The marrow is one of the three layers in a living bone. If we cut though the compact bone, we find that its cylinder surrounds and protects the spongy bone marrow, which contains a material, much like gelatin. This cylinder is the medullary cavity, and is the central cavity of bone shafts. It is where yellow bone marrow (or adipose) is produced and stored, and also where we find red marrow. In infants, all of the marrow is red because blood cell formation is taking place within these cavities. This red marrow produces red blood cells (which carry oxygen), white blood cells (which fight infection), or platelets (that help stop bleeding). As we age, the portion of red marrow diminishes until as adults only about 50% of bone marrow is red.