Internal Intercostal Muscles

Internal Intercostal Muscles

The Intercostales interni (Internal intercostals) are eleven in number on either side. They commence anteriorly at the sternum, in the interspaces 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, whence they are continued to the vertebral column by thin aponeuroses, the posterior intercostal membranes.

Each arises from the ridge on the inner surface of a rib, as well as from the corresponding costal cartilage, and is inserted into the inferior border of the rib above. The internal intercostals are innervated by the intercostal nerve.

Their fibers are also directed obliquely, but pass in a direction opposite to those of the Intercostales externi.

The external intercostals are used in inspiration, the intercartilaginous part, the part of the muscle that lays between the cartilage portion of the superior and inferior ribs, aids in elevating the ribs and expanding the thoracic cavity. In expiration, the interosseous part, 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. The internal intercostals, however, are only used in forceful exhalation such as coughing or during exercise and not in relaxed breathing.