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.