The rectus abdominis muscle is entirely fleshy and is situated along the floor of the abdomen. The other muscles have a muscular part laterally and an aponeurotic part ventrally.
The aponeurotic parts of the abdominal muscles of the opposite side meet and form a median fibrous raphe (white fibrous cord), extending from the xiphoid cartilage to the prepubic tendon (the common tendon of all the abdominal muscles), which is inserted to the anterior border of the pubis. This white fibrous cord is the linea alba which shows a little behind its middle, a cicatrix, the umbilicus which indicates the position of the umbilical opening of the fetus.
The four abdominal muscles on either side of the linea alba, arranged in layers; these from without inwards –
This is the largest and the most superficial and extensive of all the abdominal muscles. The fibres of this muscle are directed downwards and backwards. It is having muscular and aponeurotic parts.
Origin: The lateral faces of ribs extending from the lower part of the fifth rib to the last rib above its middle and the lumbo-dorsal fascia.
Insertion: Linea alba, prepubic tendon and tuber coxae.
The aponeurosis of this muscle at its insertion to the prepubic tendon detaches a fibrous expansion -the poupart’s or inguinal ligament, which extends upwards and forwards in the medial face of the thigh and is inserted to the lateral angle of the ilium. It forms the posterior wall of the inguinal canal. In front of the pubis and about 5 cm from the median line its aponeurosis is perforated by the subcutaneous or external inguinal ring for the exit of the spermatic cord. The muscular part is attached to the lateral wall of the chest.
Action: Contraction of the muscle tends to reduce the size of the abdominal cavity and put pressure on the abdominal viscera in micturition, defecation, in forced expiration, coughing and parturition; to flex trunk (arch the back); acting singly, to flex the trunk laterally.
Blood supply: Circumflex iliac, intercostal and lumbar arteries.
Nerve supply: Last five or six intercostal and first two lumbar spinal nerves.
It is situated under the preceding and consists of muscular and aponeurotic portions. The aponeurotic part is blended with that of the external oblique muscle. Its fibres are directed downwards, forwards and inwards. It occupies the entire space bound by the last rib, the free extremities of the transverse processes of the lumbar vertebra and the external angle of the ilium.
Origin: Lateral angle of ilium and the adjacent part of the poupart’s ligament.
Insertion: Linea alba, prepubic tendon and posterior border of the last rib.
Action: Similar to the preceding.
The medial part of this muscle forms the anterior wall of inguinal canal. The upper opening of this canal, the abdominal or internal inguinal ring, which is found here, is slit like.
Blood supply: Circumflex iliac, intercostal and lumbar arteries
Nerve supply: First two lumbar and the last thoracic spinal nerves.
Rectus abdominis
The muscle is confined to the floor of the abdomen and extends from the sternum to the pubis. The two muscles are separated except at the pelvis by an interval of about 2 to 4 inches so that the abdominal wall here is entirely fibrous. Consisting the aponeuroses of the oblique and transverse muscles and abdominal tunic. It shows on its superficial face five tendinous inscriptions (linea transverse) near the second of which, is a foramen for the passage of subcutaneous abdominal vein.
Origin: Ventral and lateral parts of the sternum as forwards as the third costal cartilage.
Insertion: Pubis by means of the prepubic tendon.
Action: Similar to preceding.
Blood supply: Anterior and posterior abdominal arteries.
Nerve supply: The last six or seven thoracic and the first two or three lumbar nerves.
Transversus abdominis
This is the most medial of the abdominal muscles. It extends from the sternum to pubis and lies between the aponeurotic portion of the internal oblique muscle laterally and that of the transverse abdominis medially. It is a powerful, wide, band-like muscle.
Origin: Medial faces of the cartilages of the asternal ribs and the transverse processes of the first four lumbar vertebrae.
Insertion: Linea alba.
Action: Same as the preceding.
Blood supply: Circumflex iliac, musculo-phrenic and intercostal arteries.
Nerve supply: First two lumbar and the last seven or eight intercostal nerves.
