Abdominal Muscles

Introduction 

The abdominal muscles form the anterior and lateral abdominal wall and consist of the external abdominal obliques, the internal abdominal obliques, the rectus abdominis and the transversus abdominus . Their function is to protet the viscera, to help maintain posture and to compress the abdominal contents , thus raising intra-abdominal pressure such as in sneezing, coughing, micturating, defecating, lifting and childbirth. 

External Abdominal Oblique

Structure  

The external abdominal oblique muscle is the largest and most superficial of the four muscles and lies on the sides and front of the abdomen. It is broad and thin with it's muscular portion occupying the side and it's aponeurosis the anterior wall of the abdomen. It arises fromt he external surface and inferior borders of the lower eight ribs. The fibres from the lowest ribs pass nearly vertically downward and are inserted into the anterior half of the iliac crest, the middle and upper fibres,  directed inferiorly and anteriorly, end in an aponeurosis at approximately the mid-clavicular lineand insert into the xiphoid process, the linea alba, the pubic crest and the pubic tubercle.

Function 

Both sides acting together, the external abdominal oblique flex's the vertebral column by drawing the pubis toward the xiphoid process. Acting unilaterally it results in ipsilateral side flexion and contralateral rotation of the trunk. 

Internal Abdominal Oblique

Structure

The internal abdominal oblique muscle is also a broad thin muscular sheet that lies deep to the external oblique muscle. it arises from the thoracolumbar fascia, the anterior two-thirds of the iliac crest and the lateral two-thirsd of the inguinal ligament. the muscle fibres radiate superomedially and insert into the inferior borders of the lower three ribs and their costal cartilages , the xiphoid process, the linea alba and the symphsis pubis. Near their insertion the lowest tendinous fibres are joined with similar fibres from the transversus abdomins to form the conjoint tendon. 

Function

Acting unilaterally, contraction of the internal oblique results in ipsilateral side flexion and rotation of the trunk. It acts with the external oblique muscle of the opposite side to achieve this torsional movement of the trunk. It also acts to compress the abdominal viscera, pushing them up into the diaphragm, resulting in a forced expiration. 

Transversus Abdominis

Structure

The transversus abdominis muscle is the deepest of the abdominal muscles , lying internally to the internal abdominal obliques. It is a thin sheet of muscle whose fibres run horizontally anteriorly . It arises as fleshy fibres from the deep surface of the lower six costal cartilages, the lumbar fascia, the anteror two-thirds of the iliac crest and the lateral third of the inguinal ligament. it inserts into the xiphoid process, the linea alba and the symphsis pubis. the lowest tendinous fibres join similar fibres from the interior obliques to form the conjoint tendon which is fixed to the pubic crest and the pectineal line.