Anatomy of Perineum in a nutshell

Anatomy of Perineum

• Boundaries
  • Superior = pubic symphysis
  • Inferior = coccyx
  • Laterally by ischial tuberosities.
• Is diamond shape.
• Is divided by line joining ischeal tuberosities
  • Posterior triangle is anal triangle. Same in both sexes.
  • Anterior triangle is urogenital triangle
• In anatomical position, the perineum is just a cleft between medial aspects of proximal thighs.
• It becomes triangular in the “lithotomy position”
• Scrotum and penis belong to urogenital triangle and are diverticuli (extensions) in the perineum.

Dermatome map exists in the perineum.

• S3-5 are represented.
• S3 is on inner thighs (majority of area)
• S4 is small portion in anal triangle surrounding the smallest portion, S5, around the anus.

Anal triangle:

• Large muscle: Gluteus maximus. Is not involved in the anal triangle.  It impinges on the area.
• Anus is the midline opening.
  • Anal canal = lst 2-3 cm of GI.
  • Anus is the aperture itself.
• Since pelvic diaphragm is sloping toward midline, the area on either side of anus has depth.
  • It is not a flat plane, it is pyramidal in shape.
  • The base of the pyramid is skin
  • The apex is deep into recess on either side of anal canal.
• These are the ischioanal fossae (Right and Left).
• They are filled with fat which helps to compress anal canal.
• There is a communication posterior to anus between the two fossa. Apex are fibers of diaphragm taking origin from abdominal wall.

• Obturator Internis (373)
  • Muscle of lower limb.
  • The muscle lies in the pelvis and leaves through lesser sciatic foramen via tendon.
  • The fascia of the obturator intermis is very thick

• Provides attachment for the origin of pelvic diaphragm.
• This is where the apex of pyramid is for ischioanal fossae

• External anal sphincter:
  • Voluntary muscle surrounding the anal canal.
  • Used to override the peristaltic function.
  • Has a somatic nerve supply.
  • Has subcutaneous , superficial, and deep parts.

Pudendal canal:

• The obturator internis muscle passes medial to the ischial fossa.  On the medial side of this muscle, its fascia forms the Pudendal canal
• The nerves and vessels for the perineum come though here.
• Have pudendal nerve, internal pudendal artery / vein that give off branches as they go through the canal.
• Pudendal nerve:
  • Inferior rectal nerve:
• First branch of pudendal nerve.
• Pierces wall of canalà goes through fat to reach the external anal sphincter.

• Has somatic innervation for skin as well ( perianal skin).

• Has companion branches of artery / vein (called inferior rectal artery / vein).
• Thus, the only structures in the fat are the inferior rectal vein, artery, nerve.
• The inferior rectal nerve may have multiple branches.

1. • The vein, artery,nerve in the pudendal canal will continue on to supply the urogenital triangle.
   • Pathway:

1. 1. • Pudendal nerve through greater sciatic foramen. Innervates nothing in gluteal region
      • goes right into lesser sciatic foramen to go to the perineum.
      • It passes through 3 regions: pelvis (part of sacral plexus)→ leaves through greater sciatic foramen→ passes through lesser sciatic foramenà into perineum.

• It gives off inferior rectal branch and branches to urogenital triangle.

Anal canal:

• Rectum ends when it encounters the rectal floor
• The portion of the rectum that pierces musculature is the anal canal.
• At the junction, have longitudinal ridges = anal columns.
  • These are in the mucosa, and are caused by arteries and veins that lie in the mucosa.
  • These are associated with the superior rectal artery and vein (The superior rectal artery is a continuation of the inferior mesenteric artery)
  • These are only on the upper portion of the anal canal.
• At the lower edge of each anal canal, have anal valves:
  • Little sac-like folds of mucus membrane that connect lower portion of adjacent columns.
  • Mucus is forced out of these to allow feces to pass.
• Pectinate line: the line of junction between portion of anal canal derived from endoderm of hindgut versus that derived from ectoderm (mucus membrane lining lower part of anal canal)
  • Upper portion: derived from endoderm

• The portion with anal columns
• Has no pain, pressure, tempà only stretch. Thus, different innervation between upper / lower parts and also have different blood supply.

• • Innervation is autonomic.

• Gets blood from the superior rectal artery.
• Veins draining the upper part are tributaries of the portal veinous system.
• Lymphatic drainage upper half is into pelvic lymph nodes.

1. • Lower partion:

• Gets blood from inferior rectal artery (branch of internal pudendal).
• Venous drainage is part of cabal system.

• • Lower half lymph drains into inguinal lymph nodes.

Internal hemorrhoids:

• Vessels in upper half of anal canal (esp. veins) can become distended (varicose).
• Veins may project into columnà hard stool may cause bleeding.
• Distension of veins that are a part of the portal system.
• Prolapse:
  • The venous enlargements have become so enlarged that they are projecting through the anus.
  • Are covered by mucous membrane (difference between internal and external hemorrhoids).
  • When they are exposed to external atmosphere, they may itch, but they aren’t painful.
• Considered submucosal.

External hemorrhoids:

• Just deep to perianal skin.
• Associated with inferior rectal vein.
• They are only covered by skin, they are considered subcutaneous.
• These are not as much of a problem.

Tears in lining of anal canal:

• Anal fissure: tear in the mucous membrane lining in the anal canal.
  • Depending on where fissure is will determine where pain is.
  • Usually are in inferior aspect.
  • They spontaneously heal like other tears.
• Problem is infection in the fat of ischeal fossa.
  • This creates an abcess that will “point” which means it reaches the surface through a fistula.
  • Abscess may reach surface of skin.

The urogenital triangle has 2 “pouches”

• Superficial pouch
• Deep pouch
• Separation is made based on location of fascia

Fascia:  Urogenital Triangle (UG)

• Anterior abdominal wall: the superficial fascia has 2 layers, a fatty camper’s and membranous scarpa’s layer.
• The Superficial fascia continues into the perineum (since we have skin there, must have superificial fascia).  It continues into UG triangle.
• Male:
  • Most of the fat in the fatty layer (extension of campers, but not called campers) is lost.
  • That fat is then picked up again when move to anal triangle.
  • There is no fat deep to skin on penis or scrotum, only loose CT
  • In the scrotum, there is also a layer of smooth muscle = dartos muscle.
• These muscles insert into the dermis of the skin, thus when they contract, it pulls on the skin.
• Will cause a change in texture of scrotum. Contraction causes “orange peel” apperance to decrease the size of the scrotal sac.

• • Decreasing the size regulates the distance of testicles from body wall.

• • • Also, cremaster muscles in spermatic cord pulls on testicle itself to change its elevation. About a 3 degree difference between body and testicles.
    • On the shaft of the penis, it is impossible to differentiate between the membranous portion of the superficial fascia (now called Colles’ fascia) and Dartos layer. Consider it to be lined by just “dartos layer (tunic)”.

Female:

• The fat persists, has enough fat to form ridges in the skin called labia majora.
• T=They are fat filled elevations of the skin.  The fat increases / decreases with age.
• There is no fat deep to skin over clitoris (same concept as penile skin in male)

• Colles’ fascia is continuous with scarpa’s fascia. It is only a name change.

Perineal Body:

• Membranous layer of superficial fascia in the UG triangle of the perineum: called Colles’ fascia.  This is not distinguished from Dartos in the penis / clitoris.
• In both sexes, have a  fibromuscular (collagen, muscle fibers, etc.) tuft of tissue that lies anterior to anus = perineal body.
  • Colles’ fascia fuses with this perineal body at the midline.
  • This establishes a barrier between the UG triangle and the anal triangle.
  • Fluid will not flow between these two triangles.
• Becomes an issue in the male because the urethra comes through pelvis / pelvic floor
• Urine runs out tip of penis where ends as external urethral orifice.
• Trauma to base of penis (bike seat injury) may cause a tear in the urethra. This means that every time urinating, urine will escape (called extravasation of urine) which will gather in the UG triangle.  It may line the scrotal sac, line shaft of penis, line abdominal wall.
• 1. Analogous trauma for female (bike seat) will not injure urethra.
• The superficial perineal pouch (space) is the space created by the limits of Colles’ fascia.
  • This is where urine would gather in a urethra tear.
  • Clitoris, labia, scrotum all a part of this pouch.

Pelvic Floor: (345)

• Genital hiatus: The area of the pelvic floor / outlet that is not closed by the pelvic diaphragm muscle.
• Another diaphragm covers this space = Urogenital diaphragm.
  • This diaphragm is on a flat transverse plane between the rami of the pubis.  Vagina and urethra (male just urethra) pass through this diaphragm.

• • UG diaphragm (366) spans the subpubic arch.  3 parts

• Inferior most fascia- called perineal membrane.
  1. Is the deepest part of the superficial perineal pouch (roof of the pouch).
  2. This is the barrier between the superficial and deep pouches.
  3. This is deep fascia.
• Superior most fascia- another portion of deep fascia

• Middle layer of muscle tissue between the two. (lies in deep perineal pouch)

• The deep perineal pouch is the space between the two layers of fascia and houses the muscular layer between them.
• Both layers of fascia are anchored to perineal body.

Male UG triangle: superficial pouch (between perineal membrane and colles fascia). (364).

• Can see cut edge of colles fascia
• 3 elevations, one in midline→ flanked by 2 lateral elevations
  • Muscle coating central elevation = bulbospongiosus muscle.
• This was a paired muscle that fused together.
• Has inferior most fibers inserting into perineal body.
• • Laterally, have cylinder of muscle: ichialcavernosus muscle (each side).
  • None of these extend into shaft of penis, they are only associated with root of penis.
• The perineal body is at midline between anus and base of the bulbospongiousus
• Superficial transverse perineal muscle:
  • Another paired muscle on either side inserting into the perineal body
  • In the superficial pouch, on transverse plane, is in perineum.

(365). Muscles have been removed: see 3 cylinders of erectile tissue.

• Tissue is like a sponge, cavernous system of venous sinusoids.
• Sinusoids become engorged with blood.
• Penis anchored by 2 crura and central bulb of erectile tissue that is partially encased by muscle (bulbospongiosus / ischiocavernosus muscle)
• 3 bodies attached in UG triangle:
• Bulb: central body of erectile tissue
  • The urethra comes through UG diaphragm→ through perineal membrane→ through bulb of penis.
  • It is in the core of the bulb.
  • Bulb continues into the shaft and changes name to corpus spongiosum.
  • This erectile tissue does not become as turgid as lateral bodies
• Due to urethra running through
• For ejaculate to pass through urethra (within the bulb)

1. • Bulb distally expands to form cap over the blunt tips of the corpora cavernosa.

• This tip is the glans (acorn like).
• The glans has a rich network of sensory nerve endings (richest in penis).

• Lateral erectile bodies = Right and Left Crura (Crus).
  • Right and left crura extend into shaft of penis and change name to corpora cavernosum.  These are continuations of the crus into the shaft.
  • The two corpora cavernosa end bluntly on distal end. Are fused.

On the subject of foreskin:

• Double fold of skin covers glans in uncircumcised males.
• It may form complete sac, or be variable.
• This fold of skin is the foreskin = prepuce.
• If the male is not circumcised, there are secretions in that fold of skin on deep side.
• Glands release secretions in this space, and this space may be invaded for foreign matter = smegma.

(364) Cross section of shaft of penis:

• In the flaccid penis, the dorsal surface is facing anterior.  Anatomical position is only possible when penis is erect and angles upward.
• Erectile cylinder corpus spongiosum contains portion of urethra called the spongy part of the urethra.
• Dorsal to corpus spongiosum have paired corpus cavernosum with denser array of venous sinusoid.
  • They fuse in the center.
  • Each erectile body has outer core of connective tissue (especially corpus cavernosum) called the tunica albuginea.
• External to the 3 erectile bodies, have collective investment of fascial sheath called Buck’s fascia.
  • Have spetum that separates corpus spongiosum from c. cavernosum.
  • This is deep fascia of the penis, thicker than in other areas of the penis,
• External to Buck’s fascia is superficial fascia
  •  Here referred to as Dartos tunica / layer (just deep to skin).
  • On shaft of penis, Dartos is loose connective tissue which makes a loose connection between deep fascia and skin.
• Superficial dorsal vein: Single vein on dorsal aspect of penis beneath skin.
  • Is on its own, no companion artery.
  • This has a serpantine pathway as it courses on penis.
  • Deep to this, have Buck’s fasciaà deep to this have a series of vessels:

• Deep dorsal vein. This is the vein that drains the erectile tissues.  This is flanked on either side by:
• Dorsal arteries of the penis. These branch from the internal pudendal artery (supplies all of perineum- anal triangle, UG triangle).  Each artery flanked by:

• Dorsal nerve of the penis (branch of pudendal)

• Thus, have 5 structures on dorsal surface deep to Buck’s fascia.

1. 1. Erection:

• When erectile bodies swell, they compress the deep vein, thus escaping blood is cut off from escape.
• The blood is thus held captive within the prison of the erectile tissue.

Pudendal Canal: (385) with internal artery, vein, pudendal nerve. A branch of the internal pudendal artery goes to superficial pouch = perineal artery (with companion vein).

• Inferior rectal branch of internal pudendal artery = to anal canal.
• Perineal artery of internal pudendal = structures of superficial pouch.
• The internal pudendal artery enters the deep pouch (goes superior to perineal membrane).

Pudendal nerve (from sacral plexus, S2-S4):

• Gives off inferior rectal nerve (to anal triangle) perianal skin, external anal sphincter.
• Gives off branch (perineal nerve) to all superficial pouch and all of deep pouch, thus all of UG triangle of perineum.  The sensory distribution is to skin (posterior scrotum etc.)
• Pudendal nerve gives off the perineal branch (terminal branch) and thus, no longer called pudendal nerveà continues on to the superficial pouch as dorsal nerve of penis.  This is pure sensory, only on shaft of penis.
• 3 branches to remember: inferior rectal nerve, perineal nerve, dorsal nerve of the penis.

Perineal membrane (366) is deepest part of perineal pouch.

• Thin outlines show attachment point for erectile bodies.  The base of the penis is anchored to the perineal membrane.  See outlines for bulb and R/ L crus.
• The urethra is also coming out of UG diaphragmà pierces the perineal membrane.
• Passing inferior to the pubic symphysis is a single vein that empties into venous plexus around the prostate gland (posterior venous plexus) = the deep dorsal vein of the penis.
• See the edge of the superficial transverse perineal muscle (in the superficial pouch).

Remove the perineal membrane: In the deep pouch

• See a terminal branch of the pudendal nerve (dorsal nerve of penis) approaching the pubic symphysis where it leaves the deep pouchà out through perineal membrane into superficial pouch→ onto penis.  Thus, the perineal nerve is in both deep / superficial pouches.
• The artery remains deep pudendal artery until goes onto crus of penis to become dorsal artery of penis and other branch becomes deep artery of penis.  In the center of each corpus cavernosum is a deep artery of the penisà terminal branch of the pudendal artery.
• See bulbourethral gland (paired glands) near the urethra.  They release secretions slightly before semen = called pre-ejaculate.  They neutralize acidity of remnant urine and vagina.  They are accessory reproductive glands.
• Sheet of musculature that occupies deep pouch:
  • Posterior edge = deep transverse perineal muscle.  (had superficial in superficial pouch on other side of perineal membrane).
  • The remaining musculature contributes to sphincteric action on the urethra as it passes through diaphragm.

• This is the external urethral sphincter.
• It is a circule of fibers that have voluntary action on sphincter.

• • Considered better developed in males than females.

• Muscles of pelvic floor are distended during vaginal deliverà their muscle tone may be compromised.
• Must retone those muscles to retain continence. These are Kegal exercisesà retone muscles of pelvic diaphragm and UG diaphragm.

(368). Urinary tract.

• Urethra has uniform length until it reaches the glans of the penis→ expands as the navicular fossaà opens as external urethral orifice.
• Parts of urethra:
  • Prostatic: The portion that passes through the prostate gland.
  • Membranous: small portion passing through external urethral sphincter. This is the narrowest part, least distensible.
  • Spongy: the portion that is running through the penis, traveling through the corpus spongiosum.
• A catheter will go in easy through spongy urethra, when reach pelvic floor, the external urethral spinincter compresses wall.  This segment in the deep pouch = membranous portion of the urethra.

Associated glands:

• Prostate gland is an organ of the pelvis, sits on the urogenital diaphragm.
• Bulburethral glands AKA cowpers glands which are in the deep pouch.
  • These are deep to the prostate
  • Deep to bulbourethral glands, find spongy urethra.
  • Each cowpers gland has a duct that empties into the spongy urethra, thus the duct must pierce the perineal membrane to reach the bulb of the penis.
  • Gland itself is in deep pouch, but it empties into the superificial pouch.

Female

Opening comments:

Pudendal nerve: Gives off the inferior rectal nerve and then continues toward UG triangle.  As it approaches it, it divides into 2 terminal branches.

• Perineal nerve goes into the superficial pouch. Innervates all structures in superficial pouch and small branches innervate structures in the deep pouch.
• Dorsal nerve of penis / clitoris runs in deep pouch. This is pure sensory, passes through deep pouch to get to distribution center on penis.

Action of bulbous spongiosus:

• Anchors bulb of penis to perineal membrane.
• Helps force last drops of urethra out of spongy urethra.
• Also contracts involuntary during ejaculation.
• This muscle contracts during sexual arousal to help force blood into shaft.

Ischiocavernosus muscle (surrounding crus):

• Helps anchor erectile body.
• Contracts during sexual arousal around crus to maintain erection.

Female Fascia:

• Has labial folds in place of scrotum.
• The colles’ fascia is associated with labia majora.
• In the female, lack dartos fascia, but does have persistent fat.
• All descriptions of superficial fascia are the same except now have labia majora replacing the scrotum.

Female structures:  (359)

• Mons Pubis:
  • Situated Anteriorly in female UG triangle over pubic symphysis. the female has fat deposit subcutanously over pubic symphysis
  • It is continuous with camper’s fascia of anterior abdominal wall.
• Labia Majora:
• • 2 skin folds that exend posteriorly.
  • They meet anteriorly at mons pubis and course toward anus.
  • They meet again just anterior to the anus.
  • Posteriorly they have less fat.
  • Labia majora have hair on lateral border but not on medial border.
• Perineal body: Just deep to the skin between anus and vaginal opening.
• Vulva:
  • Refers to all of the external genitalia of the female.
  • Includes everything between the two labia majora.
• During reproductive years, fat is deposited into the labia majora.
  • Fat is less after menopause and before puberty.
  • Labia minora project outward in post menopausal woman because majora have regressed due to fat loss.
• Labia minora:
  • 2 folds of very vascular hairless thin skin.
  • The core of the labia minora are considered “pseudoerectile tissue”.
  • During arousal, they do become engorged with blood and more rigid.
  • They also meet posteriorly and anteriorly.
  • Anteriorly, they come into association with the clitoris.
• They reflect over the glans of the clitoris to form the prepuce (foreskin) over the clitoris.
• It is very variable just as in the male.
• • Come together posteriorly and meet to form a ridge called the frenulum of labia minora.
  • There is also an anterior frenulum called the frenulum of the clitoris which is where labia minora have portion passing beneath the clitoris forming the frenulum.
  • Labia minora will help direct the stream of urine as it comes out of urethral orifice.
• Vestibule:
  • This is the region flanked by the labia minora.
  • Glans of the clitoris is the most anterior structure in the vestibule.
  • Posteriorly, find the external urethral orifice.
  • Vaginal orifice.
• Varies with age / sexual activity.
• Have a thin fold of mucous membrane that projects there in virgin = hymen.
  • It narrows the lumen, not completely occlude.
  • It formation, it was an imperforate membrane.

• • Greater Vestibular Glands: (AKA Bartholin’s glands)

• At posterior edge of the vaginal orifice, have small openings of their ducts
• Secretes small amount of mucous regulary, but releases more during sexual arousal.

1. • Also have small ducts associated with urethra that secrete mucous as well.  Very hard to see.

Deeper female structures (361) -in the superficial pouch.

• Ischiocavernosus muscle:
  • Along each pubic ramus on each side, running on ichiopubic ramus.
  • Terminates as erectile body enters the clitoris.
  • Very similar to male.
• Bulbospongiosus muscle:
  • 2 distinct muscles near the midline.
  • Remember, male bulbuspongiosus muscle was initially paired and then came together.
  • They won’t come together in the female because of the vestibule, thus have 2 distinct bulbospongiosus muscles on either side of the vestibule.
• Bulbs of the vestibule:
  • Deep to the bulbospongiosus muscle, find erectile tissue.
  • In the male, these tissues fuse to form bulb.
  • In female, have erectile body on either side of the vestibule= “bulbs of the vestibule”.
• Thus, have 4 total erectile bodies.
  • 2 crura and 2 bulbs of the vestibule.
  • Each is associated with overlying muscle.
• Can also find a gland in the superficial pouch on posterior edge of the vestibule = Greater vestibular gland (Bartholin’s gland).

(384).  Vessels:

• Are the same in both sexes.
• The internal pudendal artery branches the same way as it does in the male
  • Internal pudendal in lateral wall of anal canal gives off inferior rectal artery and comes anterorly to give off perineal artery to superficial pouch
  • Goes on as internal pudendal artery into deep pouch where it divides into terminal branches

• Dorsal artery of clitoris
• Deep artery to the crus (deep artery of the clitoris) which continues out into body of the clitoris.

• Nerves: same distribution, same branching.  Inferior rectal, perineal nerve, dorsal nerve of clitoris, all have same patterns.

Differences in sexes: related to body of clitoris

• Clitoris has 2 erectile bodies in the shaft (not 3).
  • These are the continuations of the crura (beneath ischiocavernosus).
  • These enter the shaft of the clitoris to become the “corpora cavernosa” in the clitoris.
• Bulb:
  • in male it continues in shaft of penis.
  • In female, it does not continue into shaft of clitoris.
  • 2 bulbs come anteriorly, meet, and stop.
  • They do not extend into shaft of clitoris.
  • Glans of clitoris is not connected to any erectile body-  It is a cap over the distal ends of the corpora cavernosa.
• Termination of urethra: in female, it ends in the vestibule, it is much shorter than in male.
• Function of the clitoris:
  • Pure sensory organ.
  • Sexual arousal only.
  • Is rather enigmatic to most men.
• Dorsal surface of the clitoris:
  • Faces anteriorly when it isn’t aroused (same as in male).
  • Will see same arrangement: superifical dorsal vein under skin, paired deep arteries around deep vein, and nerves as well (see male arrangement).
• Have superficial transverse perineus muscle in superficial pouch, is very small.

Perineal membrane of the Urogenital diaphragm:

• Perineal membrane is inferior layer of fascia of UG diaphragm
• Deep to the perineal membrane is the deep pouch of the perineum
• In the deep pouch, have muscular layer.
  • Deep transverse perineal muscle
  • external urethral sphincter
  • Sphincter Vagini:

• These muscles in the deep pouch are associated with vaginal wall
• Have spincter action on walls of vagina.

• Biggest gender difference here is that vagina is passing through urogenital diaphragm.
• Deep pouch has same vessel / nerve distribution.

Perineal body (361).

• Attachment for:
  • Bulbospongiosa posterior fibers
  • Transverse perineus (both superificial / deep)
  • External anal sphincter all attach here.
• This acts as central tendon.
• Common for incision in posterior vaginal wall to the perineal body: avoids tearing of the vaginal opening.
  • This is called an episiotomy.
  • May be midline or lateral.
  • Most do lateral to avoid vaginal / anal fistula.

(362). Female Urinary Tract

• Urinary bladder:
  • Sits on pelvic floor (no gland to rest on as the prostate in the male).
  • It is supported by muscles of the pelvic floor.
• Female urethra has membranous portion: as it passes through pelvic diaphragm = membranous urethra.
  • This is the most narrow part due to action of urethral sphincter on it.
  • Musculature surrounds it prior to entering the deep pouch.
• Associated structures:
  • Glands distributed along the female urethra.  The female urethra may be embedded in the vaginal wall.  Some of the CT in the area has bound them together.
  • In the anterior vaginal wall near the vestibule, have collection of sensory nerve endings.
• Stimulation of this collection of nerve endings can lead to a female orgasm. This would specifically be a vaginal orgasm as opposed to a clitoral orgasm.
• This is the “G spot”.

• • Closely associated with these nerve endings are some urethral and pareurethral glands (called Skene’s glands).

• The urethral dump right into urethra whereas paraurethral are a unification of many glands that empty near opening of urethra.
• Some women can have a release of fluid from these glands which is considered to be female ejaculate. It is not urine, but it passes through the urethra.

Miscellaneous:

• Round ligament of the uterus comes into the fat of the labia majora.  It is just fibrous strands by the time it enters the labia majora.
• Colles’ fascia is beneath the subcutaneous fat layer of labia majora.
  • In superifical pouch, have erectile body on either side of vestibule that is flanked by bulbuspongiosa muscle.
  • See superior fascia of UG diaphragm which marks the end of the boundary between the pouches.

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