Examining a long bone. Bone density is a major health concern, because the density can increase the risk of fractures and the need for surgical repair. Osteoporosis can occur, as shown here in the trabeculae of spongy bone. But compact bone can also become osteoporotic. This video will describe the location of compact and spongy bone in the long bone, and indicate the locations of the membranes of the bone that act as coverings. In general, long bones, such as the femur, are longer than they are wide. The shaft of the bone is called the diaphysis. The diaphysis is composed mostly of compact bone. The superficial membrane that covers the diaphysis is the periosteum. We will use a model to illustrate how the periosteum surrounds the compact bone. The model shown here represents a section of the compact bone of the diaphysis. The periosteum is shown in light blue. The perforating fibers of the periosteum penetrate the bone, and are continuous with tendons, ligaments, and aponeuroses. Also notice the osteons on the bone model. They are organizational structures composed of concentric rings of bone matrix called lamellae. The osteons are pulled out on the model for illustration purposes. Let's check your understanding. The membrane that covers the shaft of the long bone is the osteon, diaphysis, periosteum, or lamella? The periosteum is the external membrane that forms a covering around the diaphysis-- that's like the perimeter of a building. Lamella are layers of bone matrix that form the osteon, the organizational unit of compact bone. The ends of the long bone are called epiphyses, singular epiphysis. The proximal epiphysis of the femur forms the head. Each epiphysis has a layer of compact bone. The compact bone encloses the spongy bone. Spongy bone is composed of passageways called trabeculae. The trabeculae shown here are magnified 300 times. On their outer surface, the epiphyses have articular cartilage composed of hyaline cartilage, as shown with this chicken bone. This cartilage serves to absorb shock and reduce friction during movement. Let's check your understanding. Which of the following is mismatched? Spongy bone to trabeculae, compact bone to osteon, or articular cartilage to elastic cartilage? Articular cartilage is composed of hyaline cartilage. When bones are still growing in length, there will be a thin layer of hyaline cartilage in between the epiphysis and the diaphysis. It is called an epiphyseal plate, or growth plate. Returning to the sectioned long bone, we see that the growth plate is converted into bone and forms the epiphyseal line. The hollow portion of the interior of the shaft is called the medullary cavity. It is lined with a thin layer of spongy bone. In adult bones, the medullary cavity is filled with yellow marrow, which consists mostly of adipose tissue, as shown here. In children, it is filled mostly with red bone marrow and produces red blood cells. In both adults and children, red bone marrow is found in between the trabeculae. In adults, the trabecular spaces also store adipose tissue. The endosteum is a membrane that covers the trabeculae, forming spongy bone found in the diaphysis and the epiphysis. In the image shown here, the endosteum is covering the trabeculae in the epiphysis. Although we have been examining a femur, all long bones have the same general locations of compact bone, spongy bone, and membrane coverings. You can gain an appreciation of these features by examining a long bone, such as the femur or the humerus, illustrated here.