External abdominal oblique muscle

The external oblique muscle of the abdomen is the most superficial of the abdominal muscles. Very broad and flat, it is composed of a fleshy dorso-cranial part and a ventro-caudal aponeurotic part. It originates from the lateral surfaces of the ribs (varying according to species, generally from the caudal ribs) and inserts into the linea alba and pelvic structures via a broad aponeurosis.

Origin: From the ventral quarter of the ribs (except the first ones), with species variations, and from the thoracolumbar fascia caudally.

Insertion: Caudally, the fibers attach to the ilium. All other terminations belong to the aponeurosis, which attaches to the linea alba, the prepubic tendon, and the inguinal arch.

Reports: The external oblique muscle of the abdomen is covered by the abdominal tunic, the presence of which varies among species. Cranially, it is related to the ascending pectoral muscle; more caudally, to the cutaneous trunci muscle and the skin, and at its caudal extremity, to the tensor fasciae latae muscle. Its fleshy part inserts on the ventral quarter of the ribs, covers the intercostal muscles, and interdigitates with the serratus ventralis thoracis. Ventrally and cranially, it overlies the rectus abdominis, and caudally it lies adjacent to the internal oblique. Its aponeurosis, closely fused with that of the internal oblique, contributes to the formation of the rectus sheath.

Action: Acting with the other abdominal muscles, it compresses the abdominal viscera. This action, known as the abdominal press, is involved in vital functions such as expiration, urination, defecation, and parturition. The muscle also contributes to flexion of the vertebral column during bilateral contraction, and to lateral bending during unilateral contraction.

Blood supply: By the dorsal intercostal, lumbar, and circumflex iliac arteries. In some species, such as carnivores, pigs, and rabbits, also by the cranial abdominal artery and branches of the musculophrenic, cranial, and caudal epigastric arteries.

Nerve: By the intercostal nerves of the spaces corresponding to the fleshy attachments, and by the ventral branches of the first two or three lumbar nerves.

Variation:

The obliquus externus abdominis is divided in two different parts (not listed in the Vth NAV) in carnivores:

• A costal part (Obliquus Externus Abdominis (Pars Costalis))

  • Origin: From the middle parts of the fourth to twelfth ribs and the adjacent trunk fascia.

  • Insertion: Pubic symphyses and xiphoid process via the linea alba.

  • Action: Along with other abdominal muscles, compression of the abdominal viscera. This action, known as abdominal press, aids in such vital functions as expiration, urination, defecation, and parturition. Flexion of the vertebral column when fellow muscles contract. Lateral bending of the vertebral column.

  • Nerve: Lateral branches of the last eight or nine intercostales and the lateral branches of the costoabdominalis, iliohypogastricus, and ilioinguinalis.

  • A lumbar part (Obliquus Externus Abdominis (Pars Lumbalis))

• Origin: Last rib.

  • Insertion: Into twelfth rib.

  • Action: Along with other abdominal muscles, compression of the abdominal viscera. This action, known as abdominal press, aids in such vital functions as expiration, urination, defecation, and parturition. Flexion of the vertebral column when fellow muscles contract. Lateral bending of the vertebral column.

  • Nerve: Lateral branches of the last eight or nine intercostales and the lateral branches of the costoabdominalis, iliohypogastricus, and ilioinguinalis.