Placed beneath the muscles last described. These also occupy the spaces between the ribs, and in addition extend between the sternal cartilages below. They resemble the external intercostals in their general form, but their fibres take an opposite course, viz. downward and forward, so that the two sets cross each other like the lines of the letter X.

Origin. - From the anterior borders of the ribs.

Insertion. - Into the posterior borders of the ribs in front.

Action.-To assist in expiration.