The longissimus thoracis muscle or musculus longissimus thoracis in latin (‘longissimus’ meaning longest, and ‘thoracis’ meaning thorax) is part of the longissimi muscles that belong to the sacro-spinal system which consists of longissimus cervicis and capitus muscles as well. This group of muscles belongs to the intermediate layer of the intrinsic back muscles that stretch along the trunk wall.
The longissimus thoracis is the largest of the erector spinae (or sacro-spinalis) muscles. It is located in the middle column of the upper lumbar region, and consists of many small bundles of muscle fibers. These bundles are aggregated in such a way that produces a very long, and in some places thick, muscle. The adjacent spinalis thoracis laterally blends with it, and is considered a component of the muscle by some.
The thoracic segment of the longissimus muscles originates from the sacrum (os sacrum) and iliac crest (crista iliaca) through the iliacostalis lumborum tendon and the lumbosacral aponeurosis. It also arises from the spinous processes of the lumbar vertebrae (processus spinosus L1-L5) and the transverse processes of the lower thoracic vertebrae. It inserts into the transverse processes of all the thoracic vertebrae, and lateral to the tubercles of the medial half of the second till twelfth ribs (costae II-XII).
The lateral branches of the dorsal spinal rami innervate the longissimus thoracis muscle (as well as the iliocostalis muscles). Its blood supply is provided by the dorsal branches of the posterior intercostal arteries that arise from posterior aspect of the descending thoracic aorta and are distributed in between the lower intercostal spaces.
The longissimus thoracis is one of the powerful primary extensors of the vertebral column and head. Acting bilaterally, it extends the back and returns it into an upright position. It also participates in facilitating both lateral and backward flexion of the spine by a series of coordinated muscle contractions and relaxations. On the other hand, unilateral contraction bends the spine towards the same side. It also rotates the face to the actively contracting ipsilateral side.
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