Anterior scalene

• Originates on the transverse processes of C3-C7
• Inserts on the superior surface of the first rib.
• Elevates the first rib during forced inhalation, or, when the rib is fixed, flexes the neck (moves the head closer to the chest or shoulder).

Middle scalene

• Originates on the transverse processes of C2-C7.
• Inserts with the anterior scalene on the first rib.
• Elevates the first rib and flexes the neck.

Posterior scalene

• Originates on the transverse processes of C4-C6.
• Inserts on the second rib.
• Elevates rib 2; when this rib is fixed, it flexes the neck.

Diaphragm

• Originates on the inferior internal surface of the sternum, costal cartilages of ribs 6-12, and the lumbar vertebrae L1-L3.
• Inserts into the central tendon.
• Expands the thoracic cavity to decrease intrapulmonary pressure and induce inspiration (inhalation).

Three layers of intercostal muscles:

External intercostals

• Originate on the inferior border of the ribs.
• Insert into the superior border of the ribs below.
• Elevate the ribs during inhalation.

Internal and innermost intercostals

• Originate on the superior border of the ribs.
• Insert into the inferior border of the ribs above.
• Depress ribs during forced exhalation (normal exhalation requires no active effort).

Transversus thoracis

• Originates on the posterior surface of the sternum (body and xiphoid process).
• Inserts into the internal surfaces of ribs 2-6.
• Depresses ribs during exhalation.