The ribs consist of 12 paired bones which form a protective cage around the heart and lungs. This cage is anchored dorsally to the vertebrae. Ventrally, the ribs are anchored to the sternum by way of cartilage. Males and females do not differ in their typical number of ribs, although some individuals may have 11 or 13 ribs on a side. Ribs are numbered from the first to the twelfth starting with the most superior rib.
The distal end of each ribs has a cartilaginous extension known as a costal cartilage. Ribs 1-7 are known as the sternal ribs because the costal cartilages connect directly to the sternum. Ribs 8-10 connect to a common costal cartilage which then joins up with the sternum. These are known as asternal ribs. Finally, ribs 11-12 have free floating ends which do not connect with the sternum. These ribs are termed floating ribs.
Generalized Rib Morphology
Each rib has a head on the dorsal end which articulates with the bodies of two thoracic vertebrae. The head of each rib bears two demifacets, one for each thoracic vertebra. Just distal to the head, on the inferior surface, is the tubercle, which marks the location of the rib’s articulation with the vertebral transverse process. Distal to the tubercle, the rib shaft takes a sharp bend at the angle. The ventral end of the rib, opposite the head, is termed the sternal end. Sternal ends connect to costal cartilage and resemble concave cups: their morphology changes with age and can be a useful guide to age estimation. The long, curved shaft of each rib bears a smooth and convex superior surface. The inferior shaft surface is marked by a sharp inferior margin and a shallow costal groove.
See more rib specimens in the Digital Teaching Collection.
The first rib is the most distinctive. It is extremely short and the shaft is flattened superior-inferiorly. The superior surface bears two shallow grooves while the inferior surface is smoother, lacking the costal groove typical of other ribs. The head of the first rib points inferiorly.
The second rib is longer than the first. It has a somewhat superior-inferiorly flattened shaft, but less so than the first rib.
Ribs 3-9 are “typical” ribs, sharing the description of the generalized rib given above. Ribs 3-7 increase in length, but length decreases starting with rib 8, and this decrease continues through the last rib. Rib 10 is similar to a typical rib, but has a single articular facet instead of demifacets. Rib 11 is similar to rib 10 but lacks a tubercle.
The rib shown to the right is a right Rib 6.
Ribs 3-9 are “typical” ribs, sharing the description of the generalized rib given above. Ribs 3-7 increase in length, but length decreases starting with rib 8, and this decrease continues through the last rib. Rib 10 is similar to a typical rib, but has a single articular facet instead of demifacets. Rib 11 is similar to rib 10 but lacks a tubercle.
The rib shown to the right is a right Rib 7.
Ribs 3-9 are “typical” ribs, sharing the description of the generalized rib given above. Ribs 3-7 increase in length, but length decreases starting with rib 8, and this decrease continues through the last rib. Rib 10 is similar to a typical rib, but has a single articular facet instead of demifacets. Rib 11 is similar to rib 10 but lacks a tubercle.
The rib shown to the right is a right Rib 11.
Rib 12 is extremely short (sometimes shorter than the first rib). It resembles rib 11 but lacks a costal groove, and is the least curved of all the ribs.
For most ribs, the superior shaft surface is smooth and convex, while the inferior shaft surface is grooved and bears a sharp ridge. The tuberosity is found on the inferior surface. The head and tubercle are dorsal while the rugged cup-shaped sternal end is ventral.
The first rib represents a special case. It is most easily sided by noting the inferior plunge of the head. The superior surface is marked by two shallow grooves. The rounded head is on the ventral end of the rib.