The diaphysis is the main portion of a long bone. The ends of a long bone are called the epiphysis. There are several functions to the diaphysis. One example is the formation of red blood cells in the medullary (bone marrow) cavity, which is the central portion of the diaphysis. The medullary cavity is protected by two layers of tissue. These layers are named periosteum & endosteum. The periosteum is the thin outer layer of tissue around bone. This structure is usually affected when a stress fracture occurs. The endosteum is the inner lining on the bone comprised of thin connective tissue.
The diaphysis can also serve as the attachment site for muscle. This typically occurs in the lower extremities. Examples include the tibia (tibialis anterior), fibula (peroneal group), and femur (vastus muscle group).