The ribs and intercostal spaces

Abstract

Of the 12 ribs, the first 7 are ‘true’ and articulate via their costal cartilages with the sternum; ribs 8 to 10 articulate with each other through their costal cartilages (false ribs), while ribs 11 and 12 have costal cartilages that end in the abdominal wall muscles (floating). In inspiration, ribs 2 to 6 move upwards and forwards to increase the antero-posterior diameter of the thoracic cage (pump handle action), while ribs 7 to 10 move upwards and outwards, to increase the side-to-side diameter (bucket-handle action). Inspiratory movement of the chest wall is effected by the intercostal muscles, arranged in three layers: the external, internal, and (incomplete) innermost intercostals. Between the first and second layer, lying protected by the subcostal groove of the upper rib in the space, lies the intercostal neurovascular bundle, made up of the intercostal nerve, artery and vein from below upwards. The posterior intercostal vessels anastomose with anterior intercostal vessels. The neurovascular bundles from 7th downwards continue into the anterior abdominal wall to supply both the skin and anterior abdominal muscles. The muscles of the anterior abdominal wall relax in inspiration to allow full descent of the diaphragm; inspiration is inhibited by a tight corset. In full expiration the anterior abdominal muscles contract, thus raising the intra-abdominal pressure to elevate the relaxed diaphragm. Other accessory muscles are the scalenes and sternocleidomastoid, which elevate the first rib, and muscles of the limb girdles acting, from their insertions to their attachments, on the chest wall, which include the pectorals, latissimus dorsi and serratus anterior.