Anterolateral Abdominal Wall
Before describing the walls of
the abdomen, it is necessary to mention different ways in which the word abdomen
is used. In some cases, abdomen is synonymous with abdominopelvic cavity,
but in other cases, it is used in a more specific sense to refer to that
portion of the body cavity between the diaphragm and the pelvis minor (true
pelvis). Abdomen is also used more loosely to refer to a general region of the
body.
For purposes of specificity, it seems advisable to name that portion of
the body cavity below the diaphragm the “abdominopelvic cavity” and then to
divide this into the abdominal cavity proper and the pelvic cavity (pelvis
minor), separated from each other by the plane of the pelvic inlet (the plane passing
through the sacral promontory and the pubic crests). It must be remembered,
however, that certain structures that are ordinarily referred to as abdominal
structures (some of the coils of small intestine, for example) usually hang
into the pelvic cavity, and that the inferior and posterior inferior support of the abdominal viscera is furnished by the
walls of the pelvic cavity and not by the theoretical plane at the pelvic
inlet. It is convenient to divide the borders of the abdominopelvic cavity into
four general parts—the anterolateral abdominal wall, the posterior wall of the
abdominal cavity, the diaphragm (superior wall or roof of the abdominal and
abdominopelvic cavities), and the bowl of the pelvic cavity, which can be
loosely called the floor of the abdominopelvic cavity. However, the limits of
each boundary are not sharp, because we are dealing with curved contours, and
certain arbitrary limits need to be defined for descriptive purposes. This has
been done in part above and will be completed as necessary at appropriate
places in the following descriptions.
The anterolateral abdominal wall fills in the gap in the
bony-cartilaginous framework between the costal margin superiorly and the hip
bones inferiorly. Following the curve of the body laterally, several muscles,
nerves, vessels, and fascial layers will be encountered. For the present work,
the quadratus lumborum muscle and the structures medial to it will be included
with the posterior wall of the abdominal cavity. Owing to its muscular
components, the anterolateral abdominal wall can contract and relax and, thus,
help to accommodate the size of the abdominopelvic cavity to changes in volume
of the contained viscera and to control intraabdominal pressure. The surgical
approach to the abdominopelvic cavity is commonly made through this wall.
Starting from the outside, the layers of the anterolateral abdominal wall
are skin, subcutaneous fat (superficial fascia), outer investing layer of deep
fascia, the muscles with their related fasciae, transversalis fascia,
extraperitoneal fascia, and parietal peritoneum. Abdominal skin is of average
thickness (thicker posteriorly than anteriorly and laterally) and rather
loosely attached to the underlying
layers, except in the area of the umbilicus.
The subcutaneous fat is soft, movable, and contains a variable
amount of fat, depending mostly on the state of nutrition of the individual and
varying to some extent in distribution. The thickness of this layer can be
roughly estimated by picking up a fold, the thickness of which, minus the
double thickness of the skin, would be about twice the thickness of the layer.
The superficial fascia, particularly
of the part of the wall inferior to the level of the umbilicus, has been
classically described as having a superficial fatty layer, called the Camper
fascia, and a deep membranous layer (to some extent discontinuous), called
the Scarpa fascia. This classical description is somewhat of a
simplification of the actual situation, in which the layering is not always as
clear-cut as indicated, but it serves as a means of description if this is kept
in mind. The Camper layer is continuous with
the fatty layer of surrounding areas, such as the superficial fascia of the
thigh. The Scarpa layer fuses with the fascia lata along a line parallel to and
just inferior to the inguinal ligament. Medial to the pubic tubercle, both
layers continue into the urogenital region. This is significant in relation to
the path that extravasated urine takes after injuries to the urethra or neck of
the bladder. When entering the fasciae in the perineal region, this urine and
blood may escape superiorly into the anterolateral abdominal wall. In the male,
the two layers continue into the scrotum and blend into a single, smooth
muscle-containing layer, the fat being rather abruptly lost as they enter into
the formation of the scrotum. Just above the symphysis pubis a considerable
addition of closely set strong bands to the Scarpa fascia form the fundiform
ligament of the penis, which extends down onto the dorsum and sides of the
penis.
The outer investing layer of the deep fascia (not readily distinguished from the muscular fascia on the external
surface of the external abdominal oblique muscle and its aponeurosis)
is easily demonstrable over the fleshy portion of the muscle but is much more
difficult to separate from the aponeurotic portion of the muscle. This layer is
attached to the inguinal ligament and blends with the fascia coming out
from under this ligament to form the fascia lata. It also joins with the fascia
on the inner surface of the external oblique at the superficial inguinal
ring to form the external spermatic fascia. External to the inferior
end of the linea alba, the outer investing layer is thickened into the suspensory
ligament of the penis, which anchors the penis to the symphysis pubis and
the inferior pubic ligament. It is also continuous with the deep fascia
investing the penis.
The external abdominal oblique muscle typically arises by eight
digitations from the external surfaces of the lower eight ribs lateral to the
costochondral junction, the middle group of digitations arising at a greater
distance lateral to the junction than the ones above and below them. The upper
five slips interdigitate with the serratus anterior muscle, and the
lower three slips inter-digitate with the latissimus dorsi muscle. The
general direction taken by the fibers of this muscle is anteroinferior from
their site of origin, and this leads the fibers from the lower two or three
digitations to a fleshy insertion on the anterior half of the outer lip of the
crest of the ilium, this portion of the muscle having a free posterior border
that forms the anterior side of the lumbar triangle. The muscular portion from
the remainder of the origin becomes the strong aponeurosis of this
muscle along a line that courses vertically inferiorly through about the tip of
the ninth costal cartilage to the level of the anterior superior iliac spine,
where it curves rather sharply laterally to course toward this spine. The aponeurosis
passes in front of the rectus abdominis muscle (where it partly fuses with the aponeurosis of the internal
oblique) to blend with the one of the opposite side in the midline linea alba,
gaining attachment to the xiphoid process at the upper end of the linea alba
and to the pubis at the lower end. The lower margin of the aponeurosis is
folded backward and slightly upward upon itself between the anterior superior
iliac spine and the pubic tubercle. The folded edge, together with an extremely variable number of fibrous
strands running along it, is called the inguinal ligament.
The nerve supply of the external abdominal oblique muscle is derived from
the ventral rami of the 6th to 12th thoracic spinal nerves. The 6th to the 11th
are intercostal nerves, which continue from the intercostal spaces into the
anterolateral abdominal wall to lie in the plane between the internal abdominal
oblique and transversus abdominis
muscles. The 12th thoracic nerve is the subcostal nerve, and it follows a
course similar to the intercostal nerves above. The iliohypogastric nerve from
the anterior ramus of L1 also contributes to the supply. The nerves have a
segmental distribution corresponding to the primitive segmental condition of
the muscle, with the 10th thoracic extending toward the umbilicus and the 12th
toward a point about halfway between the umbilicus and the symphysis pubis.
The external abdominal oblique muscle has several actions in common with
the other large muscles of the anterolateral abdominal wall. These are to (1)
support the abdominal viscera and, by compressing them, help to expel their
contents; (2) depress the thorax in expiration; (3) flex the spinal column; and
(4) assist in rotation of the thorax and pelvis in relation to each other. With
the pelvis fixed in place, contraction of the external oblique of one side
produces a rotation that brings the shoulder of the same side anteriorly.
The internal abdominal oblique muscle, smaller and thinner than
the external oblique, arises from the posterior layer of the thoracolumbar
fascia, from the anterior two thirds or more of the intermediate line (lip) of
the iliac crest and the lateral one half to two thirds of the folded-under edge
of the external oblique aponeurosis, together with the immediately adjacent and
closely related iliac fascia. The majority of the fibers from the thoracolumbar
fascia and the iliac crest course superiorly and medially, which means that
their direction is perpendicular to the general direction of the fibers of the
external oblique. The most posterior fibers insert on the inferior borders of
the lower three (or four) ribs and their costal cartilages. The rest of these
fibers end in an aponeurosis along a line which extends inferiorly and medially
from the 10th costal cartilage toward the crest of the pubis. In the upper two
thirds (to three fourths) of the abdomen, the aponeurosis splits at the lateral
margin of the rectus into a posterior layer, which passes posterior to the
rectus abdominis muscle, and an anterior layer, which passes anterior to it.
These two layers join medial to each of the two rectus abdominis muscles and
blend with those of the opposite side in the linea alba. In the lower
one third of the abdomen, the aponeurosis of the internal abdominal oblique
does not split but passes entirely anterior to the rectus abdominis muscle to
reach the linea alba. The fibers arising from the margin of the external
oblique aponeurosis and the related iliac fascia are paler and less compact and
course downward and medially, arching superior to the spermatic cord in the
male (round ligament in the female). This portion of the internal oblique is
generally closely blended with the related portion of the transversus abdominis
muscle and tends to fuse with it to create a common, more or less aponeurotic,
insertion that passes anterior to the insertion of the rectus muscle on the
pubic crest and for a variable distance on the pecten pubis as the conjoint tendon (inguinal falx). The nerve
supply of the internal abdominal oblique is by way of the lowest two or three
intercostal nerves, as well as the subcostal, iliohypogastric, and ilioinguinal
nerves. The actions of the internal oblique are similar to those of the
external oblique (see above), except that contraction of the muscle of one side
would help to produce a rotation that would bring the shoulder of the same side
posteriorly if the pelvis were fixed in place.
The cremaster muscle is well developed only in the male because it
is an extension of the lower border of the internal abdominal oblique muscle
that travels into the spermatic cord. Laterally it is thicker and fleshier and
attaches to about the middle of the turned-under edge of the external abdominal
oblique aponeurosis and to the inferior edge of the internal abdominal oblique
muscle. From here, the somewhat scattered muscle fibers spread over the
spermatic cord along with connective
tissue (cremasteric fascia) running between them to end at the pubic
tubercle and the anterior layer of the rectus sheath. The nerve supply of
this muscle is from the genital branch of the genitofemoral nerve and also a
branch from the ilioinguinal nerve. The action of the cremaster muscle is to
lift the testis toward the superficial inguinal ring.
The transversus abdominis is a broad thin muscle that takes a
nearly horizontal course around the inner side of the anterolateral abdominal
wall. It arises from (1) the inner surfaces of the costal cartilages of the
lower six ribs by fleshy slips, which interdigitate with the slips that make up
the costal origin of the diaphragm; (2) an aponeurosis formed by the union at
the lateral border of the erector spinae muscle of the layer of the thoracolumbar
fascia attached to the tips of the transverse processes of the lumbar vertebrae
and the layer of this fascia attached to the tips of the spinous processes of
the same vertebrae (an indirect origin from the lumbar vertebrae); (3) the
anterior one half to three fourths of the internal lip of the iliac crest; and
(4) approximately the lateral one third of the folded-under margin of the
external oblique aponeurosis and the closely related portion of the iliac
fascia. The muscular fibers terminate in a strong (for most of its extent)
aponeurosis along a line that extends from deep to the rectus muscle above and
courses interiorly and slightly laterally to emerge lateral to the rectus at
about the level of the umbilicus and then to extend variably toward the middle
of the inguinal ligament. In the upper two thirds to three fourths of the
abdomen, the aponeurosis passes posterior to the rectus muscle, fusing with the
posterior layer of the aponeurosis of the internal abdominal oblique muscle,
and ends by meeting the one of the opposite side in the linea alba. Insertion
occurs also on the xiphoid process at the upper end of the linea alba. In the
lower one fourth to one third of the abdomen, the aponeurosis passes anterior
to the rectus muscle to reach the linea alba. The lower fibers of the
transversus abdominis muscle have a common insertion with the lower fibers of
the internal oblique, as described with the insertion of the latter muscle
above. The transversus abdominis muscle is often described as having an
inferior free border that arches over the spermatic cord in the male (round
ligament in the female) from the origin on the external oblique aponeurosis to
the pubic attachment. The nerve supply of the transversus muscle comes from the
anterior rami of the lower five or six intercostal and subcostal nerves as well
as the iliohypogastric, ilioinguinal, and genitofemoral nerves. The actions of
the transversus muscle are the same as those listed as being common to the
external oblique and other large muscles of the abdomen. Unilateral contraction of one side of the transversus abdominis muscle will not produce
appreciable rotation.
The rectus abdominis is a flat, vertical muscle, located just lateral to the anterior midline,
which is wider and thinner superiorly and becomes narrower and thicker inferiorly. It has a superior and an
inferior attachment, each of which is called the origin of the muscle by some
authors and the insertion by others. Several incomplete, zigzag, transversely
running tendinous bands are present in the muscle, creating its
distinctive appearance. These are better developed on the anterior surface of
the muscle and are closely attached to the anterior wall of the rectus sheath.
The one at the level of the umbilicus is segmentally related to the 10th rib.
Two are usually present between the
umbilicus and the xiphoid process, and, in about one third of the instances,
one is found below the level of the umbilicus. The superior attachment of the
rectus muscle is to the anterior surfaces of the fifth, sixth, and seventh
costal cartilages, the xiphoid process, and the costoxiphoid ligament. These attachments
fall more or less in a horizontal line. The inferior (caudal) or pubic
attachment of the rectus muscle is by a short tendon, a broader lateral portion
of which ends on a roughened area on
the pubic crest, extending from the pubic tubercle to the pubic symphysis. The narrower
medial portion of the tendon is attached to the front of the symphysis, where
it interdigitates with the one of the opposite side. The nerve supply of the
rectus abdominis muscle comes from the anterior branches of the anterior rami
of the lower six or seven intercostal nerves that enter the deep surface of the
muscle near its lateral edge to send cutaneous branches obliquely through the
muscle as muscular branches enter into the formation of an intramuscular
plexus. The branch from the 10th thoracic nerve usually enters the muscle below
the tendinous inscription at the level of the umbilicus. The rectus abdominis
muscle generally acts in conjunction with the previously described muscles to
compress the abdominal organs and during respiratory expiration. However, it is
particularly involved in pro- ducing flexion of the vertebral column, bringing
the xiphoid and pubic bones closer together.
The pyramidalis is a small and seemingly unimportant muscle that
is absent in 20% to 25% of the population. It arises from the crest of the
pubis, just anterior to the line of attachment of the rectus muscle, and its
fibers run superiorly and toward the linea alba, into which they insert as high
as one third of the distance to the umbilicus. The pyramidalis is supplied by a
branch from the subcostal nerve and, sometimes, also the iliohypogastric or
ilioinguinal nerves. No biomechanical importance is ascribed to this muscle,
although it does tense the linea alba, anchoring it to the pubic bones.
The rectus abdominis and pyramidalis muscles are wrapped in a sheath
formed, for the most part, by the aponeuroses of the three large flat muscles
of the anterolateral abdominal wall, the make-up of which differs in the lower
one fourth to one third of the abdomen from the make-up of the rest of its
length. In the upper two thirds to three fourths of the abdomen, the
aponeurosis of the external abdominal oblique muscle fuses with the anterior
lamella of the aponeurosis of the internal abdominal oblique muscle to form the
anterior layer of the rectus sheath, and the aponeurosis of the transversus
abdominis muscle fuses with the posterior lamella of the internal oblique
aponeurosis to form the posterior layer of the rectus sheath. The anterior and
posterior layers of the sheath fuse medial to the rectus muscle in the linea
alba, and, at the lateral margin of the rectus muscle, the anterior and
posterior layers come together at the line of the splitting of the aponeurosis
of the internal oblique. The posterior layer of the sheath does not extend
superior to the costal margin, so that the uppermost part of the rectus muscle
lies directly on the chest wall. In the lower part of the abdomen, the
aponeurosis of the internal oblique muscle does not split into two layers, and
both it and the greater part of the aponeurosis of the transversus muscle pass
anterior to the rectus muscle, so that only the transversalis fascia forms
the posterior layer of the rectus sheath in this area. Usually, the
inferior margin of the definitely aponeurotic part of the posterior layer of
the sheath is an obvious margin, called the arcuate line.
The transversalis fascia is thin and adherent in some areas and thickened and more independent
in others. At the arched lower border of the transversus muscle, the transversalis fascia is thought to
fuse with the fascia on the external surface of the transversus and to form a
sheet extending to the inguinal ligament. This fascia extends deep to the inguinal
ligament to form the anterior wall of the femoral sheath. Lateral to this, in
the area where the transversus abdominis arises from the turned-under edge of
the external oblique aponeurosis and the related iliac fascia, the
transversalis fascia fuses with the iliac fascia.
The extraperitoneal fascia (subserous fascia) is thin and comparatively
free from fat on the roof and anterolateral abdominal wall, except in the
lowest portion, where it is loose and fatty to allow for the expansion of the
bladder. In contrast to the situation on the roof and most of the anterolateral
abdominal wall, the extraperitoneal tissue on the posterior wall of the abdominal
cavity is large and quite fatty, particularly around the great vessels and kidneys.
