The ribs form the main structure of the thoracic cage protecting the thoracic organs, however their main function is to aid respiration.
There are 12 pairs of ribs which are separated by intercostal spaces. The first seven ribs progressively increase in length, the lower five ribs then begin to decrease in length. Ribs are highly vascular and trabecular with a thin outer layer of compact bone.
Ribs can also be divided into true, false and floating ribs:
- true ribs: the first seven pairs of ribs are true ribs as they are attached to the sternum directly by costal cartilages anteriorly
- false ribs: the 8 to 10 ribs converge anteriorly to each other via costal cartilages and eventually to the seventh rib, therefore, their connection to the sternum is indirect and hence the term false ribs
- floating ribs: the 11 and 12 ribs have no anterior direct or indirect sternal attachments and therefore are classified as floating ribs; these ribs are often of variable length
The costal groove runs along the inferior border of each rib. It is situated on the internal aspect of the rib. Each costal groove accommodates an intercostal vein, artery and nerve in that order downwards.
The internal intercostal muscle extends inferoposteriorly from the costal groove to the superior border of the rib below
The costal margin is the lower margin of the thoracic wall.
Anteriorly, cartilage from the 7, 8, 9 and 10 ribs form the margin by extending inferolaterally from the xiphisternal joint. The apex of the angle forms the infrasternal or sternal angle. Posteriorly, cartilage from the 11 and 12 ribs form the margin. At its lowest point, cartilage from the 10 rib defines the margin.
The greater part of the liver lies below the right costal margin. In gross hepatomegaly, the liver will extend inferiorly from the right costal margin. The stomach extends below from the left costal margin. The pathologic spleen may extend inferomedially from below the left costal margin.
The ribs are attached to corresponding thoracic vertebrae posteriorly. They articulate at the costovertebral joints at the head of the rib and at the costotransverse joints with the tubercle. Anteriorly, most are attached directly to the sternum. They articulate at the costochondral joints with some exceptions.
There are a number of muscles related to ribs.
- intercostal muscles: situated in the intercostal spaces.
- diaphragm: arises from the inner surfaces of the costal cartilages on the sixth rib
- serratus anterior: originates anterolaterally from the 1 to 8 ribs
- pectoralis major and minor muscles arise from superior anterior ribs
- latissimus dorsi: originates from the 9 to 12 ribs
- scalenus anterior, posterior and medius muscles have attachments on the first and second ribs
- rectus abdominis: inserts at the xiphisternum and the 5 to 7 costal cartilages
In 35-70% of the population, the 10 rib may also be floating. Additional variation is associated with ethnicity.