The ribs (Latin: costae) are long, flat, curved bones that form the rib cage. There are twelve pairs of ribs, all of which articulate with the vertebral column, while only the first seven ribs directly articulate with the sternum. The rib cage forms the majority of the thoracic skeleton and provides protection for the internal thoracic organs, including the lungs and the heart. The ribs also provide attachment sites for thoracic muscles.
Based on articulations there are three types of ribs:
Based on their structure ribs can be classified as either typical or atypical. The typical ribs have generalized structure making these ribs similar, while the atypical ribs have variations of this structure.
Ribs 3 to 9 are considered typical ribs. Major landmarks of a typical rib are the following: head, neck, tubercle, and body of a rib.
The head of each rib is wedge-shaped and has two articular facets, which are separated by the crest of the head. The larger of the two facets is for articulation with the superior costal facet of its corresponding thoracic vertebral body, while the smaller facet articulates with the inferior costal facet on the body of the vertebra above.
The neck of the rib is a flat part of the bone that connects the head of the rib with its body.
The tubercle is a bony prominence located on the posterior side of each typical rib at the junction between the neck and the body. It consists of two parts, a smooth articular part for articulation with the transverse process of the corresponding vertebra, and a rough non-articular part which provides an attachment site for the costotransverse ligament.
The body of each rib is thin, flat, and curved. The most prominent part of the curve is the costal angle, which also provides attachment sites for some deep back muscles to the ribs. The internal surface of the costal body is concave and contains the costal groove for the passage and protection of the intercostal nerve and blood vessels.
Ribs 1, 2, 10, 11, and 12 are considered atypical ribs, because they have features that are not common with most of the ribs.
The 1st rib is the shortest, widest and has the sharpest curve of all the ribs. There is only one articulatory surface on the head of the first rib for articulation with the body of the 1st thoracic vertebra. The tubercle is like that of typical ribs, as it has a facet for articulation with the transverse process of the corresponding vertebra. The distinguishing features of the first rib are found on its superior surface, and these are two grooves for the passage of the subclavian blood vessels. These grooves are separated by the scalene tubercle, which serves as the attachment site for the anterior scalene muscles.
The 2nd rib is thinner and significantly longer than the first. There are two articular facets on the head of the second rib for articulation with the 1st thoracic (superior) and 2nd thoracic (corresponding) vertebrae. The main distinguishing feature of the 2nd rib is a roughened tuberosity on its superior surface, which provides part of the origin for the serratus anterior muscle.
The 10th rib only has one facet, which serves for articulation with its corresponding thoracic vertebra.
The 11th and 12th ribs are short, they have no necks or tubercles, and these ribs have only one facet - for articulation with their corresponding vertebrae.