Abdominal Wall & Cavity

Anterior Abdominal Wall

The anterior abdominal wall extends from the xiphoid and lower six costal cartilages to the anterior aspect of the pelvic bones. It is composed of several layers, including skin, superficial fascia, subcutaneous fat, anterolateral and midline muscle groups, transversalis fascia, extraperitoneal fat and peritoneum. The anatomy is well demonstrated on CT and MRI.

Ultrasound is useful in evaluating focal masses in the anterior abdominal wall but does not demonstrate the anatomy as well as CT or MRI. Conventional plain radiography has no place in the evaluation of the anterior abdominal wall.

The superficial fascia of the superior and central abdominal wall is a single layer containing a variable amount of fat. Inferiorly it divides into two layers, superficial and deep, between which run vessels, nerves and lymphatics.

The superficial layer is thick and contains areolar tissue and variable amounts of fat. It passes over the inguinal ligament to fuse with the underlying fascia of the thigh.

Inferomedially it forms the superificial perineal fascia by passing over the penis and scrotum, in males, and into the labia majora in females.

External Oblique

Origin: ribs 5-12

Insertion: iliac crest and pubic tubercle:

The posterior fibres, from the lower ribs, pass vertically to insert into the anterior half of the liac crest.

The middle and upper fibres pass anteriorly and forwards to end in the muscle’s aponeurosis.

The aponeurosis forms a strong tendinous sheet which medially contributes to the rectus sheath and linea alba.

Functions: Contralateral rotation of the torso.

Innervation: Thoracoabdominal nerves (T7-T11) and subcostal nerve (T12).

The free margin between the anterior superior iliac spine and pubic tubercle forms the inguinal ligament.

The medial part of the inguinal ligament forms the medial margin of the femoral ring. The superfical inguinal ring is a hiatus in the external oblique aponeurosis lying superolaterally to the pubic crest. It transmits the spermatic cord in males, the round ligament of the uterus in females and the ilioinguinal nerve in both.

Internal Oblique

The internal oblique lies internal to the external oblique and is thinner and less bulky than the external oblique

Origins: lateral two-thirds of inguinal ligament, anterior two-thirds of iliac crest and thoracolumbar fascia

Insertion: ribs 10-12.

posterior fibres pass upwards and laterally to the inferior borders of the lower three or four ribs, where they are continuous with the intercostal muscles.

Innervation: Thoracoabdominal nerves (T7-T11), subcostal nerve (T12) and branches of the lumbar plexus.

Functions: Bilateral contraction compresses the abdomen, while unilateral contraction ipsilaterally rotates the torso.

The rest of the fibres diverge and end in an aponeurosis which gradually broadens from below upwards. In its upper two-thirds this aponeurosis splits to form part of the anterior and posterior layers of the rectus sheath. Superiorly, the aponeurosis is attached to the cartilages of the seventh, eighth and ninth ribs. In the lower part of the abdominal wall the whole aponeurosis helps form the linea alba medially.

Transversus Abdominis

Origin: lateral third of the inguinal ligament, inner two-thirds of the iliac crest, the thoracolumbar fascia between the iliac crest and the 12th rib and costal cartilages 7-12

Insertion: conjoint tendon, xiphoid process, linea alba and the pubic crest.

Innervation: Thoracoabdominal nerves (T7-T11), subcostal nerve (T12) and branches of the lumbar plexus.

Functions: Compression of abdominal contents.

*The muscle ends in an aponeurosis, part of which inserts into the pubis forming the cojoint tendon and the rest passes medially to the rectus to help form the linea alba medially.

The transversalis fascia is a thin areolar membrane lying between the transversus abdominis and the extraperitoneal fat. The spermatic cord in the male, or round ligament in the female, passes through the transversalis fascia at the deep inguinal ring.

Rectus Abdominis

The rectus abdominis (commonly called the “sit-up” muscles) are paired paramedian strap muscles that extend from the ventral lower thorax to the pubis, separated from each other by the linea alba. These muscles are narrow and thick inferiorly, becoming broader and thinner superiorly.

They arise from two tendons inferiorly, the larger lateral tendon which originates on the crest of the pubis and the medial tendon which interlaces with the contralateral tendon and is continuous with the ligamentous fibres covering the front of the symphysis pubis.

The muscle fibres are enclosed by the aponeuroses of the oblique and transverse muscles, which form the rectus sheath. Each muscle is segmented by three transverse bands of collagen fibers called the tendinous intersections. This results in the look of “six-pack abs,” as each segment hypertrophies on individuals at the gym who do many sit-ups.

Origin: Pubic crest

Insertion: xiphoid process of the sternum and the costal cartilage of ribs 5-7.

Innervation: Thoracoabdominal nerves (T7-T11).

Functions: Assist flat muscles in compressing the abdominal viscera, stabilises the pelvis during walking, and depresses the ribs