Muscle · Thoracic spine

Semispinalis thoracis

Musculus semispinalis thoracis

A deep layer of the mid-back, lying under the erector spinae columns, made of thin fascicles that each bridge about five or six vertebrae — from a transverse process low down to a spinous process well above it. That long, shallow line of pull gives it very little leverage for lifting the trunk, so it works instead as a segment-by-segment guy-rope, holding the thoracic spine steady and turning it away from the side that is contracting. It runs the whole height of the thoracic spine and is continuous above with semispinalis cervicis, so it is one uninterrupted layer rather than a discrete belly — and it lies beneath the erector spinae columns, which means no part of it can be reached, or told apart, through the muscle covering it.