The deltoid is a large and triangular muscle, which arises from the lateral third of the clavicle, the acromion, and the scapular spine. The deltoid is divided into three portions: The anterior, middle, and posterior portions, according to their origin. These portions converge and attach to the deltoid tubercle on the lateral aspect of the humeral shaft. The deltoid has at least seven segments according to the intramuscular tendons of its origin and insertion. The deltoid is innervated by the axillary nerve, which passes through the quadrilateral space and travels around the humeral neck. The blood supply to the deltoid muscle is provided by several arteries, i.e., the thoracoacromial artery, the anterior and posterior circumflex humeral arteries. The dominant one is the posterior circumflex humeral artery. The cephalic vein is a dominant vein and travels in the deltopectoral groove. The deltoid is a main abductor of the shoulder joint as well as the flexor, extensor, internal and external rotators, and adductor with some contribution by the surrounding muscles.
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