The Intercostal Muscles
These muscles are found between the ribs, and there are two kinds: the internal and external intercostals. The internal intercostal muscles (in the inside of the ribcase) extend from the front of the ribs, and go around back, past the bend in the ribs. The external intercostal muscles (on the outside of the ribcase) wrap around from the back of the rib almost to the end of the bony part of the rib in front.
The image above is seen from the inside. The yellowish bones are the sternum in the centre and the ribs and clavicle (at the very top). Between the ribs can be seen the internal intercostals. In the front of the ribcase, looking from the bottom of each muscle (i.e., the top of each rib), the muscles go diagonally INWARD. At the bottom of the sternum can be seen the tranversus thoracis muscle which originates there and inserts onto the inner surfaces of the second to the sixth costal cartilages. You can really feel these muscles when you cough, as you press your sternum in hard. At the bottom of the images can be seen the edge of the diaphragm.
The external intercostals (not shown), take the opposite path, that is they go downward and OUTWARD when viewed from the back. It is important to understand that having the muscles on diagonals increases the amount of work they can do - a longer muscle can become shorter on contraction than can a shorter, vertical muscle, as it contracts along the full length of the muscle fibers.
Now that we know what they are, it's time to learn how they work...
Back to Respiration
More on The Intercostals
The Thoracic & Abdominal Wall, viewed from the inside.
This large image shows the relationship of the diaphragm, the abodominals and the internal intercostals.