Everyday Muscle.

Spinalis Thoracis

Back
Spinalis thoracis is the most medial erector spinae muscle in the thoracic region, sitting closest to the spinous processes. It is often blended with nearby back muscles, so it is not always cleanly separated during dissection.
Function
Extends the thoracic spine when both sides contract. Assists with same-side lateral flexion when one side contracts. Helps maintain upright posture and stabilize the vertebral column.
Origin
Spinous processes of T11–L2.
Insertion
Spinous processes of the upper thoracic vertebrae, commonly T2–T8.
Innervation
Lateral branches of the dorsal/posterior rami of spinal nerves, mainly thoracic and upper lumbar levels.
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Latissimus Dorsi Erector Spinae Iliocostalis Spinalis Multifidus Rotatores Interspinales Intertransversarii Serratus Posterior Inferior Serratus Posterior Superior