large, triangular muscle that covers the shoulder and serves mainly to raise the arm laterally. The deltoid, as it is commonly known, originates on the outer front third of the clavicle (collarbone) and the lower margin of the spine of the scapula (shoulder blade). Its fibres unite to form a thick tendon that inserts at the deltoid tuberosity, a rough spot above the middle of the outer surface of the humerus (upper arm bone).