This muscle is a long, narrow and thin muscle located along the lateral border of the longissimus dorsi and across the ribs. It has distinct lumbarand costal parts.
Origin:Lumbar part from the transverse processes of lumbar vertebrae and the lateral angle of the ilium and the costal part from the anterior borders and lateral surfaces of last six or seven ribs.
Insertion:Costal part is inserted to the posterior border of ribs from first to the tenth and transverse process of the last cervical vertebra and lumbar part is to the posterior border of the last rib.
Action:Expiratory muscle.
Blood supply: Lumbar and intercostal arteries.
Nerve supply: Lumbar and dorsal spinal nerves.
Longissimus dorsi
This is the longest and most powerful of all the muscles in the body. It extends from the sacrum and the ilium to the middle of the cervical spine. It is situated medial to the preceding and is not distinctly separable from it in the lumbar region.
Origin:Crest and the lateral angle of the ilium, summits of sacral spines, lumbo-dorsal spines and supraspinous ligament.
Insertion:Transverse and articular processes of lumbar vertebrae, transverse processes of dorsal vertebrae and upper surfaces of ribs. Anteriorly about the level of the 7th rib it divides into two parts- the dorsal part (Spinalis et semispinalis) and ventral part (longissmus cervicis)between which emerges the complexus muscle. The dorsal part passes under the complexus and is inserted to the neural spines of the last three or four cervical vertebrae and the ventral part is situated to the transverse process of the last cervical vertebrae.
Three sets of fasciculi are recognized in this muscle:
The superficial or spinal fasciculi attached to the spinous process.
The deep or transverse fasciculi attached to the articular and transverse processes of the lumbar and transverse process of the thoracic and cervical vertebra;
The costal fasciculi turn outwards to become attached to the lateral surface of the ribs.
Action:The two sides of the muscle acts as extensors of the back and loin and singly to flex the spine to the same side. By its attachment to the ribs it acts as an expiratory muscle.
Blood supply: Dorsal, deep cervical, vertebral, intercostal and lumbar arteries.
Nerve supply: Cervical, dorsal and lumbar spinal nerves.
