There are four main muscles of abdominal wall. There are four large muscles on either side of the midline.
- External oblique
- Internal oblique
- Transversus abdominis
- Rectus abdominis.
Apart from these, two muscles namely cremaster and the pyramidalis are also present.
The external oblique and the transversus abdominis end in aponeurosis that reaches the midline. The aponeuroses of right and left sides decussate to form a median band called the linea alba in the midline.
The rectus abdominis runs vertically on either side of the linea alba. It is enclosed in a sheath formed by the aponeuroses of the flat muscles named above.
The muscle arises by eight fleshy slips from the lower eight ribs. The fibres run downwards, forwards and medially.
It inserts as following
- Most of the of the muscle fibers end in a broad aponeurosis through which they are inserted into the xiphoid process, the linea alba, the pubic symphysis, the pubic crest and the pectineal line of the pubis.
- The lower fibres of the muscle are inserted directly into anterior two-thirds of the outer lip of the iliac crest.
The muscle is supplied by lower six thoracic nerves.
The upper four slips of origin of the muscle interdigitate with those of the serratus anterior; and the lower four slips with those of the latissimus dorsi.
Between the anterior superior iliac spine and the public tubercle the aponeurosis has a free lower end that it folded on itself to from the inguinal ligament.
Between the linea semilunaris and the linea alba, the aponeurosis helps to form the rectus sheath.
Just above the public crest the aponeurosis of the external oblique muscle presents a triangular aperture called the superficial inguinal ring.
The muscle has free posterior and upper borders.
The muscle arises from:
- The lateral two-thirds of the inguinal ligaments.
- The anterior two-thirds of the intermediate area of the iliac crest, and
- The thoracolumbar fascia.
From this origin the fibers run upwards, forwards and medially and inserts as following
The uppermost fibres are inserted directly into the lower three or four ribs and their cartilages.
The greater part of the muscle ends in an aponeurosis through which it is inserted into to the 7th 8th, 9th costal cartilages, the xiphoid process, the linea alba, the public crest and the pectineal line of the pubis.
The muscle is supplied by lower six thoracic nerves and the first lumbar nerve.
Below a level midway between the umbilicus and the pubic symphysis the aponeurosis remains a single layer. It Passes in front of the rectus abdomonis to reach the linea alba.
Above this level the aponeurosis splints into an anterior lamina that passes medially in front of the rectus abdominis and a posterior lamina that lies behind the rectus. The posterior lamina ends below in a free curved margin called the arcuate line. The line is concave downwards.
The muscle has a originates from
- The lateral one-third of the inguinal ligament.
- The anterior two-thirds of the inner lip of the iliac crest.
- The thoracolumbar fascia.
- The inner surfaces of the lower six costal cartilages.
The fibres are directed horizontally forwards and end in a broad aponeurosis which is inserted into xiphoid process, the linea alba, the pubic crest, and the pectineal line of the pubis.
The lowest fibres of the muscle fuse with the lowest fibres of the internal oblique to from the conjoint tendon.
This muscle is supplied by lower six thoracic nerves, and first lumbar nerve.
The neurovascular plane of the abdominal wall lies between the internal oblique and transversus muscles. This plane is continuous the neurovascular plane of the thoracic wall. Various nerves and vessels run in this plane.
The muscle arises by two tendinous heads. The lateral head arises from the lateral part of the pubic crest and the medial head from the anterior pubic ligament.
The fibers run vertically upwards. and inserts on the wall of the thorax, along a horizontal line passing laterally from the xiphoid process cutting the 7th, 6th and 5th costal cartilages.
It is supplied by the lower six or seven thoracic nerves.
This muscle is enclosed in a sheath formed mainly by their aponeuroses of the three flat muscles of the abdominal wall and has tendinous intersections
These are three transverse fibrous bands which divide the muscle into smaller parts. One lies opposite the umbilicus, the second opposite the free end of the xiphoid process, and the third in between the two. One or two incomplete intersection may be present below the umbilicus. The intersections are actually zigzag in course traverse only anterior half of the muscle, and are adherent to the anterior wall of the rectus sheath.
The inguinal is formed by the lower border of the external oblique aponeurosis which is thickened and folded on itself. It extends from the anterior superior iliac spine to the pubic tubercle, and lies beneath the fold of the groin. Its lateral half is rounded and oblique. Its medial half is grooved upwards and is more horizontal.
The fascia lata is attached to the lower border. Traction of this fascia makes the ligaments convex downwards. The upper surface of the ligaments gives origin to the internal oblique from its lateral two-thirds, to the transversus abdominis from its lateral one-third, and to the cremaster muscle from middle its part.
Significance and Function of Anterior Abdominal Wall
Muscles of anterior abdominal wall provide protection to abdominal viscera by providing a firm but elastic support for the abdominal viscera against gravity. This is chiefly due to the tone of the oblique muscles, especially the internal oblique.
The oblique muscle, assisted by the transversus, can compress the abdominal viscera and thus help in all expulsive act, like micturition, defecation, parturition, vomiting, etc. This is one of the most important action of the abdominal muscles.
They are also important for forceful expiratory act.
They are also important for trunk movements.
Flexion of the trunk is brought about mainly by the rectus abdominis. Lateral flexion of the trunk is done by one sided contraction of the oblique muscles.
Rotation of the trunk is produced by a combined action of the external oblique with the opposite internal oblique.