Inspiration, or breathing in, is a muscularly complex action. Different muscles can pull at the lungs from different angles, more or less directly moving the surfaces of the lungs away from each other to allow them, or force them, to fill with air.
The key muscles of inspiration are the diaphragm, pectoralis major and minor, serratus anterior, levatores costarum, serratus posterior superior, sternocleidomastoid, and the scalenes.
The Diaphragm
The respiratory diaphragm is a large, thin, domed sheet of muscle that separates the thoracic cavity (including the heart and lungs) from the abdomen. During inhalation, it contracts and descends, compressing the abdominal organs and decompressing the lungs. It can lift and separate the ribs.
Pectoralis Minor
This relatively small muscle runs from the coracoid process of the scapula to the third, fourth and fifth ribs. It pulls the ribs upwards, causing a high, shallow breath.
Pectoralis Major
This large chest muscle runs from the upper arm to the clavicle, sternum, and first eight ribs. It lifts the ribs, especially ribs four through eight. Pectoralis major opens the costal angle (the upside-down V from the sternum to the side of the ribcage), which aids in lifting the sternum.
Serratus Anterior
The serratus anterior glides between the scapula, where it is attached to the deep surface of its vertebral border, and the ribcage. It attaches to the first ten ribs. It lifts the lower ribs, pulls the costal angle open, and creates a powerful inhalation.
Levatores Costarum
The levatores costarum are a series of small muscles that attach the spinal vertebrae to the ribs one or two levels beneath them. They lift the back of the ribs to expand the lungs.
Serratus Posterior Superior
This muscle attaches the C7-T3/4 vertebrae to the first four or five ribs. Its effect on inspiration is small, but it does elevate the ribs.
Sternocleidomastoid
The sternocleidomastoid attaches the base of the skull to the sternum and clavicle. By pulling the sternum upwards, it raises the ribs in a high inhalation.
Scalenes
The scalenes attach the upper neck to the first and second ribs. They help to achieve very high inhalation by lifting the ribs.
In normal breathing, not all of these muscles contract strongly. However, all have a purpose for specific activities and breathing practices. By changing the way in which the ribs move (from a high inhalation that lifts the upper ribs to a deep breath that expands the lower ribs, for example), different muscles can change the quality of breathing significantly.
Further reading: Blandine Calais-Germain, Anatomy of Breathing (Eastland Press: Seattle, 2006)
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