The most common bone shape in the body is the long bone shape. One of the principal features is a shaft called a(n) diaphysis that contains a hollow, cylindrical space inside called the medullary cavity. In children, this cavity contains bone marrow, which is later replaced by bone marrow in adults. An end of these bones is called a(n) . The other three bone shapes in the body generally have an external surface composed of bone and the interior is composed entirely of bone.