Deltoid m.

The deltoid m. is the muscle forming the rounded contour of the shoulder.

The deltoid originates in three distinct sets of fibers, often referred to as "heads":

1. The anterior or clavicular head arises from most of the anterior border and upper surface of the lateral third of the clavicle. The anterior head lies adjacent to the lateral fibers of the pectoralis major m. The two muscles are separated by only a small triangular space, the deltopectoral triangle, through which the cephalic vein passes.

2. The lateral or acromial head arises from the superior surface of the acromion process.

3. The posterior or spinal head arises from the lower lip of the posterior border of the spine of the scapula.

The deltoid m. is innervated by the axillary n. and is the main abductor of the humerus at the glenohumeral joint. The anterior head of the deltoid m. can also flex the humerus, whereas the posterior head of the deltoid m. can also extend the humerus.