intercostales interni

The Intercostales interni are also eleven in number on each side.

Their attachments:

• 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 backwards as far as the angles of the ribs, where each is replaced by an aponeurotic layer named the posterior intercostal membrane, which is continuous with the superior costotransverse ligament.
  

Each muscle arises from the floor of the costal groove and the corresponding costal cartilage, and is inserted into the upper border of the rib below.

Their fibers axe also directed obliquely, but at right angles to those of the External intercostal muscles.