There are 12 pairs of ribs, all of which are attached posteriorly to the respective thoracic vertebrae. The upper 7 pairs are attached to the sternum directly through their costal cartilages. Ribs 8 through 10 are attached indirectly to the sternum through the seventh costal cartilage. Their costal cartilages are also attached with each other at different points. The 11th and 12th rib do not attach anteriorly to anything and just end in the muscles of body wall. They are, for this reason, called floating ribs.
A typical rib is a long twisted bone, which is flat in outline and has smooth rounded superior border and a sharp thin inferior border. The inferior border of each rib overhangs and forms the costal groove , through which the intercostals vessels and nerves pass.
Parts of a typical rib:
A typical rib consists of 5 parts: Head, Neck, Tubercle, Shaft and Angle.
Head: It has two facets, the inferior of which articulates with the body of the numerically corresponding vertebra and the superior articulates with the vertebra immediately above.
Neck: It is a constricted portion of the bone situated between the head and the tubercle.
Tubercle: It is a prominence on the outer surface of the rib at the junction of neck and shaft. It contains a facet for articulation with the transverse process of numerically corresponding vertebra. Thus we see that a typical rib forms a total of three joints with the vertebral column.
Shaft (Body): It is thin, flattened and twisted on its long axis. The inferior border has a grove known as costal grove.
Angle: Angle is formed where the shaft of the rib bends sharply forwards. The anterior end of each rib is attached to the corresponding costal cartilage thorough which it is jointed to the sternum.
The first rib is not like the others and is known as atypical rib. It is of great importance clinically because of its close relation with the brachial plexus subclavian vessels (main vessels of the arm). Unlike other ribs, it is flattened from above downwards. It has a tubercle, known as the scalene tubercle, on the inner border. The scalenus anterior muscle is inserted into this tubercle. Anterior to this tubercle, the subclavian vein crosses over the rib while coming out of the upper limb. Posterior to the tubercle, there is a groove known as the subclavian groove through which the subclavian artery and the lower trunk of the brachial plexus cross the rib.
They are bars of hyaline cartilage that connect the ribs with the sternum. There are a total of 12 ribs divided into three categories on the basis of their articulation with the sternum. The first seven are known as true ribs because their costal cartilages articulate directly with the sternum. Ribs eight through ten are known as false ribs because their costal cartilages do not articulate with the sternum directly. They articulate with the cartilage immediately above. Ribs eleven and twelve are known as floating ribs because their costal cartilages do not articulate with the sternum at all.
The costal cartilages make a significant contribution to the elasticity and mobility of the rigid thoracic wall and permit vital movement of respiration without which the process of exchange of gases in the lungs would have impaired. In old age, as a result of superficial calcification, the costal cartilages lose their elasticity and become somewhat ossified.
In about 0.5% of human beings, there is an additional rib, known as the Cervical rib, which arises from the seventh cervical vertebra. It may articulate with the first rib on its anterior aspect or may be connected by a firbrous band. Sometimes the anterior end is free and not articulated with anything. A cervical rib is clinically important because it can cause pressure on the lower trunk of the brachial plexus producing pain down the medical side of the forearm and wasting of small muscles of hand. It can also interfere with the circulation of upper limb by exerting pressure on the subclavian vessels that pass close to it.