The sternum, commonly known as the breastbone, is a long, flat bone situated in the center of the chest, forming the anterior part of the rib cage. This bone is divided into three distinct segments that function together to protect internal organs, such as the heart and lungs. The superior, or uppermost, portion of this bone is the manubrium.
The Manubrium and Its Features
The manubrium is the broad, thick, and somewhat quadrangular upper part of the sternum, often resembling the knot of a necktie. It is the thickest segment of the sternum, which helps it manage the load from the upper limbs.
A distinct landmark on the manubrium’s upper border is the suprasternal notch, also known as the jugular notch, a noticeable concave depression easily felt at the base of the neck. Flanking the jugular notch are the clavicular notches. These specialized articular surfaces connect with the medial ends of the clavicles, forming the sternoclavicular joints.
The manubrium serves as an attachment point for the first pair of ribs, articulating directly with their costal cartilages. It also contributes a half-facet for the articulation of the second pair of ribs. The posterior surface of this segment provides attachment for muscles like the sternohyoid and sternothyroid.
Completing the Breastbone Structure
The manubrium connects inferiorly to the central and longest segment, known as the body of the sternum (or the gladiolus). This articulation point forms a slight projection that is visible and palpable on the anterior chest wall. The junction between the manubrium and the body is called the sternal angle, or the Angle of Louis.
The sternal angle is an important anatomical landmark because it is the exact point where the costal cartilages of the second pair of ribs attach to the sternum. Healthcare professionals use this distinct ridge as a starting point to identify and count the remaining ribs. The sternal angle also marks the approximate horizontal plane separating the superior and inferior parts of the mediastinum, the region containing the heart and major blood vessels.
The body of the sternum is longer and thinner than the manubrium, featuring four transverse ridges that mark the fusion lines of the bone segments during development. This central section connects directly with the costal cartilages of ribs three through six, and partially with ribs two and seven.
The most inferior and smallest segment of the sternum is the xiphoid process, a structure that is often triangular or sword-shaped. This segment remains largely cartilaginous in youth and may not fully ossify until an individual is around 40 years old. The xiphoid process is an attachment site for several muscles, including the diaphragm and the rectus abdominis, one of the primary abdominal muscles.