Scapular spinal part of deltoid muscle

At a glance

English

Scapular spinal part of deltoid muscle

Latin (TA2)

Pars spinalis scapularis musculi deltoidei

System

Muscle

Region

Upper limb

Side

Paired, left and right

About the scapular spinal part of deltoid muscle

The deltoid is the thick, triangular muscle that gives the shoulder its rounded outline. It has three parts: a clavicular part in front, an acromial part at the side and a spinal or scapular part behind. This page shows the posterior, spinal part, which covers the back of the shoulder joint and the upper parts of the rotator cuff muscles below the scapular spine.

The spinal part takes origin along the scapular spine, from the inferior lip of its crest along most of its length behind the acromion, and from the fascia over infraspinatus. Its fibres run forward, outward and downward, converging with the other two parts on a thick tendon. All three attach to the deltoid tuberosity, the rough V-shaped area halfway down the lateral side of the humerus. The fibres of this part are long and parallel, unlike the multipennate middle part.

Acting alone, the spinal part extends the arm and rotates it laterally. It is the main muscle for drawing the raised arm backward in the horizontal plane, as in a rowing pull or the backswing of a throw. During abduction it works with the other parts to keep the humeral head centred, and it helps stop the arm from being pulled forward. The whole deltoid is supplied by the axillary nerve, and blood reaches this part mainly from the posterior circumflex humeral artery, with branches from the circumflex scapular artery.

The axillary nerve winds round the surgical neck of the humerus, so a fracture there or a shoulder dislocation can injure it, causing wasting of the deltoid and numbness over the outer upper arm. Pushing the arm back against resistance tests this part directly.

Attachments, action and supply

Origin

Inferior lip of the scapular spine crest and the fascia over infraspinatus.

Insertion

Deltoid tuberosity of the humerus, with the other parts of the muscle.

Action

Extends and laterally rotates the arm; draws the raised arm backward and helps steady the humeral head.

Nerve supply

Axillary nerve (C5, C6).

Blood supply

Posterior circumflex humeral artery, with branches of the circumflex scapular artery.