Transversus abdominis muscle
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The transversus abdominis muscle, named according to the direction of its muscle fibers, is one of the flat muscles that form the anterior abdominal wall. It is deep to the internal oblique muscle and ends in the anterior aponeurosis, which ultimately blends with the linea alba.
- origin: thoracolumbar fascia, inner lip of the anterior 2/3rd of the iliac crest, lateral 1/3rd grooved upper surface of inguinal ligament, 12th rib, and costal cartilages of 7th-12th ribs where it interdigitates with fibers of the diaphragm.
- insertion: anterior aponeurosis, ending in the linea alba and pubic crest
- innervation: anterior rami of spinal nerves T7-L1
- blood supply: posterior intercostal and subcostal arteries, superior and inferior epigastric arteries, superficial and deep circumflex iliac arteries, posterior lumbar arteries
- action: flat muscle which forms part of abdominal wall, compresses abdominal cavity
The transversus abdominis is the deepest of the lateral abdominal muscles (the others being the external and internal oblique muscles). It contributes mainly to maintain abdominal tone and can increase intra-abdominal pressure when it contracts.
It attaches to the lateral one-thirds of the inguinal ligament and the associated iliac fascia, the inner lip of the anterior 2/3rds of the iliac crest, the thoracolumbar fascia between the iliac crest and the 12th rib, and the costal cartilages of ribs 7-12. It ends in the anterior aponeurosis, with the majority of its fibers blending into the linea alba. The lower fibers curve medially and posteriorly to form the roof of the inguinal canal with those of the internal oblique muscle to insert into the pectineal line and pubic crest which forms the conjoint tendon. The posterior and inferior aspect contributes fibers to the interfoveolar ligament.
As a flat muscle, the vascular supply of the transversus abdominis includes the posterior intercostal and subcostal arteries, superior and inferior epigastric arteries, superficial and deep circumflex arteries and posterior lumbar arteries.
It is innervated by the terminal branches of intercostal nerves T7-T12, the subcostal nerve (T12), the iliohypogastric nerve (L1) and ilioinguinal nerves (L1). These nerves arise from the ventral rami of the spinal nerves of T7-L1, and this is the simplified description of innervation described in anatomy textbooks.