Muscle · Chest

Internal intercostals

Musculi intercostales interni

The second layer in each rib space, lying directly beneath the external intercostals with their fibres crossing them at right angles — downward and BACKWARD, the direction of the internal oblique they continue into. They do two opposite things depending on where along the space you are: the fibres between the bony ribs pull the ribs down and help you breathe out forcefully, while the fibres between the costal cartilages at the front lift the ribs and help you breathe in. Like their outer partner, they are part of the rib-cage stiffness the founder's document blames for a breath that goes up into the chest instead of out into the ribs.