineum is provided by the superior and inferior hypogastric plexuses. The superior one receives the sympathetic and parasympathetic impulses from the lumbar splanchnic nerve and X, respectively (Fig. 6-49).

Successively, the inferior hypogastric plexus receives the sympathetic impulse from the hypogastric nerve (origin: superior hypogastric plexus). In addition, the inferior hypogastric plexus receives sympathetic impulse from the sacral splanchnic nerves (Fig. 7-18).

 

Fig. 7-18.BMP

Fig. 7-18. Sacral and pelvic splanchnic nerves for inferior hypogastric plexus.

 

The sacral splanchnic nerves originate from the five sacral ganglia that are paravertebral ganglia on the sympathetic trunk (Route C in Fig. 3-18).

The terminal station of the sympathetic trunk is the ganglion impar which is the fusion of bilateral coccygeal ganglia. The ganglion “impar” means ganglion “without pair.” This term has us recall the “azygos” vein, a vein “without pair” (Fig. 5-45).

It is the pelvic splanchnic nerves that provide parasympathetic impulse for the inferior hypogastric plexus (Fig. 7-17). The pelvic splanchnic nerves originate from SN2–SN4 and pass the anterior sacral foramina like other anterior rami of sacral nerves.

 

 

It is easy to remember that S (sacral) is S (sympathetic), while P (pelvic) is P (parasympathetic) for the inferior hypogastric plexus (Fig. 7-17).

The superior and inferior hypogastric plexuses give off the postganglionic fibers of the sympathetic nerve and the preganglionic fibers of the parasympathetic nerve (Fig. 3-17). All the fibers are distributed to the smooth muscles in the pelvic cavity and the perineum. The inferior hypogastric plexus is closer to the internal iliac artery (Fig. 7-17), so that its fibers comfortably travel along with the arterial branches (Figs. 7-20,21).

In order to send additional sympathetic impulse to the perineum, the postganglionic fibers from sacral ganglia go through the gray rami communicantes to join the anterior rami of SN2–SN4 (Figs. 3-18,19, 7-18). The pudendal nerve from these anterior rami conveys sympathetic impulse to the perineum (Figs. 7-15,16).


< Arteries, lymph nodes >

 

 

Like the common carotid artery (Fig. 3-27), the common iliac artery divides into the external and internal branches. The external iliac artery is thicker than the internal iliac artery because the external iliac artery leads to the far toes.

 

Fig. 7-19.BMP

Fig. 7-19. Two branches of external iliac artery.

 

Also like the common carotid artery (Fig. 3-27), the common iliac artery has no branches. The external iliac artery gives off only two branches. These are the deep circumflex iliac artery that travels along the iliac crest and the inferior epigastric artery (Fig. 6-30) that forms the lateral umbilical fold (Fig. 6-28).

The branches of the internal iliac artery can be categorized either by their origins (from anterior division, from posterior division) or by their destinations (for pelvic wall, for pelvic cavity). This book follows the latter categorization.

 

Fig. 7-20.BMP

Fig. 7-20. Branches of internal iliac artery to pelvic wall (medial view).

 

Among the six branches for the pelvic wall, the obturator artery, superior and inferior gluteal arteries, and internal pudendal artery accompany their corresponding nerves (Fig. 7-15).

Unlike the pudendal nerve, the pudendal artery requires the adjective “internal” because there also exists “external” pudendal artery from the femoral artery (Fig. 8-12). Nevertheless, the branches of internal pudendal artery (e.g., inferior rectal artery) are identical with the branches of pudendal nerve (Figs. 7-16,28).

The remaining two branches among the six are the “iliolumbar” artery going to the “iliacus” and quadratus “lumborum” (Fig. 6-54), and the lateral sacral artery entering the sacral canal through anterior sacral foramina (Fig. 7-18).

 

Fig. 7-21.BMP

Fig. 7-21. Branches of internal iliac artery to pelvic cavity (medial view).

 

There are three branches for male, and four for female, of the internal iliac artery that go to the pelvic cavity. The inferior vesical artery of the male is homologous to the vaginal artery of the female. The female also has the uterine artery, which has no counterpart in the male.

In both sexes, there are the umbilical artery and the middle rectal artery. The umbilical artery gives off the superior vesical artery going to the urinary bladder, then becomes the medial umbilical ligament (Fig. 6-36) inside the medial umbilical fold (Fig. 6-28).

Eventually, all three rectal arteries have been introduced: the superior rectal artery from the inferior mesenteric artery (Fig. 6-44), the middle rectal artery directly from the internal iliac artery, and the inferior rectal artery indirectly from the internal iliac artery (by way of internal pudendal artery) (Fig. 7-20).

The branches of the internal iliac artery can be memorized by a sentence. “I Like Getting Ultimate Item In My Very Own Underwear.” The words stand for Internal pudendal, Lateral sacral, Gluteal (superior and inferior), Uterine, Iliolumbar, Inferior vesical, Middle rectal, Vaginal, Obturator, Umbilical arteries. Students should be careful to not be confused by the three Is and two Us of acronyms.

 

Fig. 7-22.BMP

Fig. 7-22. Lymph drainage of lower limb, pelvis, perineum, abdominal wall.

 

The lymph in the lower limb ascends to the external iliac node via the deep inguinal node (Fig. 8-10) or the superficial inguinal node (Fig. 8-3). The lymph in the pelvis and perineum goes to the internal iliac node. The entire lymph flows to the common iliac node and then the lumbar node where it encounters the lymph from the abdominal wall. Eventually the lymph from the abdominal wall, pelvis, perineum, and lower limbs passes the lumbar lymphatic trunk to empty into the chyle cistern (Fig. 6-47). The intestinal lymphatic trunk (from abdominal cavity) has been introduced in the abdomen chapter (Fig. 6-46).


< Male urogenital system >

 

Fig. 7-23.BMP

Fig. 7-23. Urinary bladder, male urethra.

 

We are ready to talk about the organs in the pelvic cavity and perineum. The urinary bladder is the first.

Like the inside of the heart (Figs. 5-15,17,18,19), the inside of the urinary bladder has rough and smooth areas. The small smooth area, named the trigone of bladder, is continuous with two ureters and one urethra. The extension of trigone of bladder is the urethral crest in urethra (Fig. 7-30).

 

 

In anatomy, any hollow, expandable organs such as the urinary bladder and stomach (Fig. 6-31) are described in their empty states.

 

 

Male urethra, which is responsible for the passage of both urine and semen, is affiliated not only with the urinary system but also with the genital system.

 

Fig. 7-24.BMP

Fig. 7-24. Male urogenital system.

 

Male urethra is divided into the intramural part, prostatic urethra, membranous urethra, and spongy urethra.

 

Fig. 7-25.BMP

Fig. 7-25. Male pelvis, perineum (midsagittal plane).

 

In the midsagittal plane, the spongy urethra is curved twice. The distal curve is due to the suspensory ligament of penis that suspends the border between the root and body of the penis.

At ordinary times, we keep urinary continence by contraction of the internal urethral sphincter (smooth muscle) around the intramural part and the external urethral sphincter (skeletal muscle) (Fig. 7-14) around the membranous urethra. However during urination, we relax these sphincters; simultaneously, we contract the detrusor (smooth muscle) in the urinary bladder (Fig. 7-23) and the external oblique muscle, etc. (skeletal muscle) in the anterolateral abdominal wall (Figs. 6-2,3,5).

For both male and female (Fig. 7-30), harmonious contraction and relaxation of the smooth and skeletal muscles are essential in urinary continence and urination. If such contraction and relaxation do not take place, voluntary and controlled urination would be difficult.

During ejaculation, the internal urethral sphincter contracts while the external urethral sphincter relaxes (Fig. 7-23).

 

Fig. 7-26.BMP

Fig. 7-26. Urinary bladder, prostate, ductus deferens.

 

 

The ductus deferens passes the inguinal canal (Figs. 6-9, 7-24) and reaches the back of the urinary bladder. Strangely, the ductus deferens hooks around the distal end of the ureter. This is because of the upward migration of the kidney during the developmental stage.

The ductus deferens then encounters the duct of seminal vesicle to become the ejaculatory duct. The short ejaculatory duct penetrates the prostate to enter the prostatic urethra (Fig. 7-24).

 

Fig. 7-27.BMP

Fig. 7-27. Prostatic urethra.

 

The bilateral ejaculatory ducts open on the seminal colliculus, the expanded part of the urethral crest (Fig. 7-23). The seminal colliculus includes the prostatic utricle also. Surprisingly, the seminal colliculus and prostatic utricle of male are homologous to the hymen and vagina of female (Fig. 7-32). Unlike the female structures, the two male structures are small and not of importance.

The several prostatic ducts open into the prostatic sinuses, on both sides of the seminal colliculus. Notably, the two depressed sites (“prostatic” utricle and “prostatic” sinus) start with the word “prostatic.”

During the ejaculation, the prostate as well as the seminal vesicle discharge their secretions. The secretions help sperms meet the ovum.

 

 

In clinics, the ductus deferens is called the vas deferens. Therefore, the excision of the ductus deferens for contraception is vasectomy. The amount of the semen is barely reduced after the vasectomy, because the secretions from the seminal vesicle and prostate are much larger than sperms in volume.

The “bulbourethral” gland is located in the deep perineal pouch, close to the “bulb” of the penis. It secretes fluid into the spongy “urethra” (Fig. 7-24). The fluid lubricates the urethra for ejaculation.

 

Fig. 7-28.BMP

Fig. 7-28. Penis (transverse plane).

 

The spongy urethra goes through the corpus spongiosum. The other two erectile tissues are the corpora cavernosa. The corpus spongiosum is smaller than a single corpus cavernosum. If the corpus spongiosum were large, its erection would obliterate the spongy urethra located inside, which would interrupt the ejaculation.

 

 

In etymology, both “spongiosum” and “cavernosum” mean sponge. Yet these two must be distinguished.

Corpora cavernosa are encircled by a thick connective tissue, called the tunica albuginea. The tunica albuginea prevents blood from leaking when the penis is erected. The erectile tissue is like a bag filled with sponge.

 

 

Simply put, the erection is caused by the widened artery to penis and the narrowed vein from penis.

Smooth muscles in the artery and vein of the penis are innervated by the sympathetic nerve (Fig. 7-18) which has passed the inferior hypogastric plexus (Fig. 7-17); another route is the pudendal nerve (Fig. 7-16). Smooth muscles are also controlled by the parasympathetic nerve (Fig. 7-18) which has passed the inferior hypogastric plexus too.

 

 

A man urinates in a peaceful state with no one else around. It is because parasympathetic nerve induces urination. The parasympathetic nerve is for Pee.

On the other hand, a man ejaculates in a war (?) state with someone. It signifies that sympathetic nerve results in ejaculation. Reversely, erection is affected by the parasympathetic nerve.

 

Fig. 7-29.BMP

Fig. 7-29. Body and root of penis.

 

At the root of penis, the straight corpus spongiosum (proximal end: bulb of penis) is a bit shorter than the oblique corpus cavernosum (proximal end: crus of penis). Instead, at the body of penis, the corpus spongiosum (distal end: glans penis) is longer. The glans penis is surrounded by skin called the prepuce.

The muscle that surrounds the “bulb” and corpus “spongiosum” is the “bulbospongiosus.” This muscle squeezes urine and semen to the body of penis so as to finish urination and ejaculation voluntarily. It also squeezes blood to the body of penis to contribute to erection.

The bulbospongiosus and the superficial transverse perineal muscle hold the perineal body in place (Fig. 7-13). The superficial and deep transverse perineal muscles (Fig. 7-14) can be found in the superficial and deep perineal pouches, respectively.

The muscle that originates from the “ischial” tuberosity and enclose the crus of penis (corpus “cavernosum”) is called the “ischiocavernosus.” The ischiocavernosus squeezes blood to the body of penis in order to assist erection voluntarily.

As told, all muscles in the root of penis are innervated by the perineal nerve (Fig. 7-16).

 

 

Unlike the root of penis, the body of penis does not have any skeletal muscle. Therefore, no exercise can make the body of penis hypertrophy.


< Female urogenital system >

 

Fig. 7-30.BMP

Fig. 7-30. Urinary bladder, female urethra.

 

 

Without the prostate and the penis, a female has neither a prostatic urethra nor a spongy urethra. Therefore, the female urethra, just encircled by the internal and external urethral sphincters, is relatively short and is not divided into different parts.

The female urethra belongs only to the urinary system. Therefore, urologists and gynecologists deal with the urinary and genital systems of female separately.

 

Fig. 7-31.BMP

Fig. 7-31. Parts of uterine tube, ovulation.

 

We have moved to the female genital system. The uterine tube is divided into the fimbriae, infundibulum, ampulla, isthmus, and the intramural part. One of the fimbriae is the ovarian fimbria which is in contact with the ovary. The ampulla is the longest and widest part which provides a good environment for the fertilization of the ovum and the sperm.

The intramural part is often regarded as the part of the uterus just like the intramural part of the urethra is regarded as the part of the urinary bladder (Fig. 7-24). Students are suggested to answer that the intramural parts belong to the uterine tube and urethra by their official affiliation.

 

Fig. 7-32.BMP

Fig. 7-32. Female genital system.

 

Size of the uterus is nearly equivalent to that of a woman’s fist. The uterus consists of the body and cervix. The border between the body and cervix is the isthmus where the uterine cavity narrows.

The body of uterus includes fundus, while the body of stomach does not include fundus (Fig. 6-31). Sometimes, the anatomy is distant from the consistency.

The cervix protrudes into the vagina to form the luminal corner (vaginal fornix) (Fig. 7-33). (In neuroanatomy, students will learn the fornix of the hippocampus. The fornix is not a lumen but a bunch of neurons; another inconsistency.)

 

 

Inside of the uterus is lined by a simple columnar epithelium which is efficient for the repetitive destruction and reconstruction of the uterus wall that occur during menstrual cycles. However, the vagina and the vaginal part of cervix are lined by stratified squamous epithelium which serves as a protective barrier during coitus (Fig. 7-32). Consequently, the cervix is the transitional zone of the two kinds of epithelium and is thus prone to cancer.

 

Fig. 7-33.BMP

Fig. 7-33. Female pelvis, perineum (midsagittal plane).

 

 

The uterus is anteverted to make an angle with the vagina. Accordingly, the uterus covers the urinary bladder from above. This is why a pregnant woman feels the urge to urinate frequently.

In a midsagittal plane, peritoneum around the uterus forms the rectouterine and vesicouterine pouches that are parts of the peritoneal cavity. The rectouterine pouch is close to the vaginal fornix, both of which are simultaneously palpable in female cadaver by two hands (Fig. 7-33). In case of male, only rectovesical pouch exists (Fig. 7-25); (rectouterine – uterus) + (vesicouterine – uterus) = rectovesical.

 

 

In the terms “rectouterine pouch” and “vesicouterine pouch,” the term “uterus” always comes after the terms “rectum” and “urinary bladder.”

 

 

The rectovesical pouch contains liquid (serous fluid), while the deep and superficial perineal pouches contain solid (muscles, etc.) (Fig. 7-13).

 

Fig. 7-34.BMP

Fig. 7-34. Peritoneum in abdominal and pelvic cavities.

 

A non-pregnant uterus is situated in the pelvic cavity, and the uterus is surrounded by the peritoneum and the peritoneal cavity (Fig. 7-33). This proves that the peritoneal cavity is present not only in the abdominal cavity but also in the pelvic cavity. The pelvic inlet between the abdominal and pelvic cavities is not blocked (Fig. 7-1).

 

 

It is a common misunderstanding that the peritoneal cavity is included only in the abdominal cavity.

 

 

We can make our upper limb mimic the uterine tube (Fig. 7-31). At that time, fingers should face backward like the fimbriae are directed to the ovary which is posterior (Fig. 7-35).

 

Fig. 7-35.BMP

Fig. 7-35. Broad ligament of uterus.

 

In the figure above, one can easily identify the mesenteries that hold the uterus (mesometrium) and the uterine tube (mesosalpinx). Between the two mesenteries, another mesentery (mesovarium) holds the posterior ovary. Those three mesenteries are collectively called the broad ligament of uterus.

The ovary and uterus are directly connected by the ligament of ovary in the mesovarium. The ligament is continuous with the round ligament of uterus in the mesometrium. Remember that the round ligament of uterus occupies the female inguinal canal (Fig. 6-13).

Obviously, the three structures (uterus, uterine tube, and ovary) are intraperitoneal organs (Fig. 7-34). The ovary makes and sends ovum regularly (ovulation). Just after the ovulation, the ovum strangely enters the peritoneal cavity. Then the ovum is swallowed by the uterine tube, the entrance of which is opened to the peritoneal cavity (Fig. 7-32). Fimbriae of the uterine tube move to swallow the ovum (Fig. 7-31).

 

 

One may think that when a woman is in a swimming pool, the water flows through her vagina, uterus, and uterine tube to enter the peritoneal cavity. However, this is misunderstanding because the lumina are too thin and long for water to get in. In the case of uterine tube, it allows only the passing of the ovum. The size of ovum is only 0.1 mm even if it is the biggest human cell.

At the inferior end of the vagina, there are three structures: the hymen, labia minora which are mucosa, and the labia majora which are skin. The space between the bilateral labia minora is the “vestibule” of vagina; the greater “vestibular” glands open into this space (Figs. 7-32,36). The glands are not easy to see grossly. Moreover, the lesser vestibular glands are not visible at all during the dissection.

 

 

The bulbourethral gland (male) and greater vestibular gland (female) are homologous, even though the former is in the deep perineal pouch (Fig. 7-24) and the latter is in the superficial perineal pouch (Fig. 7-37). The secretion of the bulbourethral gland is a lubricant for ejaculation, while the greater vestibular gland’s secretion is a lubricant for coitus.

 

Fig. 7-36.BMP

Fig. 7-36. Vestibule of vagina, adjacent structures.

 

Anterior to the hymen (ruptured in the figure), opening of urethra and glans clitoris are located in order. The glans clitoris is another term for the body of clitoris, although the glans penis is a part of the body of penis. The glans clitoris is covered by prepuce like the glans penis (Fig. 7-29).

 

Fig. 7-37.BMP

Fig. 7-37. Female superficial perineal pouch.

 

Dissection reveals that the bulb of vestibule is located deep to the labium minus. The bulb of vestibule is equivalent to the male corpus spongiosum (Fig. 7-29). However, they are quite different: The bulb of vestibule is double and has no urethra inside unlike the male corpus spongiosum.

The clitoris bears more similarities to the equivalent male structure, the corpus cavernosum. The body of clitoris is continuous with the root of clitoris on both sides; like the male corpus cavernosum (Fig. 7-29). The border between the body and root is held by the suspensory ligament of clitoris, just as in the male (Fig. 7-25).

This book presents six suspensory ligaments in total: the suspensory ligaments of breast (Fig. 2-6), axilla (Fig. 2-20), lens (Fig. 4-44), duodenum (Fig. 6-32), penis (Fig. 7-25), and clitoris.

Fortunately, the superficial perineal pouch of the female has muscles with identical names as those of the male (Fig. 7-29).

“Bulbospongiosus” which covers the “bulb” of vestibule (corpus “spongiosum”) helps the constriction of vagina. This action is supported by the external urethral sphincter in female (Figs. 7-14,32,33). “Ischiocavernosus” which originates from the “ischial” tuberosity and covers the root of clitoris (corpus “cavernosum”) squeezes the blood and helps the body of clitoris erect. The superficial transverse perineal muscle assists the fixation of the perineal body like that of the males (Fig. 7-29). These muscles are voluntarily contracted by the perineal nerve (Fig. 7-16).


< Rectum, anal canal >

 

Fig. 7-38.BMP

Fig. 7-38. Rectum, anal canal.

 

The pelvic cavity includes the sigmoid colon and rectum, whereas the perineum includes the anal canal (Fig. 7-10). The rectum and the anal canal of the cadaver are usually viewed from the posterior.

Rectum is the reservoir of feces, just as the urinary bladder keeps urine. The rectum has transverse folds, unlike the colon which does not have such folds. The transverse folds of rectum prevent small amount of feces from migrating toward the anal canal. In other words, the transverse folds limit the frequency of defecation.

 

Fig. 7-39.BMP

Fig. 7-39. Anal canal.

 

To keep the fecal continence, the internal anal sphincter (smooth muscle by visceral motor nerve) and the external anal sphincter (skeletal muscle by inferior rectal nerve (Fig. 7-16)) contract. The internal anal sphincter is an expansion of the internal circular muscle. During defecation, the sphincters relax; concurrently, the large intestine muscle (smooth muscle) contracts for peristalsis and the anterolateral abdominal wall muscles (skeletal muscle) contract (Figs. 6-2,3,5). Recall that similar harmonious muscle movements are needed for urinary continence and urination (Figs. 7-24,30).

The anal columns are structures that bulge out due to the blood vessels inside. The inferior ends of the anal columns are connected by the anal valves. The anal valve creates the anal sinus, just as the aortic valve builds the aortic sinus (Fig. 5-24). However, the anal valves are too tiny to be functional.

Anal valves used to be the anal membrane between the hindgut and the proctodeum of an embryo. The proctodeum is the subsided skin like the stomodeum (primitive oral cavity) (Figs. 6-16,29). Anal valves and the inferior ends of anal columns form the pectinate line. The pectinate line is of great significance, because it serves as a border, prior and post to which many differences arise.

Above the pectinate line, the simple columnar epithelium (for gastrointestinal tract) lines, while below it, the stratified squamous epithelium (for skin) lines.

Above the pectinate line, a branch of inferior hypogastric plexus (visceral sensory nerve) senses (Fig. 7-17), while below it, the inferior rectal nerve (somatic sensory nerve) senses (Fig. 7-16).

 

 

The superior rectal artery (Fig. 6-44) supplies the rectum and the anal canal above the pectinate line, while the inferior rectal artery (Fig. 7-20) supplies the anal canal below the pectinate line. Additionally, both areas are supplied by the middle rectal artery (Fig. 7-21). The artery names do not include the anal canal.

The anal canal is only as long as 3–4 cm. When index finger is inserted into the anal canal, it soon reaches the rectum. Therefore, this medical procedure is called digital rectal examination, not digital anal examination. The anal canal is habitually disregarded.

 

Fig. 7-40.jpg

Fig. 7-40. Portal and systemic circulations from anal canal.

 

The superior, middle, and inferior rectal veins have the same distribution as the arteries. Only blood above the pectinate line is drained into the superior rectal vein, and then to the inferior mesenteric vein, and the portal vein (Fig. 6-45). Blood below the pectinate line is drained into the inferior rectal vein (excluding the middle rectal vein), and then to the internal pudendal vein, then the internal iliac vein; such pattern follows that of the arteries (Fig. 7-20). Above the pectinate line is called the portal circulation; below the pectinate line is the systemic circulation. The portal and systemic circulations anastomose in the anal canal; in normal state, the anastomosis is grossly undetectable.

However, when the liver is pathologically hardened (e.g., liver cirrhosis), the blood pressure in the portal vein rises. It congests the venous blood in the anal canal above the pectinate line and activates the blood flow in the anastomosis between the portal and systemic circulations. Similar venous blood congestion happens in the esophagus, umbilicus, etc. where similar anastomosis pattern exists between the portal and systemic circulations.

The lymph above the pectinate line is drained into the internal iliac node (Fig. 7-22), while the lymph below the line is drained into the external iliac node by the way of the superficial inguinal node (Figs. 7-22, 8-3).


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