11.11: The Thoracic Cage: Sternum
The thoracic or rib cage forms the body's thorax (chest) portion. Its primary function in the body is to protect vital organs in the thoracic cavity, such as the heart and the lungs. It consists of 12 pairs of ribs with their costal cartilages and the sternum. The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1-T12).
The sternum is the elongated bony structure on the anterior side of the thoracic cage. It consists of three parts: the manubrium, the body, and the xiphoid process. The manubrium is the broader, superior portion of the sternum where the first rib pair attaches. The top of the manubrium has a shallow, U-shaped depression called the jugular (suprasternal) notch. It can be easily felt at the anterior base of the neck, between the medial ends of the clavicles. The clavicular notches are the shallow depressions on either side along the superior-lateral margins of the manubrium. They act as the site of the sternoclavicular joint between the sternum and clavicle.
The elongated, central portion of the sternum is termed the body. The manubrium and body join together at the sternal angle: a slightly bent junction. The second rib attaches to the sternum at the sternal angle. Since the first rib is hidden behind the clavicle, the second rib is the highest rib that can be identified by palpation. Thus, the sternal angle and second rib are essential landmarks for identifying and counting the lower ribs. Ribs 3–7 attach to the sternal body. The inferior tip of the sternum is the xiphoid process. This small structure is cartilaginous early in life but gradually becomes ossified with age.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 7.4 The Thoracic Cage (Sternum)
