Longissimus Thoracis belongs to the intermediate column of the Sacrospinalis group of muscles.
The common broad, thick tendon with Iliocostalis Lumborum, fibers from the transverse and mamillary processes of the lumbar vertebrae and lumbrosacral aponeurosis
The tips of transverse process of all thoracic vertebrae and the lower 9 or 10 ribs between the tubercles and angles 
Dorsal rami of thoracic and lumbar spinal nerves (T7 to L5) 
Dorsal branches of the posterior intercostal arteries from the thoracic aorta. 
- acting unilaterally, extension of vertebral column and flexion to the same side
- acting bilaterally, extension and hyperextension of vertebral column
Draws ribs down to help with breathing
Recent Related Research (from Pubmed)
- Gray, Henry. Anatomy of the Human Body. Philadelphia: Lea & Febiger, 1918; Bartleby.com, 2000. www.bartleby.com/107/.