FEMALE EXTERNAL GENITAL ORGANS
The female external genitalia (the vulva) include the
mons pubis, labia majora, labia minora, clitoris, vestibule, the greater
vestibular glands (Bartholin’s glands) and bulbs of the vestibule
• The mons pubis is a rounded pad of fat lying anterior
to the symphysis pubis. It is covered with pubic hair from the time of puberty.
• The labia majora (‘greater lips’) are two folds of fat and areolar tissue which are
covered with skin and pubic hair on the outer surface and have a pink, smooth
inner surface.
• The labia minora (‘lesser lips’) are two
small subcutaneous folds, devoid of fat, that lie between the labia majora.
Anteriorly, each labium minus divides into two parts: the upper layer passes
above the clitoris to form along with its fellow a fold, the prepuce, which
overhangs the clitoris. The prepuce is a retractable piece of skin which
surrounds and protects the clitoris. The lower layer passes below the clitoris
to form with its fellow the frenulum of the clitoris.
• The clitoris is a small rudimentary sexual
organ corresponding to the male penis. The visible knob-like portion is located
near the anterior junction of the labia minora, above the opening of the
urethra and vagina. Unlike the penis, the clitoris does not contain the distal
portion of the urethra and functions solely to induce the orgasm during sexual
intercourse.
• The vestibule is the area enclosed by the
labia minora in which the openings of the urethra and the vagina are situated.
• The urethral orifice lies 2.5 cm posterior to
the clitoris and immediately in front of the vaginal orifice. On either side
lie the openings of the Skene’s ducts, two small blind-ended tubules 0.5 cm
long running within the urethral wall
• The vaginal orifice, also known as the
introitus of the vagina, occupies the posterior two-thirds of the vestibule.
The orifice is partially closed by the hymen, a thin membrane that tears during
sexual intercourse. The remaining tags of hymen are known as the ‘carunculae
myrtiformes’ because they are thought to resemble myrtle berries.
• The greater vestibular glands (Bartholin’s
glands) are two small glands that open on either side of the vaginal
orifice and lie in the posterior part of the labia majora. They secrete mucus,
which lubricates the vaginal opening. The duct may occasionally become blocked,
which can cause the secretions from the gland to accommodate within it and form
a cyst.
• The bulbs of the vestibule are two elongated
erectile masses flanking the vaginal orifice.
Blood supply
The blood supply comes from the internal and the external
pudendal arteries. The blood drains through corresponding veins.
Lymphatic drainage
Lymphatic drainage is mainly via the inguinal glands.
Innervation
The nerve supply is derived from branches of the
pudendal nerve
THE PERINEUM
The perineum corresponds to the outlet of the pelvis
and is somewhat lozenge-shaped. Anteriorly, it is bound by the pubic arch,
posteriorly by the coccyx, and laterally by the ischiopubic rami, ischial
tuberosities and sacrotuberous ligaments. The perineum can be divided into two
triangular parts by drawing an arbitrary line transversely between the ischial
tuberosities. The anterior triangle, which contains the external urogenital
organs, is known as the urogenital triangle and the posterior
triangle, which contains the termination of the anal canal, is known as the anal
triangle.
The urogenital triangle
The urogenital triangle is bound anteriorly and laterally
by the pubic symphysis and the ischiopubic rami. The urogenital triangle has
been divided into two compartments: the superficial and deep perineal spaces,
separated by the perineal membrane which spans the space between the
ischiopubic rami. The levator ani muscles are attached to the cranial surface
of the perineal membrane.
The vestibular bulb and clitoral crus lie on the
caudal surface of the membrane and are fused with it. These erectile tissues
are covered by the bulbospongiosus and the ischiocavernosus muscles.
Superficial muscles of the perineum
Superficial transverse perineal muscle
The superficial transverse muscle is a narrow slip of
a muscle that arises from the inner and forepart of the ischial tuberosity and
is inserted into the central tendinous part of the perineal body.
The muscle from the opposite side, the external anal sphincter (EAS) from behind,
and the bulbospongiosus in the front, all attach to the central tendon of the
perineal body.
Bulbospongiosus muscle
The bulbospongiosus (previously known as bulbocavernosus)
muscle runs on either side of the vaginal orifice, covering the lateral aspects
of the vestibular bulb anteriorly and the Bartholin’s gland posteriorly.
Some fibres merge posteriorly with the superficial
transverse
perineal muscle and the EAS in the central fibromuscular perineal body.
Anteriorly, its fibres pass forward on either side of the vagina and insert
into the corpora cavernosa clitoridis, a fasciculus crossing over the body of
the organ so as to compress the deep dorsal vein. This muscle diminishes the
orifice of the vagina and contributes to the erection of the clitoris.
Ischiocavernosus muscle
The ischiocavernosus muscle is elongated, broader at
the middle than at either end and is situated on the side of the lateral
boundary of the perineum. It arises by tendinous and fleshy fibres
from the inner surface of the ischial tuberosity, behind the crus clitoridis,
from the surface of the crus and from the adjacent portions of the ischial
ramus.
Innervation
The nerve supply is derived from branches of the
pudendal nerve.
The anal triangle
This area includes the anal canal, the anal sphincters
and the ischioanal fossae.
Anal canal
The rectum terminates in the anal canal.
The anal canal is attached posteriorly to the coccyx by the anococcygeal
ligament, a midline fibromuscular structure that runs between the posterior
aspect of the EAS and the coccyx. The anus is surrounded laterally and
posteriorly by loose adipose tissue within the ischioanal fossae, which is a
potential pathway for spread of perianal sepsis from one side to the other. The
pudendal nerves pass over the ischial spines at this point and can be accessed
for injection of local anaesthetic into the pudenal nerve at this site.
Anteriorly, the perineal body separates the anal canal
from the vagina. The anal canal is surrounded by an inner epithelial lining, a vascular
subepithelium, the internal anal sphincter
(IAS), the EAS and fibromuscular supporting tissue.
The lining of the anal canal varies along its length
due to its embryologic derivation. The proximal anal canal is lined with rectal
mucosa (columnar epithelium) and is
arranged in vertical mucosal folds called the columns
of Morgagni. Each column contains a terminal radical of the superior
rectal artery and vein. The vessels are largest in the left-lateral,
right-posterior and right[1]anterior
quadrants of the wall of the anal canal where the subepithelial tissues expand
into three anal cushions. These cushions seal the anal canal and help maintain continence
of flatus and liquid stools. The columns are joined together at their inferior
margin by crescentic folds called anal valves. About 2 cm from the anal
verge, the anal valves create a demarcation called the dentate line.
Anoderm covers the last 1–1.5 cm of the distal canal below
the dentate line and consists of modified squamous epithelium that lack skin
adnexal tissues such as hair follicles and glands, but contains numerous
somatic nerve endings. Since the epithelium in the lower canal is well supplied
with sensory nerve endings, acute distension or invasive treatment of
haemorrhoids in this area causes profuse discomfort, whereas treatment can be carried
out with relatively few symptoms in the upper canal lined by insensate columnar
epithelium. As a result of tonic circumferential contraction of the sphincter,
the skin is arranged in radiating folds around the anus and is called the anal
margin. These folds appear to be flat or ironed out when there is underlying
sphincter damage.
The junction between the columnar and squamous epithelia
is referred to as the anal transitional zone, which is variable in height and
position and often contains islands of squamous epithelium extending into
columnar epithelium. This zone probably has a role to play in continence by
providing a highly specialized sampling mechanism.
Anal sphincter complex
The anal sphincter complex consists of the EAS and IAS
separated by the conjoint longitudinal coat (Fig. 3.3). Although they form a
single unit, they are distinct in structure and function
External anal sphincter
The EAS comprises of striated muscle and appears red
in colour (similar to raw red meat). As the EAS is normally under
tonic contraction, it tends to retract when completely torn. A defect of the
EAS can lead to urge faecal incontinence.
%20which%20is%20red%20in%20colour%20and%20appears%20like%20raw%20red%20meat.webp "An intact external anal sphincter (E) which is red in colour and appears like raw red meat") |
| An intact external anal sphincter (E) which is red in colour and appears like raw red meat |
Internal anal sphincter
The IAS is a thickened continuation of the circular
smooth muscle of the bowel and ends with a well-defined rounded edge 6–8 mm
above the anal margin at the junction of the superficial and subcutaneous part
of the EAS. In contrast to the EAS, the IAS has a pale appearance to the naked
eye. Defect of the IAS can lead to passive soiling of stools and
flatus incontinence.
%20is%20pale%20in%20colour%20and%20appears%20like%20raw%20white%20meat.webp "The internal anal sphincter (I) is pale in colour and appears like raw white meat") |
| The internal anal sphincter (I) is pale in colour and appears like raw white meat |
The longitudinal layer and the conjoint longitudinal coat
The longitudinal layer is situated between the EAS and
IAS and consists of a fibromuscular layer, the conjoint longitudinal coat and
the intersphincteric space with its connective tissue components.
Innervation of the anal sphincter complex
The nerve supply is derived from branches of the pudendal nerve.
Vascular supply
The anorectum receives its major blood supply from the
superior haemorrhoidal (terminal branch of the inferior mesenteric artery) and
inferior haemorrhoidal (branch of the pudendal artery) arteries, and to a
lesser degree, from the middle haemorrhoidal artery (branch of the internal iliac),
forming a wide intramural network of collaterals.
The venous drainage of the upper anal canal mucosa,
IAS and conjoint longitudinal coat passes via the terminal branches of the
superior rectal vein into the inferior mesenteric vein. The lower anal canal
and the EAS drain via the inferior rectal branch of the pudendal vein into the internal
iliac vein.
Lymphatic drainage
The anorectum has a rich network of lymphatic
plexuses. The dentate line represents the interface between the two different
systems of lymphatic drainage. Above the dentate line (the upper anal canal),
the IAS and the conjoint longitudinal coat drain into the inferior mesenteric
and internal iliac nodes. Lymphatic drainage below the dentate line, which
consists of the lower anal canal epithelium and the EAS, proceeds to the
external inguinal lymph nodes.
The ischioanal fossa (previously known as the
‘ischiorectal fossa’) extends around the anal canal and is bound ante[1]riorly
by the perineal membrane, superiorly by the fascia of the levator ani muscle
and medially by the EAS complex at the level of the anal canal. The ischioanal
fossa contains fat and neurovascular structures, including the pudendal nerve
and the internal pudendal vessels.
The perineal body
The perineal body is the central point between the urogenital
and the anal triangles of the perineum. Within the perineal body
there are interlacing muscle fibres from the bulbospongiosus, superficial transverse
perineal and EAS muscles. Above this level there is a contribution from the
conjoint longitudinal coat and the medial fibres of the puborectalis muscle.
Therefore, the support of the pelvic structures, and to some extent the hiatus
urogenitalis between the levator ani muscles, depends upon the integrity of the
perineal body
THE PELVIC FLOOR
The pelvic floor is a musculotendinous sheet that
spans the pelvic outlet and consists mainly of the symmetrically paired levator
ani muscle (LAM), which is a broad muscular sheet of variable
thickness attached to the internal surface of the true pelvis. Although there
is controversy regarding the subdivisions of the muscle, it is broadly accepted
that it is subdivided into parts according to their attachments, namely the
pubovisceral (also known as pubococcygeus), puborectal and iliococcygeus. The pubovisceral
part is further subdivided according to its relationship to the viscera, i.e.,
puboperinealis, pubovaginalis and puboanalis. The puborectalis muscle is located
lateral to the pubovisceral muscle, cephalad to the deep
component of the EAS, from which it is inseparable posteriorly.
The muscles of the levator ani differ from most other skeletal
muscles in that they:
• maintain constant tone, except during voiding, defaecation
and the Valsalva manoeuvre
• have the ability to contract quickly at the time of acute
stress (such as a cough or sneeze) to maintain continence;
• distend considerably during parturition to allow the
passage of the term infant and then contract after birth to resume normal
functioning.
Until recently, the concept of pelvic floor trauma was
attributed largely to perineal, vaginal and anal sphincter injuries. However,
in recent years, with advances in magnetic resonance imaging and
three-dimensional ultrasound, it has become evident that LAM injuries form an important
component of pelvic floor trauma. LAM injuries occur in 13–36% of women who
have a vaginal birth.
Injury to the LAM is attributed to vaginal birth
resulting in reduced pelvic floor muscle strength, enlargement of the vaginal
hiatus and pelvic organ prolapse. There is inconclusive evidence to support an
association between LAM injuries and stress urinary incontinence and there seems
to be a trend towards the development of faecal incontinence.
Innervation of the levator ani
The levator ani is supplied on its superior surface by
the sacral nerve roots (S2–S4) and on its inferior surface by the perineal
branch of the pudendal nerve.
Vascular supply
The levator ani is supplied by branches of the
inferior gluteal artery, the inferior vesical artery and the pudendal artery
THE PUDENDAL NERVE
The pudendal nerve is a mixed motor and sensory nerve and
derives its fibres from the ventral branches of the second, third and fourth
sacral nerves and leaves the pelvis through the lower part of the greater
sciatic foramen. It then crosses the ischial spine and re-enters the pelvis
through the lesser sciatic foramen. It accompanies the
internal pudendal vessels upward and forward along the lateral wall of the
ischioanal fossa, contained in a sheath of the obturator fascia termed Alcock’s
canal. It is presumed that during a prolonged second stage of labour,
the pudendal nerve is vulnerable to stretch injury due to its relative
immobility at this site.
The inferior haemorrhoidal (rectal) nerve then
branches off posteriorly from the pudendal nerve to innervate the EAS. The
pudendal nerve then divides into two terminal branches: the perineal nerve and
the dorsal nerve of the clitoris. The perineal nerve divides into posterior
labial and muscular branches. The posterior labial branches supply the labium
majora. The muscular branches are distributed to the superficial transverse
perineal, bulbospongiosus, ischiocavernosus and constrictor urethræ muscles.
The dorsal nerve of the clitoris, which innervates the clitoris, is the deepest
division of the pudendal nerve.
THE PELVIS
Knowledge of anatomy of a normal female pelvis is key
to midwifery and obstetrics practice, as one of the ways to estimate a woman’s
progress in labour is by assessing the relationship of the fetus to certain
bony landmarks of the pelvis. Understanding the normal pelvic anatomy helps to
detect deviations from normal and facilitate appropriate care.
The pelvic girdle
The pelvic girdle is a basin-shaped cavity and
consists of two innominate bones (hip bones), the sacrum and the coccyx. It is
virtually incapable of independent movement except during childbirth as it
provides the skeletal framework
of the birth canal. It contains and protects the bladder, rectum and internal
reproductive organs. In addition
it provides an attachment for trunk and limb muscles.
Some women experience pelvic girdle pain in pregnancy and
need referral to a physiotherapist
Innominate bones
Each innominate bone or hip bone is made up of three bones
that have fused together: the ilium, the ischium and the pubis. On
its lateral aspect is a large, cup shaped acetabulum articulating with the
femoral head, which is composed of the three fused bones in the following
proportions: two-fifths ilium, two-fifths ischium and one-fifth pubis. Anteroinferior to this is the large oval or triangular obturator foramen.
The bone is articulated with its fellow to form the pelvic girdle.
The ilium has an upper and lower part. The smaller lower
part forms part of the acetabulum and the upper part is the large flared-out
part. When the hand is placed on the hip, it rests on the iliac crest, which is
the upper border. A bony prominence felt in front of the iliac crest is known
as the anterior superior iliac spine. A short distance below it is the anterior
inferior iliac spine. There are two similar points at the other end of the
iliac crest, namely the posterior superior and the posterior inferior iliac spines.
The internal concave anterior surface of the ilium is known as the iliac fossa.
The ischium is the inferoposterior part of the innominate
bone and consists of a body and a ramus. Above it forms part of the acetabulum.
Below its ramus ascends anteromedially at an acute angle to meet the descending
pubic ramus and complete the obturator foramen. It has a large prominence known
as the ischial tuberosity, on which the body rests when sitting. Behind and a
little above the tuberosity is an inward projection, the ischial spine. This is
an important landmark in midwifery and obstetric practice, as in labour, the
station of the fetal head is estimated in relation to the ischial spines
allowing assessment of progress of labour.
The pubis forms the anterior part. It has a body and
two oar-like projections, the superior ramus and the inferior ramus. The two
pubic bones meet at the symphysis pubis and the two inferior rami form the
pubic arch, merging into a similar ramus on the ischium. The space enclosed by
the body of the pubic bone, the rami and the ischium is called the obturator
foramen.
The sacrum
The sacrum is a wedge-shaped bone consisting of five fused
vertebrae, and forms the posterior wall of the pelvic cavity as it is wedged
between the innominate bones. The caudal apex articulates with the coccyx and
the upper border of the first sacral vertebra (sacral promontory)
articulates with the first lumbar vertebra. The
anterior surface of the sacrum is concave and is referred to as the hollow of
the sacrum. Laterally the sacrum extends into a wing or ala. Four pairs of
holes or foramina pierce the sacrum and, through these, nerves from the cauda equina
emerge to innervate the pelvic organs. The posterior surface is roughened to
receive attachments of muscles.
The coccyx
The coccyx is a vestigial tail. It consists of four
fused vertebrae, forming a small triangular bone, which articulates with the
fifth sacral segment.
Pelvic joints
There are four pelvic joints: one symphysis pubis, two
sacroiliac joints and one sacrococcygeal joint.
The symphysis pubis is the midline cartilaginous joint uniting the rami
of the left and right pubic bones.
The sacroiliac joints are strong, weight-bearing synovial joints with
irregular elevations and depressions that produce interlocking of the bones.
They join the sacrum to the ilium and as a result connect the spine to the
pelvis. The joints allow a limited backward and forward movement of the tip and
promontory of the sacrum, sometimes known as ‘nodding’ of the sacrum.
The sacrococcygeal joint is formed where the base of the coccyx articulates
with the tip of the sacrum. It permits the coccyx to be deflected backwards
during the birth of the fetal head.
Pelvic ligaments
The pelvic joints are held together by very strong
ligaments that are designed not to allow movement. However, during pregnancy
the hormone relaxin gradually loosens all the pelvic ligaments allowing slight
pelvic movement providing more room for the fetal head as it passes through the
pelvis. A widening of 2–3 mm at the symphysis pubis during pregnancy above the
normal gap of 4–5 mm is normal but if it widens significantly, the degree of
movement permitted may give rise to pain on walking.
The ligaments connecting the bones of the pelvis with each
other can be divided into four groups:
• those connecting the sacrum and ilium – the sacroiliac
ligaments;
• those passing between the sacrum and ischium – the sacrotuberous
ligaments and the sacrospinous ligaments (Fig. 3.10);
• those uniting the sacrum and coccyx – the sacrococcygeal
ligaments;
• those between the two pubic bones – the inter-pubic ligaments
The pelvis in relation to pregnancy and childbirth
The term pelvis is applied to the skeletal ring formed
by the innominate bones and the sacrum, the cavity within and even the entire
region where the trunk and the lower limbs meet. The pelvis is divided by an
oblique plane which passes through the prominence of the sacrum, the arcuate
line (the smooth rounded border on the internal surface of the ilium), the
pectineal line (a ridge on the superior ramus of the pubic bone) and the upper
margin of the symphysis pubis, into the true and the false pelves.
The true pelvis
The true pelvis is the bony canal through which the
fetus must pass during birth. It is divided into a brim, a cavity and an outlet.
The pelvic brim
The superior circumference forms the brim of the true pelvis,
the included space being called the inlet. The brim is round except where the
sacral promontory projects into it.
Midwives need to be familiar with the fixed points on the
pelvic brim that are known as its landmarks. Commencing posteriorly, these are:
• sacral promontory (1)
• sacral ala or wing (2)
• sacroiliac joint (3)
• iliopectineal line, which is the edge formed at the inward
aspect of the ilium (4)
• iliopectineal eminence, which is a roughened area formed
where the superior ramus of the pubic bone meets the ilium (5)
• superior ramus of the pubic bone (6)
• upper inner border of the body of the pubic bone (7)
• upper inner border of the symphysis pubis (8).
The pelvic cavity
The cavity of the true pelvis extends from the brim
superiorly to the outlet inferiorly. The anterior wall is formed by the pubic
bones and symphysis pubis and its depth is 4 cm. The posterior wall is formed
by the curve of the sacrum, which is 12 cm in length. Because there is such a difference
in these measurements, the cavity forms a curved canal. With the woman upright,
the upper portion of the pelvic canal is directed downward and backward, and
its lower course curves and becomes directed down[1]ward and forward. Its
lateral walls are the sides of the pelvis, which are mainly covered by the
obturator internus muscle.
The cavity contains the pelvic colon, rectum, bladder and
some of the reproductive organs. The rectum is placed posteriorly, in the curve
of the sacrum and coccyx; the bladder is anterior behind the symphysis pubis.
The pelvic outlet
The lower circumference of the true pelvis is very irregular;
the space enclosed by it is called the outlet. Two outlets are described: the
anatomical and the obstetrical.
The anatomical outlet is formed by the lower borders
of each of the bones together with the sacrotuberous ligament. The obstetrical
outlet is of greater practical significance because it includes the narrow
pelvic strait through which the fetus must pass. The narrow pelvic strait lies between
the sacrococcygeal joint, the two ischial spines and the lower border of the
symphysis pubis. The obstetrical outlet is the space between the narrow pelvic
strait and the anatomical outlet. This outlet is diamond-shaped.
The false pelvis
It is bounded posteriorly by the lumbar vertebrae and laterally
by the iliac fossae, and in front by the lower portion of the anterior
abdominal wall. The false pelvis varies considerably in size according to the
flare of the iliac bones. However, the false pelvis has no significance in midwifery.
Pelvic diameters
Knowledge of the diameters of the normal female pelvis
is essential in the practice of midwifery because contraction of any of them
can result in malposition or malpresentation of the presenting part of the
fetus.
Diameters of the pelvic inlet
The brim has four principal diameters: the
anteroposterior diameter, the transverse diameter and the two oblique diameters.
The anteroposterior or conjugate diameter extends from
the midpoint of the sacral promontory to the upper border of the symphysis
pubis. Three conjugate diameters can be measured: the anatomical (true)
conjugate, the obstetrical conjugate and the internal or diagonal conjugate.
The anatomical conjugate, which averages 12 cm, is measured
from the sacral promontory to the uppermost point of the symphysis pubis. The
obstetrical conjugate, which averages 11 cm, is measured from the sacral promontory
to the posterior border of the upper surface of the symphysis pubis. This
represents the shortest anteroposterior diameter through which the fetus must
pass and is hence of clinical significance to midwives. The obstetrical
conjugate cannot be measured with the examining fingers or any other technique.
The diagonal conjugate is measured anteroposteriorly from
the lower border of the symphysis to the sacral promontory.
The transverse diameter is constructed at right-angles
to the obstetric conjugate and extends across the greatest width of the brim;
its average measurement is about13 cm.
Each oblique diameter extends from the iliopectineal eminence
of one side to the sacroiliac articulation of the opposite side; its average
measurement is about 12 cm.
Each takes its name from the sacroiliac joint from
which it arises, so the left oblique diameter arises from the left sacroiliac
joint and the right oblique from the right sacroiliac joint.
Another dimension, the sacrocotyloid (see Fig. 3.11), passes
from the sacral promontory to the iliopectineal eminence on each side and
measures 9–9.5 cm. Its importance is concerned with posterior positions of the
occiput when the parietal eminences of the fetal head may become
caught (see Chapter 20).
Diameters of the cavity
The cavity is circular in shape and although it is not
possible to measure its diameters exactly, they are all considered to be 12 cm.
Diameters of the outlet
The outlet, which is diamond-shaped, has three
diameters: the anteroposterior diameter, the oblique diameter and the transverse
diameter.
The anteroposterior diameter extends from the lower border
of the symphysis pubis to the sacrococcygeal joint.
It measures 13 cm; as the coccyx may be deflected backwards
during labour, this diameter indicates the space available during birth.
The oblique diameter, although there are no fixed points,
is said to be between the obturator foramen and the sacrospinous ligament. The
measurement is taken as being 12 cm.
The transverse diameter extends between the two
ischial spines and measures 10–11 cm. It is the narrowest diameter in the
pelvis. The plane of least pelvic dimensions is said to be at the level of the
ischial spines.
Orientation of the pelvis
In the standing position, the pelvis is placed such
that the anterior superior iliac spine and the front edge of the symphysis
pubis are in the same vertical plane, perpendicular to the floor. If the line
joining the sacral promontory and the top of the symphysis pubis were to be extended,
it would form an angle of 60° with the horizontal floor. Similarly, if a line
joining the centre of the sacrum and the centre of the symphysis pubis were to
be extended, the resultant angle with the floor would be 30°. The angle of
inclination of the outlet is 15°. When in the recumbent position,
the same angles are made as in the vertical position; this fact should be kept
in mind when carrying out an abdominal examination.
Pelvic planes
Pelvic planes are imaginary flat surfaces at the brim,
cavity and outlet of the pelvic canal at the levels of the lines described
above.
 |
| Fetal head entering plane of pelvic brim |
Axis of the pelvic canal
A line drawn exactly half-way between the anterior
wall and the posterior wall of the pelvic canal would trace a curve known as
the curve of Carus. The midwife needs to become familiar with this concept in
order to make accurate observations on vaginal examination and to facilitate the
birth of the baby.
The four types of pelvis
The size of the pelvis varies not only in the two
sexes, but also in different members of the same sex. The height of the
individual does not appear to influence the size of the pelvis in any way, as
women of short stature, in general, have a broad pelvis. Nevertheless, the
pelvis is occasionally equally contracted in all its dimensions, so much so
that all its diameters can measure 1.25 cm less than the average.
This type of pelvis, known as a justo minor pelvis,
can result in normal labour and birth if the fetal size is consistent with the
size of the maternal pelvis. However, if the fetus is large, a degree of
cephalopelvic disproportion will result. The same is true when a
malpresentation or malposition of the fetus exists.
The principal divergences, however, are found at the brim and affect the relation of the anteroposterior to the transverse
diameter. If one of the measurements is reduced by 1 cm or more from the normal,
the pelvis is said to be contracted and may give rise to difficulty in labour
or necessitate caesarean section.
Classically, pelves have been described as falling
into four categories: the gynaecoid pelvis, the android pelvis, the anthropoid
pelvis and the platypelloid pelvis.
The gynaecoid pelvis
This is the best type for childbearing as it has a
rounded brim, generous forepelvis, straight side walls, a shallow cavity with a
well-curved sacrum and a sub-pubic arch of 90°.
The android pelvis
The android pelvis is so called because it resembles
the male pelvis. Its brim is heart-shaped, it has a narrow forepelvis
and its transverse diameter is situated towards the back. The side walls
converge, making it funnel-shaped, and it has a deep cavity and a straight
sacrum. The ischial spines are prominent and the sciatic notch is narrow. The sub-pubic
angle is less than 90°. It is found in short and heavily built women, who have
a tendency to be hirsute.
Because of the narrow forepelvis and the fact that the
greater space lies in the hindpelvis the heart-shaped brim favours an
occipitoposterior position. Furthermore, funnelling in the cavity may hinder
progress in labour. At the pelvic outlet, the prominent ischial spines sometimes
prevent complete internal rotation of the head and the anteroposterior diameter
becomes caught on them, causing a deep transverse arrest. The narrowed
sub-pubic angle cannot easily accommodate the biparietal diameter and this displaces the head backwards. Because of these factors, this type of
pelvis is the least suited to childbearing.
The anthropoid pelvis
The anthropoid pelvis has a long, oval brim in which
the anteroposterior diameter is longer than the transverse diameter. The side
walls diverge and the sacrum is long and deeply concave. The ischial spines are
not prominent and the sciatic notch and the sub-pubic angle are very
wide. Women with this type of pelvis tend to be tall,
with narrow shoulders. Labour does not usually present any difficulties, but a
direct occipitoanterior or direct occipitoposterior position is often a feature
and the position adopted for engagement may persist to birth.
The platypelloid pelvis
The platypelloid (flat) pelvis has a kidney-shaped
brim in which the anteroposterior diameter is reduced and the transverse
diameter increased. The sacrum is flat and the cavity shallow. The ischial
spines are blunt, and the sciatic notch and the sub-pubic angle are both wide.
The head must engage with the sagittal suture in the transverse diameter, but
usually descends through the cavity without difficulty. Engagement may
necessitate lateral tilting of the head, known as asynclitism, in order to allow
the biparietal diameter to pass the narrowest anteroposterior diameter of the
brim.
Other pelvic variations
High assimilation pelvis occurs when the 5th lumbar vertebra
is fused to the sacrum and the angle of inclination of the pelvic brim is
increased. Engagement of the head is difficult but, once achieved, labour
progresses normally.
Deformed pelvis may result from a developmental anomaly,
dietary deficiency, injury or disease.
Deformed pelves
Developmental anomalies
The Naegele’s and Robert’s pelves are rare
malformations caused by a failure in development. In the Naegele’s pelvis, one
sacral ala is missing and the sacrum is fused to the ilium causing a grossly
asymmetric brim. The Robert’s pelvis has similar malformations which are bilateral.
In both instances, the abnormal brim prevents engagement of the fetal head.
Dietary deficiency
Deficiency of vitamins and minerals necessary for the formation
of healthy bones is less frequently seen today than in the past but might still
complicate pregnancy and labour to some extent.
A rachitic pelvis is a pelvis deformed by rickets in
early childhood, as a consequence of malnutrition. The weight of the upper body
presses downwards on to the softened pelvic bones, the sacral promontory is
pushed downwards and forwards and the ilium and ischium are drawn outwards
resulting in a flat pelvic brim similar to that of the platypelloid pelvis. The sacrum tends to be straight, with the coccyx bending acutely
forward.
Because the tuberosities are wide apart, the pubic
arch is wide. The clinical signs of rickets are bow legs and spinal deformity.
If severe contraction is present, caesarean section is
required to deliver the baby. The fetal head will attempt to enter the pelvis
by asynclitism.Osteomalacic pelvis. The disease osteomalacia is rarely encountered
in the United Kingdom. It is due to an acquired deficiency of calcium and
occurs in adults. All bones of the skeleton soften because of gross calcium deficiency.
The pelvic canal is squashed together until the brim becomes a Y-shaped slit.
Labour is impossible.
In early pregnancy, incarceration of the gravid uterus
may occur because of the gross deformity.
Injury and disease
Trauma. A pelvis that has been fractured will develop callus
formation or may fail to unite correctly. This may lead to reduced measurements
and therefore to some degree of contraction.
Conditions sustained in childhood such as fractures of
the pelvis or lower limbs, congenital dislocation of the hip and poliomyelitis
may lead to unequal weight-bearing, which will also cause deformity.
Spinal deformity. If kyphosis (forward angulation) or scoliosis
(lateral curvature) is evident, or is suggested by a limp or deformity, the
midwife must refer the woman to a doctor. Pelvic contraction is likely in these
cases.
THE FEMALE REPRODUCTIVE SYSTEM
The female reproductive system consists of the
external genitalia, known collectively as the vulva, and the internal reproductive
organs: the vagina, the uterus, two uterine tubes and two ovaries. In the
non-pregnant state, the internal reproductive organs are situated within the
true pelvis.
The vagina
The vagina is a hollow, distensible fibromuscular tube that extends from the vestibule to the cervix. It is approximately 10 cm in length and 2.5 cm in diameter (although there is wide anatomical variation. During sexual intercourse and when a woman gives birth, the vagina temporarily widens and lengthens.
The vaginal canal passes upwards and backwards into the
pelvis with the anterior and posterior walls in close contact along a line
approximately parallel to the plane of the pelvic brim. When the woman stands
upright, the vaginal canal points in an upward-backward direction and forms an
angle of slightly more than 45° with the uterus.
Function
The vagina allows the escape of the menstrual fluids, receives
the penis and the ejected sperm during sexual intercourse, and provides an exit
for the fetus during birth.
Relations
Knowledge of the relations of the vagina to other
pelvic organs is essential for the accurate examination of the pregnant woman
and the safe birth of the baby.
• Anterior to the vagina lie the bladder and the urethra,
which are closely connected to the anterior vaginal wall.
• Posterior to the vagina lie the pouch of Douglas,
the rectum and the perineal body, which separates the vagina from the anal
canal.
• Laterally on the upper two-thirds are the pelvic fascia
and the ureters, which pass beside the cervix; on either side of the lower
third are the muscles of the pelvic floor.
• Superior to the vagina lies the uterus.
• Inferior to the vagina lies the external genitalia.
Structure of the vagina
The posterior wall of the vagina is 10 cm long whereas the anterior wall is only 7.5 cm in length; this is because the cervix projects into its upper part at a right-angle.
The upper end of the vagina is known as the vault.
Where the cervix projects into it, the vault forms a
circular recess that is described as four arches or fornices. The posterior
fornix is the largest of these because the vagina is attached to the uterus at
a higher level behind than in front. The anterior fornix lies in front of the
cervix and the lateral fornices lie on either side.
Layers
The vaginal wall is composed of three layers: mucosa, muscle
and fascia. The mucosa is the most superficial layer and consists of
stratified, squamous non-keratinized epithelium, thrown in transverse folds
called rugae. These allow the vaginal walls to stretch during intercourse and childbirth.
Beneath the epithelium lies a layer of vascular connective tissue. The muscle
layer is divided into a weak inner coat of circular fibres and a stronger outer
coat of longitudinal fibres. Pelvic fascia surrounds the vagina and adjacent
pelvic organs and allows for their independent expansion and contraction.
There are no glands in the vagina; however, it is moistened
by mucus from the cervix and a transudate that seeps out from the blood vessels
of the vaginal wall.
In spite of the alkaline mucus, the vaginal fluid is strongly
acid (pH 4.5) owing to the presence of lactic acid formed by the action of
Doderlein’s bacilli on glycogen found in the squamous epithelium of the lining.
These lactobacilli are normal inhabitants of the vagina. The acid deters the
growth of pathogenic bacteria.
Blood supply
The blood supply comes from branches of the internal iliac
artery and includes the vaginal artery and a descending branch of the uterine
artery. The blood drains through corresponding veins.
Lymphatic drainage
Lymphatic drainage is via the inguinal, the internal
iliac and the sacral glands.
Nerve supply
The nerve supply is derived from the pelvic plexus.
The vaginal nerves follow the vaginal arteries to supply the vaginal walls and
the erectile tissue of the vulva.
The uterus
The uterus is a hollow, pear-shaped muscular organ located
in the true pelvis between the bladder and the rectum. The position of the uterus
within the true pelvis is one of anteversion and anteflexion. Anteversion means
that the uterus leans forward and anteflexion means that it bends forwards upon
itself. When the woman is standing,
the uterus is in an almost horizontal position with the fundus resting on the
bladder if the uterus is anteverted.
Function of the uterus
The main function of the uterus is to nourish the
developing
fetus prior to birth. It prepares for pregnancy each month and following
pregnancy expels the products of conception.
Relations
Knowledge of the relations of the uterus to other pelvic organs is desirable, particularly when giving women advice about bladder and bowel care during pregnancy and childbirth.
• Anterior to the uterus lie the uterovesical pouch
and the bladder.
• Posterior to the uterus are the recto-uterine pouch
of Douglas and the rectum.
• Lateral to the uterus are the broad ligaments, the uterine
tubes and the ovaries.
• Superior to the uterus lie the intestines.
• Inferior to the uterus is the vagina.
Supports
The uterus is supported by the pelvic floor and
maintained in position by several ligaments, of which those at the level of the
cervix (Fig. 3.24) are the most important.
• The transverse cervical ligaments fan out from the sides
of the cervix to the side walls of the pelvis. They are sometimes known as the
‘cardinal ligaments’ or ‘Mackenrodt’s ligaments’.
• The uterosacral ligaments pass backwards from the cervix
to the sacrum.
• The pubocervical ligaments pass forwards from the cervix,
under the bladder, to the pubic bones.
• The broad ligaments are formed from the folds of peritoneum,
which are draped over the uterine tubes. They hang down like a curtain and
spread from the sides of the uterus to the side walls of the pelvis.
• The round ligaments have little value as a support but
tend to maintain the anteverted position of the uterus; they arise from the
cornua of the uterus, in front of and below the insertion of each uterine tube,
and pass between the folds of the broad ligament, through the inguinal canal,
to be inserted into each labium majus.
• The ovarian ligaments also begin at the cornua of the
uterus but behind the uterine tubes and pass down between the folds of the
broad ligament to the ovaries. It is helpful to note that the round ligament,
the uterine tube and the ovarian ligament are very similar in appear[1]ance
and arise from the same area of the uterus. This makes careful identification
important when tubal surgery is undertaken.
Structure of the uterus
The non-pregnant uterus is 7.5 cm long, 5 cm wide and 2.5
cm in depth, each wall being 1.25 cm thick (see Fig. 3.25). The cervix forms
the lower third of the uterus and measures 2.5 cm in each direction. The uterus
consists of the following parts:
• The cornua are the upper outer angles of the uterus where
the uterine tubes join.
• The fundus is the domed upper wall between the insertions
of the uterine tubes.
• The body or corpus makes up the upper two-thirds of
the uterus and is the greater part.
• The cavity is a potential space between the anterior
and posterior walls. It is triangular in shape, the base of the triangle being
uppermost.
• The isthmus is a narrow area between the cavity and the
cervix, which is 7 mm long. It enlarges during pregnancy to form the lower
uterine segment.
• The cervix or neck protrudes into the vagina. The upper
half, being above the vagina, is known as the supravaginal portion while the
lower half is the infravaginal portion.
• The internal os (mouth) is the narrow opening between
the isthmus and the cervix.
• The external os is a small round opening at the lower
end of the cervix. After childbirth, it becomes a transverse slit.
• The cervical canal lies between these two ostia and is a continuation of the uterine cavity. This canal is shaped like a spindle, narrow at each end and wider in the middle.
Layers of the uterus
The uterus has three layers: the endometrium, the myometrium
and the perimetrium, of which the myometrium, the middle muscle layer, is by
far the thickest.
The endometrium forms a lining of ciliated epithelium (mucous
membrane) on a base of connective tissue or stroma. In the uterine cavity, this
endometrium is constantly changing in thickness throughout the menstrual cycle
(see Chapter 5). The basal layer does not alter, but provides the foundation
from which the upper layers regenerate. The epithelial cells are cubical in
shape and dip down to form glands that secrete an alkaline mucus.
The cervical endometrium does not respond to the hormonal
stimuli of the menstrual cycle to the same extent.
Here the epithelial cells are tall and columnar in
shape and the mucus-secreting glands are branching racemose glands. The
cervical endometrium is thinner than that of the body and is folded into a
pattern known as the ‘arbor vitae’ (tree of life). This is thought to assist
the passage of the sperm. The portion of the cervix that protrudes into the
vagina is covered with squamous epithelium similar to that lining the vagina.
The point where the epithelium changes, at the external os, is termed the
squamo-columnar junction.
The myometrium is thick in the upper part of the
uterus and is sparser in the isthmus and cervix. Its fibres run in all
directions and interlace to surround the blood vessels and lymphatics that pass
to and from the endometrium.
The outer layer is formed of longitudinal fibres that
are continuous with those of the uterine tube, the uterine ligaments and the
vagina.
In the cervix, the muscle fibres are embedded in collagen
fibres, which enable it to stretch in labour.
The perimetrium is a double serous membrane, an extension
of the peritoneum, which is draped over the fundus and the anterior surface of
the uterus to the level of the internal os. It is then reflected onto the
bladder forming a small pouch between the uterus and the bladder called the uterovesical
pouch. The posterior surface is covered to where the cervix protrudes into
the vagina and is then reflected onto the rectum forming the recto-uterine pouch.
Laterally the perimetrium extends over the uterine tubes forming a double fold,
the broad ligament, leaving
the lateral borders of the body uncovered.
Blood supply to the uterus
The uterine artery arrives at the level of the cervix
and is a branch of the internal iliac artery. It sends a small branch to the
upper vagina, and then runs upwards in a twisted fashion to meet the ovarian
artery and form an anastomosis with it near the cornu. The ovarian artery is a
branch of the abdominal aorta, leaving near the renal artery. It supplies the
ovary and uterine tube before joining the uterine artery. The blood drains
through corresponding veins.
Lymphatic drainage
Lymph is drained from the uterine body to the internal
iliac glands and from the cervical area to many other pelvic lymph glands. This
provides an effective defence against uterine infection.
Nerve supply
The nerve supply is mainly from the autonomic nervous system,
sympathetic and parasympathetic, via the inferior hypogastric or pelvic plexus.
Uterine malformations
The prevalence of uterine malformation is estimated to
be 6.7% in the general population. The female genital tract is formed in early
embryonic life when a pair of ducts develops. These paramesonephric or
Müllerian ducts come together in the midline and fuse into a Y-shaped canal.
The open upper ends of this structure lead into the peritoneal cavity and the
unfused portions become the uterine tubes. The fused lower portion forms the
utero[1]vaginal
area, which further develops into the uterus and vagina. Abnormal development
of the Müllerian duct(s) during embryogenesis can lead to uterine abnormalities.
Structural abnormality of the uterus can lead to
various problems during pregnancy and childbirth. The outcome depends on the
ability of the uterus to accommodate the growing fetus. A problem exists only
if the tissue is insufficient to allow the uterus to enlarge for a full-term
fetus lying longitudinally. If there is insufficient hypertrophy, the possible
difficulties are miscarriage, premature labour and abnormal lie of the fetus.
In labour, poor uterine unction may be experienced. Minor defects of structure cause
little problem and might pass unnoticed, with the woman having a normal outcome
to her pregnancy. Occasionally problems arise when a fetus is accommodated in one
horn of a double uterus and the empty horn has filled the pelvic cavity. In
this situation, the empty horn has grown owing to the hormonal influences of
the pregnancy, and its size and position will cause obstruction during labour.
Caesarean section would be the method of delivery
Types of uterine malformation
Various types of structural abnormality can result
from failure of fusion of the Müllerian ducts. Three of these abnormalities can
be seen in.
·
A
double uterus with an associated double vagina will develop where there has
been complete failure of fusion.
·
Partial
fusion results in various degrees of duplication. A single vagina with a double
uterus is the result of fusion at the lower end of the ducts only.
·
A
bicornuate uterus (one with two horns) is the result of incomplete fusion at
the upper portion of the uterovaginal area. In rare cases, one Müllerian duct
regresses and the result is a uterus with one horn – termed a unicornuate
uterus
The fallopian tubes
The uterine tubes, also known as fallopian tubes,
oviducts and salpinges, are two very fine tubes leading from the ovaries into
the uterus.
Function of the fallopian tubes
The uterine tube propels the ovum towards the uterus, receives
the spermatozoa as they travel upwards and provides a site for
fertilization. It supplies the fertilized ovum with nutrition during its continued
journey to the uterus.
Position of the fallopian tubes
The uterine tubes extend laterally from the cornua of
the uterus towards the side walls of the pelvis. They arch over the ovaries,
the fringed ends hovering near the ovaries in order to receive the ovum.
Relations
• Anterior, posterior and superior to the uterine
tubes are the peritoneal cavity and the intestines.
• Lateral to the uterine tubes are the side walls of
the pelvis.
• Inferior to the uterine tubes lie the broad
ligaments and the ovaries.
• Medial to the two uterine tubes lies the uterus. Supports
The uterine tubes are held in place by their
attachment to the uterus. The peritoneum folds over them, draping down below as
the broad ligaments and extending at the sides to form the infundibulopelvic
ligaments.
Structure of the fallopian tubes
Each tube is 10 cm long. The lumen of the tube
provides an open pathway from the outside to the peritoneal cavity.
The uterine tube has four portions (Fig. 3.28):
• The interstitial portion is 1.25 cm long and lies within
the wall of the uterus. Its lumen is 1 mm wide
• The isthmus is another narrow part that extends for 2.5
cm from the uterus.
• The ampulla is the wider portion, where
fertilization usually occurs. It is 5 cm long.
• The infundibulum is the funnel-shaped fringed end that
is composed of many processes known as fimbriae. One fimbria is elongated to
form the ovarian fimbria, which is attached to the ovary
Layers of the fallopian tubes
The lining of the uterine tubes is a mucous membrane
of ciliated cubical epithelium that is thrown into complicated folds known as
plicae. These folds slow the ovum down on its way to the uterus. In this lining
are goblet cells that produce a secretion containing glycogen to nourish the
oocyte.
Beneath the lining is a layer of vascular connective tissue.
The muscle coat consists of two layers: an inner
circular layer and an outer longitudinal layer, both of smooth muscle. The
peristaltic movement of the uterine tube is due to the action of these muscles.
The tube is covered with peritoneum but the infundibulum
passes through it to open into the peritoneal cavity.
Blood supply
The blood supply is via the uterine and ovarian
arteries, returning by the corresponding veins.
Lymphatic drainage
Lymph is drained to the lumbar glands
Nerve supply
The nerve supply is from the ovarian plexus.
The ovaries
The ovaries are components of the female reproductive system
and the endocrine system.
Function of the ovaries
The ovaries produce oocytes and the hormones,
oestrogen and progesterone.
Position
The ovaries are attached to the back of the broad
ligaments within the peritoneal cavity.
Relations to the ovaries
• Anterior to the ovaries are the broad ligaments.
• Posterior to the ovaries are the intestines.
• Lateral to the ovaries are the infundibulopelvic ligaments
and the side walls of the pelvis.
• Superior to the ovaries lie the uterine tubes.
• Medial to the ovaries lie the ovarian ligaments and the
uterus.
Supports
The ovary is attached to the broad ligament but is supported
from above by the ovarian ligament medially and the infundibulopelvic ligament
laterally.
Structure of the ovaries
The ovary is composed of a medulla and cortex, coveredbwith
germinal epithelium.
The medulla is the supporting framework, which is made
of fibrous tissue; the ovarian blood vessels, lymphatics and nerves travel
through it. The hilum where these vessels enter lies just where the ovary is
attached to the broad ligament and this area is called the mesovarium (see Fig.
3.29).
The cortex is the functioning part of the ovary. It
contains the ovarian follicles in different stages of development, surrounded
by stroma. The outer layer is formed of fibrous tissue known as the tunica
albuginea. Over this lies the germinal epithelium, which is a modification of
the peritoneum.
Blood supply to the ovaries
Blood is supplied to the ovaries from the ovarian
arteries and drains via the ovarian veins. The right ovarian vein joins the
inferior vena cava, but the left returns its blood to the left renal vein.
Lymphatic drainage
Lymphatic drainage is to the lumbar glands.
Nerve supply
The nerve supply is from the ovarian plexus.
References
1. Kearney R, Sawhney R, DeLancey J O 2004. Levator
ani muscle anatomy evaluated by origin-insertion pairs. Obstetrics and
Gynecology 104: 168–73
2. A comprehensive and up-to-date description of
levator ani muscle anatomy. Schwertner-Tiepelmann N, Thakar R, Sultan A H et al
2012 Obstetric levator ani muscle injuries – current status. Ultrasound in
Obstetrics and Gynecology 39: 372–83
This review article critically appraises the diagnosis
of obstetric LAM injuries, to establish the relationship between LAM injuries
and pelvic floor dysfunction and to identify risk factors and preventive strategies
to minimize such injuries.
3. Stables D, Rankin J 2010 Physiology in childbearing:
with anatomy and related biosciences, 3rd edn. Baillière Tindall, Edinburgh
This textbook presents a comprehensive and clear
account of anatomy and physiology and related biosciences at all stages of pregnancy
and childbirth.
4. Standring S (ed) 2008 Gray’s anatomy: the
anatomical basis of clinical practice, 40th edn. Elsevier Churchill Livingston,
London
This large volume, with detailed information about the
anatomy of every part of the human body, provides the reader with much more
insight into the structure and function of the reproductive organs. This
edition includes specialist revision of topics such as the anatomy of the
pelvic floor.
5. Sultan A H, Thakar R, Fenner D E 2007 Perineal
and anal sphincter trauma: diagnosis and clinical management. Springer-Verlag,
London
This is a comprehensive text that focuses on the
maternal morbidity associated with childbirth. It is essential reading for
anyone involved in obstetric care such as obstetricians, midwives and family practitioners
but will also be of interest to colorectal surgeons, gastroenterologists, physiotherapists,
continence advisors and lawyers
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