Deltoideus

Deltoideus, commonly known as the deltoid muscle, is a large, triangular muscle that forms the rounded contour of the shoulder. In humans it is usually described as having three functional parts or heads: anterior, middle, and posterior. The name derives from the Greek word deltoeidēs, meaning triangular.

Origin and insertion: the deltoideus originates from three sites—the lateral third of the clavicle (anterior head), the acromion (middle head), and the spine of the scapula (posterior head). It inserts at the deltoid tuberosity on the lateral humerus.

Nerve and blood supply: the muscle is innervated by the axillary nerve (C5–C6). Its blood supply comes primarily from the posterior circumflex humeral artery and branches of the thoracoacromial trunk.

Function: the deltoideus acts as the principal shoulder abductor, especially the middle head, which raises the arm away from the body. The anterior head flexes and medially rotates the arm, while the posterior head extends and laterally rotates the arm. The deltoid works in concert with the rotator cuff to stabilize the glenohumeral joint, and abduction is typically effective from about 15 degrees onward, following initial movement provided by the supraspinatus.

Clinical relevance: the deltoid is a common site for intramuscular injections, typically in the upper outer quadrant a short distance below the acromion to minimize risk to nerves and vessels. Injury or nerve damage affecting the axillary nerve can lead to deltoid weakness and impaired shoulder abduction.

Variations: in humans the muscle is generally described as three heads rather than distinct muscles; some animal species show variation in size and separation of parts.