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Shoulder cap muscle
The easily palpated deltoid muscle is a thick triangular-shaped muscle that caps the shoulder joint. The deltoid is a complex muscle consisting of three principal parts. The anterior and posterior parts consist of parallel fiber bundles, while the fiber architecture of the lateral part is distinctly different from its anterior and posterior counterparts. Arising from the lateral border of the acromion four tendinous intramuscular septa descend distally into the muscle. Interdigitating with these septa are three ascending septa that arise from the distal insertion on the deltoid tuberosity. Attached to these interdigitating tendons is a dense array of short pennate muscle fibers. The deltoid develops from the posterior musculature of the embryonic limb and is a prime mover of the shoulder joint.