The Abdominal Wall

The structure of the abdominal wall is similar in principle to the thoracic wall. There are three layers, an external, internal and innermost layer. The vessels and nerves lie between the internal and innermost layers.

Surface anatomy

The abdomen can be divided into quadrants or nine abdominal regions. Pain felt in these regions may be considered to be direct or referred. The midline in the sagittal plane is the linea alba. The lateral edge of the rectus sheath is the linea semilunaris. The lower costal margin, the iliac crest and pubic tubercle can be palpated.

The fascia

Below the skin the superficial fascia is divided into a superficial fatty layer, Camper's fascia, and a deeper fibrous layer, Scarpa's fascia. The deep fascia lies on the abdominal muscles. Inferiorly Scarpa's fascia blends with the deep fascia of the thigh. This arrangement forms a plane between Scarpa's fascia and the deep abdominal fascia extending from the top of the thigh to the upper abdomen. Below the innermost layer of muscle, the transversus abdominis muscle, lies the transversalis fascia. The transversalis fascia is separated from the parietal peritoneum by a variable layer of fat.


The rectus abdominis and rectus sheath

The rectus muscle extends from the xiphoid process of the sternum and 5,6,7th costal cartilages to the pubic symphysis and pubic crest. The muscle is enclosed within the rectus sheath formed by the aponeuroses of the lateral abdominal muscles. Along the length of this strap muscle there are three fibrous intersections separating the muscle into four segments. The fibrous intersections are attached to the anterior surface of the rectus sheath, but not to the posterior surface. This allows the superior and inferior epigastric vessels to pass along the posterior surface of the muscle without encountering a barrier.


The lateral muscles

The lateral muscles arise from the lower part of the rib cage, the lumbar fascia and the iliac crest.

The external oblique muscle arises from the lower eight ribs. The fibres run downwards and forwards to form an aponeurosis anteriorly. The aponeurosis passes anteriorly to the rectus muscle to insert into the aponeurosis from the other side at the linea alba. Inferiorly the aponeurosis inserts into the anterior superior iliac spine and stretches over to the pubic tubercle, forming the inguinal ligament.





The internal oblique muscle arises from the lumbar fascia, the iliac crest and the lateral two-thirds of the inguinal ligament and runs upwards and forwards to form an aponeurosis. Above the arcuate line the aponeurosis splits to enclose the rectus muscle. Below the arcuate line the aponeurosis passes anterior to the rectus muscle. The inferior part of the aponeurosis inserts into the symphysis pubis. At this insertion the aponeurosis is fused with the aponeurosis of the transversus abdominis muscle to form the conjoint tendon.



The transversus abdominis muscle arises from the lower six costal cartilages, the lumbar fascia and the iliac crest. The fibres run forwards to form an aponeurosis. Superiorly the aponeurosis passes behind the rectus muscle. Below the arcuate line the aponeurosis passes anterior to the muscle. The inferior fibres of the aponeurosis are fused with those of the internal oblique to form the conjoint tendon.







The inguinal ligament

The inguinal ligament is formed by the aponeurotic fibres of the external oblique muscle. The ligament stretches from the anterior superior iliac spine (ASIS) to the pubic tubercle. At the medial end of the inguinal ligament, fibres are reflected backwards to insert into the superior ramus of the pubis, forming the lacunar ligament. The iliopsoas muscles, the femoral vein artery and nerve, all pass below the inguinal ligament. The inguinal canal passes obliquely through the abdominal wall above the ligament.



The inguinal canal

The inguinal canal transmits the vas deferens in the male and the round ligament in the female. The deep ring is the entrance to the inguinal canal on the inside of the abdominal wall. The deep ring is formed in the transversalis fascia. As the canal passes through the abdominal wall it receives a layer of muscle from the internal oblique, the cremaster muscle. At the superficial ring the inguinal canal passes through the external oblique aponeurosis and receives a layer from the aponeurosis, the external spermatic fascia in the male. The deep inguinal ring lies lateral to the inferior epigastric vessels. The superficial ring lies above and medial to the pubic tubercle.

A direct inguinal hernia occurs when a loop of gut pushes peritoneum and conjoint tendon through the superficial ring. An indirect hernia occurs when a loop of gut pushes peritoneum through the deep ring into the inguinal canal.

The spermatic cord

The spermatic cord passes through the inguinal canal to the testis. The vas deferens, testicular artery and veins, lymph vessels, autonomic nerves, cremasteric artery, artery of the vas and the genital branch of the femoral nerve are covered by three layers of fascia derived from the abdominal wall. The fascial covering of the spermatic cord is formed by the external spermatic fascia derved from the aponeurosis of the external oblique, the cremasteric fascia derived from the internal oblique and the internal spermatic fascia derived from the transversalis fascia.

The femoral canal

The femoral canal lies below the inguinal ligament medially and lies medial to the femoral vessels. The femoral sheath is formed by the transversalis fascia and encloses the femoral vessels and the femoral canal. The lacunar ligament forms the medial border of the femoral canal. The femoral vein lies lateral to the femoral canal.

The nerves and vessels of the abdominal wall

The lower thoracic nerves innervate the anterior abdominal wall, continuing on in the direction of the lateral intercostal space. T10 innervates the dermatome of the umbilicus. T12 innervates skin just above the inguinal ligament. The T7-12 nerves also innervate the anterior and lateral muscles. The iliohypogastric and ilioinguinal nerves innervate the lateral muscles and the skin over the inguinal ligament. The ilioinguinal nerve passes through the deep inguinal ring before piercing the anterior abdominal wall.


Hernias

Hernias occur through weak areas of the abdominal wall. It is usually the highly mobile small intestine which herniates either through the wall or into internal compartments. Since the abdominal wall is lined by parietal peritoneum it forms the lining of any hernial sac. At the point of passage through the abdominal wall the hernial sac is usually narrow, forming the neck, and expanded external to the neck.

Inguinal hernias

Inguinal hernias are classified as either direct or indirect. A direct hernia occurs when a hernial sac is pushed through the conjoint tendon directly towards the superficial ring. The hernial sac in this case is covered by a layer of peritoneum, transversalis fascia, conjoint tendon and the anterior wall of the spermatic cord. An indirect hernia occurs when a hernial sac enters the deep inguinal ring lateral to the inferior epigastric artery and passes indirectly to the superficial ring through the inguinal canal. In this case the hernial sac is covered only by the anterior wall of the spermatic cord. In both cases the hernial sac appears at the superficial ring, above and medial to the pubic tubercle.





Femoral hernia

In a femoral hernia the hernial sac is pushed into the femoral canal, below the inguinal ligament and between the lacunar ligament and the femoral vein. The hernial sac thus lies inferior and lateral to the pubic tubercle.

Umbilical hernia

At the umbilicus hernias can develop due to developmental deficiencies, congenital umbilical hernia, or may occur due to a weakness in the linea alba in the area of the umbilicus, an acquired umbilical hernia.

The diagram at the right illustrates the position of inguinal and femoral hernias relative to the pubic tubercle.




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