and the occipital bone. They have their insertion in the clavicle and scapula.
The LATISSIMUS DORSI is a broad flat muscle that covers approximately one-third of the back on each side. It rotates the arm inward and draws the arm down and back. It originates from the upper thoracic vertebrae to the sacrum and the posterior portion of the crest of the ilium. Its fibers converge to form a flat tendon that has its insertion in the humerus.
Figure 3-27.Important superficial muscles, anterior view.
The PECTORALIS MAJOR is the large triangular muscle that forms the prominent chest muscle. It rotates the arm inward, pulls a raised arm down toward the chest, and draws the arm across the chest. It originates in the clavicle, sternum, and cartilages of the true ribs, and the external oblique muscle. Its insertion is in the greater tubercle of the humerus.
The DIAPHRAGM is an internal muscle that forms the floor of the thoracic cavity and the ceiling of the abdominal cavity. It is the primary muscle of respiration, modifying the size of the thorax and abdomen vertically. It has three openings for the passage of nerves and blood vessels.
The DELTOID muscle raises the arm and has its origin in the clavicle and the spine of the scapula. Its insertion is on the lateral side of the humerus. It fits like a cap over the shoulder and is a frequent site of intramuscular injections.
Figure 3-28.Important superficial muscles, posterior view.
The BICEPS BRACHII is the prominent muscle on the anterior surface of the upper arm. Its origin is in the outer edge of the glenoid cavity and its insertion in the tuberosity of the radius. This muscle rotates the forearm outward (supination) and, with the aid of the brachial muscle, flexes the forearm at the elbow.
The TRICEPS BRACHII is the primary extensor of the forearm (the antagonist of the biceps brachii). It originates at two points on the humerus and one on the scapula. These three heads join to form the large muscle on the posterior surface of the upper arm. The point of insertion is the olecranon process of the ulna.