Pectoralis major

The pectoralis major (from Latin pectus 'breast') is a thick, fan-shaped or triangular convergent muscle of the human chest. It makes up the bulk of the chest muscles and lies under the breast. Beneath the pectoralis major is the pectoralis minor muscle.

The pectoralis major arises from parts of the clavicle and sternum, costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle; it inserts onto the lateral lip of the bicipital groove. It receives double motor innervation from the medial pectoral nerve and the lateral pectoral nerve. The pectoralis major's primary functions are flexion, adduction, and internal rotation of the humerus. The pectoral major may colloquially be referred to as "pecs", "pectoral muscle", or "chest muscle", because it is the largest and most superficial muscle in the chest area.

It arises from the anterior surface of the sternal half of the clavicle from breadth of the half of the anterior surface of the sternum, as low down as the attachment of the cartilage of the sixth or seventh rib; from the cartilages of all the true ribs, with the exception, frequently, of the first or seventh, and from the aponeurosis of the abdominal external oblique muscle.

From this extensive origin the fibers converge toward their insertion; those arising from the clavicle pass obliquely downward and outwards (laterally), and are usually separated from the rest by a slight interval; those from the lower part of the sternum, and the cartilages of the lower true ribs, run upward and laterally, while the middle fibers pass horizontally.

They all end in a flat tendon, about 5 cm in breadth, which is inserted into the lateral lip of the bicipital groove (intertubercular sulcus) of the humerus.^{[clarification needed]}

This tendon consists of two laminae, placed one in front of the other, and usually blended together below:

These deep fibers, particularly those from the lower costal cartilages, ascend the humerus insertion higher, turning backward successively behind the superficial and upper ones, so that the tendon appears to be twisted. The posterior lamina reaches higher on the humerus than the anterior one, and it gives an expansion which covers the intertubercular groove of the humerus and blends with the capsule of the shoulder-joint.

From the deepest fibers of this lamina at its insertion, an expansion is given off which lines the intertubercular groove, while from the lower border of the tendon, a third expansion passes downward to the fascia of the arm.