The medullary cavity is the cavity within a bone where a soft and flexible substance called marrow is stored. The term "medulla" is a Latin word for middle, which describes the space occupying the middle of the bone. This pit is a necessary component of the body's skeletal system, which combines with muscular features to form the musculoskeletal system.
Due to its positioning and the substance it stores, the medullary cavity is also known as marrow cavity or the cavity of the bone marrow. Its walls are made of spongy bone known as cancellous bone. This is one of the two types of osseous tissue responsible for bone formation, and it is the softer and less rigid of the two. Lining the cavity is a thin layer of connective tissue known as the endosteum.
The medullary cavity usually contains two types of marrow. The red bone marrow consists of tissue formed out of blood cellular components, and it is the only type of marrow present at birth. As one ages, however, more red bone marrow changes to yellow bone marrow. This is also known as adipose tissue, since it contains fat cells. In adulthood, marrow is virtually divided into the two types of this tissue.
The red bone marrow of the medullary cavity usually occurs in the flat bones of the body. This includes the breast and hip bones, ribs, skull and shoulder blades. The middle section of long bones such as the femur and humerus are the main sites of yellow bone marrow. The clavicle, however, is the only long bone in the body that lacks a medullary cavity.
Red bone marrow is responsible for producing red and white blood cells, which are essential for providing oxygenated blood and disease protection, respectively. It also produces platelets, which control bleeding. The yellow bone marrow stores the fat essential for the body's energy level.
In some instances, both red and yellow bone marrow share responsibilities or work together. For instance, they stop the lymph, which is an important element of the body's circulatory system, from back-flowing. Also, yellow bone marrow transforms into red bone marrow to ramp up production of blood cells so that blood loss can be curbed.
The medullary cavity is the site for a treatment method called the intramedullary (IM) rod, or IM nail. It is sometimes named after its inventor, German surgeon Gerhard Küntscher, who used it to treat the femur fractures of World War II solders. Today, physicians also use the IM nail to treat bone cancer, specifically tumors occurring in the medullary cavity.