Chest Wall Muscles

The pectoralis major is a thick, fan-shaped muscle, situated at the chest (anterior) of the body. It makes up the bulk of the chest muscles in the male and lies under the breast in the female. Underneath the pectoralis major is the pectoralis minor, a thin, triangular muscle.  This Beautiful 19th century etching shows the relation between those muscles.
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Notice in the lower figure where the pectoralis major muscle has been cut away, it has an extensive origin which includes: the anterior surface of the sternal half of the clavicle, 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. (adapted from wikipedia)

The external intercostal muscles aid in quiet and forced inhalation, are responsible for the elevation of the ribs, and expanding the transverse dimensions of the thoracic cavity.
the internal intercostal muscles, which aid in forced expiration (quiet expiration is a passive process), are responsible for the depression of the ribs decreasing the transverse dimensions of the thoracic cavity. (adapted from wikipedia)

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