The internal intercostal muscles (Musculi intercostales interni)are fibers that run from the cranial aspect of tone rib to the caudal aspect of the preceding rib in a cranioventral direction (perpendicular to the fibers of the external intercostal muscles). These muscles lie lateral to the intercostal nerve and assist expiration.
Origin: lower border of ribs
Insertion: upper border of rib below
Action: expiration, to draw the ribs together so as to narrow the thoracic cavity
Nerve: Muscular branches of intercostal nerves one to twelve.