Transversus abdominis muscle

The transversis abdominus muscle, named according to the direction of its muscle fibres, is one of the flat muscles that forms 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. 

Summary:

origin: thoracolumbar fascia, iliac crest, lateral two-thirds of inguinal ligament, costal cartilages of 7th-12th ribs. 

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 epigastrtic arteries, superficial and deep circumflex arteries, posterior lumbar arteries

action: flat muscle which forms part of abdominal wall, compresses abdominal cavity

Gross Anatomy:

The transversus abdominus 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 two-thirds of the inguinal ligament and the associated ilaic fascia, the inner lip of the anterior segment 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 fibres blending into the linea alba. The lower fibres curve inferiorly and medially with those of the internal oblique muscle and anterior aponeurosis to insert into the pectineal line which forms the conjoint tendon.

As a flat muscle, the vascular supply of the transversus abdominus includes the posterior intercostal and subcostal arteries, superior and inferior epigastrtic arteries, superficial and deep circumflex arteries and posterior lumbar arteries

It is innervated by the terminal branches of intercostal nerves T7-12, the subcostal nerve, the iliohypogastric nerve and ilioinguinal nerves. 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.