Deltoid - Musculus deltoideus
Origin: Clavicle, acromion, spine of the scapula
Insertion: Deltoid tuberosity of humerus
Artery: Primarily posterior circumflex humeral artery
Nerve: Axillary nerve
Action: Shoulder abduction, flexion and extension
Antagonist: Latissimus dorsi
The Deltoideus (Deltoid muscle) is a large, thick, triangular muscle, which covers the shoulder-joint in front, behind, and laterally.
It arises from the anterior border and upper surface of the lateral third of the clavicle (clavicular part); from the lateral margin and upper surface of the acromion (acromial part), and from the lower lip of the posterior border of the spine of the scapula (spinal part), as far back as the triangular surface at its medial end.
From this extensive origin the fibers converge toward their insertion, the middle passing vertically, the anterior obliquely backward and lateralward, the posterior obliquely forward and lateralward; they unite in a thick tendon, which is inserted into the deltoid prominence on the middle of the lateral side of the body of the humerus. At its insertion the muscle gives off an expansion to the deep fascia of the arm.
This muscle is remarkably coarse in texture, and the arrangement of its fibers is somewhat peculiar; the central portion of the muscle—that is to say, the part arising from the acromion—consists of oblique fibers; these arise in a bipenniform manner from the sides of the tendinous intersections, generally four in number, which are attached above to the acromion and pass downward parallel to one another in the substance of the muscle. The oblique fibers thus formed are inserted into similar tendinous intersections, generally three in number, which pass upward from the insertion of the muscle and alternate with the descending septa.
The portions of the muscle arising from the clavicle and spine of the scapula are not arranged in this manner, but are inserted into the margins of the inferior tendon.
Variations.—Large variations uncommon. More or less splitting common. Continuation into the Trapezius; fusion with the Pectoralis major; additional slips from the vertebral border of the scapula, infraspinous fascia and axillary border of scapula not uncommon. Insertion varies in extent or rarely is prolonged to origin of Brachioradialis.
Nerves.—The Deltoideus is supplied by the fifth and sixth cervical through the axillary nerve.
Actions.—The Deltoideus raises the arm from the side, so as to bring it at right angles with the trunk. Its anterior fibers, assisted by the Pectoralis major, draw the arm forward; and its posterior fibers, aided by the Teres major and Latissimus dorsi, draw it backward.
This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).