Spongy bone Compact bone Figure 2.6 The structure of compact bone.
of the cells and matrix produce two different types of bone—compact bone and spongy (cancellous) bone.
Compact Bone
Compact bone is found primarily in the shafts of long bones and the outer surfaces of short, fl at, and irregu- lar bones. Compact bone is built in concentric layers called lamellae. The structural units of compact bone are osteons, which consist of osteocytes located in small chambers called lacunae surrounded by bone matrix. The lacunae are linked with each other by canaliculi. Osteons are arranged in cylindrical shapes and have a central osteonic canal. These central canals (haversian canals) run lengthwise in the bone and carry nerves and blood vessels. Perforating or Volkmann’s canals traverse from one central canal to another carrying branches of blood vessels and nerves. This cylindrical arrangement around the central vessels allows the osteocytes to be in close proximity to their blood supply (Fig. 2.6).
Spongy Bone
In spongy bone, the lamellae have an irregular pattern, and osteons are not present. This less compact arrange- ment creates irregular spaces and forms bony lattices called trabeculae. Rather than being contained in canals, blood vessels run throughout the spaces of the sponge- like structure. Spongy bone is found primarily in the epiphyses of long bones, with a small amount in the shaft. It is also found in the centers of all other bones. The x-ray in Figure 2.7 shows the relationship of compact bone to spongy bone in a long bone.
Blood vessels
Figure 2.7 Both compact and spongy bone are visible in this x-ray of a long bone.
