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The manubrium is a section of the sternum or breastbone in the human chest. Situated at the top of the bone between the two clavicles or collarbones, it is the broadest portion of the sternum. With its symmetrical, many-sided shape, wider at the top than at the bottom, this bone resembles a head perched upon the body of the sternum. The many facets of the manubrium are for joining with multiple structures, including two facets for the two clavicle bones, one facet for the body of the sternum, and two facets apiece for the top and second rows of costal cartilage, which link the manubrium on either side to the first two ribs.
Flattened and curved slightly posteriorly when viewed from the side, the manubrium features a concave facet along its top aspect that does not join with any other bones. The superior facet can be felt by placing one’s index finger where the two collarbones meet at the base of the front of the neck. A semicircular cavity known as the suprasternal, or jugular, notch can be felt in the space between them. From front to back, the superior facet is the thickest edge of the manubrium. It is also where a major muscle of the anterior neck, the sternocleidomastoid, attaches before coursing upward and outward, attaching its other end to the skull just below the ear.
To each side of the suprasternal notch are similarly concave facets, angled obliquely upward and sideways. These facets, identified as the clavicular notches, are where the two clavicle bones form joints with the manubrium. Known as the sternoclavicular joints, they are a type of synovial or movable joint called an arthrodial or gliding joint, meaning that the articulating bony surfaces can slide past each other to a small degree. Specifically, the clavicles can slide up and down and front to back as well as rotate slightly relative to the sternum, which allows the arm to move in the shoulder joint, particularly when raising the arm overhead.
Below the clavicular notches on either side of the manubrium are curved facets for articulation with the first ribs, above, and below with the second ribs. These joints, the sternocostal joints, are in fact where the sternum joins with the costal cartilage, the thick cap of fibrous tissue at the medial or sternal end of each rib bone. The first sternocostal joint is known as a synarthrodial joint, meaning that the cartilage is essentially fused to the sternum and allows no true movement.
Beneath this, the second sternocostal joint is another arthrodial joint, which allows the costal cartilage to barely slide against the sternum and which is held together by ligaments. To permit such movement, there is a synovial membrane, which contains a lubricating, fluid-filled joint capsule, between the articulating surfaces. A second membrane joins the second costal cartilage to the body of the sternum.
Finally, the lower facet of the manubrium attaches to the body of the sternum. Covered in cartilage, this facet forms an articulation with the body known as an amphiarthrodial joint, meaning that the amount of movement allowed is more than that of a synarthrodial joint and less than that of an arthrodial joint. This joint is characterized by the presence of disks of a type of cartilage known as fibrocartilage, which cushion the ends of the two bones against each other and provide some flexibility to the joint but that otherwise do not permit much movement between them.