The major muscles of quiet inspiration include:


The Diaphragm


The External Intercostals

The Diaphragm

Besides being the principal muscle of inspiration, the diaphragm also separates the thoracic from the abdominal cavity to convert the thorax into a closed chamber.

The diaphragm is composed of two muscular hemidiaphragms, joined by a membranous portion at the midline. The center portion of the diaphragm normally curves upward when relaxed at end-expiration. During contraction, the central portion of the diaphragm becomes more flattened thus increasing the volume of the thoracic cavity. This increase lowers the intra-alveolar pressure to below atmospheric and air rushes in.

Motor innervation to the diaphragm originates from the spinal cord at C3 through C5. These nerves join to comprise the phrenic nerve.