The Pectoralis muscles consist of the Pectoralis Major and Pectoralis Minor muscles. The Pectoralis Major muscle is a thick, fan-shaped surface muscle located on the anterior chest of the human body. It makes up the majority of the Pectoralis muscles in the male. With women the Pectorlis Major muscle lies hidden under the breasts. Underneath the Pectoralis Major muscle is the Pectoralis Minor, which is a thin, triangular muscle.
The pectoral major muscle’s origin point is attached to the anterior surface of the sternal half of the clavicle and from the whole anterior surface of the sternum, as well as the cartliages of all true ribs as far down as the as the sixth or seventh rib. In addition to these, the pectoralis Major also has an origin point of attachment from the aponeurosis of the abdominal external oblique muscle. From these extensive origin points of attachment the pectoral major muscle fibbers converge together ending up about 5 cm wide and inserting into the lateral lip of the bicipital groove on the humerus.
There are three actions which contribute to the movement of the shoulder joint for which the pectorals major is primary responsible for:
- the flexion of the humerus, as in the lifting a child or throwing a ball.
- the adduction(moving a limb toward the mid-line of the body) of the humerus which can be demonstrated with the flapping of the hands simulating the wings of a bird.
- Rotating the humerus medially, as occurs when arm-wrestling.
The Pectoralis Major muscle is also responsible for keeping the arm attached to the trunk of the body.
The Pectoralis Minor muscle is a thin, triangular muscle, situated at the upper part of the chest, beneath the Pectoralis Major. The origin point of attachment is from the upper margins and outer surfaces of the third, fourth, and fifth ribs near their cartilages and from the apeneurosis covering on the intercostalis. The muscle fibers all converge to form a flat tendon, which is inserted into the medial border and upper surface of the scapula. In other words the minor muscle moves from the back to the front of the body. The pectoralis minor depresses the point of the shoulder, drawing the scapula inferior and medial and towards the thorax.