They commence anteriorly at the sternum, in the intercostal spaces between the cartilages of the true ribs, and at the anterior extremities of the cartilages of the false ribs, and extend backward as far as the angles of the ribs, hence they are continued to the vertebral column by thin aponeuroses, the posterior intercostal membranes.

The internal intercostal muscles originate from the costal groove of the rib and insert into the superior aspect of the rib below in a direction perpendicular to the external intercostal muscles.

In exhalation the interosseous portions of the internal intercostal muscles, (the part of the muscle that is between the bone portion of the superior and inferior ribs), depresses and retracts the ribs, compressing the thoracic cavity and expelling air.

[2] The internal intercostals, however, are only used in forceful exhalation such as coughing or during exercise and not in relaxed breathing.

[4] This article incorporates text in the public domain from page 403 of the 20th edition of Gray's Anatomy (1918)