Medical Definition of longissimus thoracis
: a muscle that arises as the middle and largest division of the sacrospinalis muscle, that is attached by some of its fibers to the lumbar vertebrae, that is inserted into all the thoracic vertebrae and the lower 9 or 10 ribs, and that depresses the ribs and with the longissimus cervicis extends the spinal column and bends it to one side—called also longissimus dorsi
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