2.2. External Intercostal Muscles
The external intercostal muscles are also known as the 'hands-in-pockets' muscles, because their fibres run in an inferomedial direction. Unsurprisingly, these muscles are the most superficial of all the intercostal muscles. Remember that there are 12 ribs on each side, and therefore only 11 intercostal spaces and therefore only 11 sets of external intercostal muscles. The precise details of these muscles are listed below.
- Origin: inferior border of ribs above.
- Insertion: superior border of rib below.
- Fibre direction: inferomedial.
- Nerves: intercostal nerves (ventral rami of T1 - T11).
- Action: move ribs upwards and outwards, increasing the AP and transverse diameters of the thorax (see Section 2.9.1.)
The external intercostal muscles reach from the rib tubercles at the back, around the curve of the ribs and they end at the costochondral junction - this is where the ribs ('costo') articulate with the cartilage ('chondral'). This means that there is no external muscles between the costochondral junction and the chondrosternal junction. Instead of muscle there is an aponeurotic membrane (basically a flattened tendon) - and it is simply called the external intercostal membrane. Look at the diagram to confirm this. Also, just be aware that the external intercostal membrane is sometimes called the anterior intercostal membrane.