Spinalis thoracis is the medial continuation of the erector spinae, lying medial to and blending with longissimus thoracis in its lower part.  

Proximal Attachment
It arises by tendons from the spinous processes of T11 to L2.  

Distal Attachment
It passes superiorly to attach by separate tendons to the spinous processes of T1 to T4 (or as low as T8).

Nerve Supply
Spinalis thoracis is supplied by the dorsal rami of T1 to L1.

Action
Working individually it laterally flexes and slightly extends the thoracic spine.  When both sides are working they extend the thoracic spine.