For bone to grow and rearrange collagen fibers and minerals in lines of stress, two processes—one that builds and another that removes—must be in place. While osteoblasts help with bone formation, another group of cells (osteoclasts) reabsorb bone. In this way, the bone retains its shape and grows without be-coming thicker.
Normally, the outer layer of bone is dense and is known as compact, or cortical, bone. Internally, the bone is less dense, with bone spicules surrounded by spaces filled with red marrow. This is the spongy, can-cellous, or trabecular bone. Spongy bone is found inlarger amounts in short, flat, and irregularly shaped bones. A bone marrow cavity, or medullary cavity,may be found at the center of the bone. Cortical bone always surrounds cancellous bone, but the quantity of each type varies. About 75% of the bones in the body are compact. Because compact bone is solid, blood vessels that supply the cells with nutrients and nerves are contained in canals. The canals that run trans-versely from the periosteum are the perforating, or Volkmann’s, canals. These canals connect with otherthe haversian canals, canals that run longitudinally through the compact bone. The collagen fibers are arranged in lamellae,concentric layers around the canals forming cylinders called osteons or haversiansystems. The osteoblasts surrounded by calcified ma-trix in the compact bone are the osteocytes. They are located in small cavities known as lacunae. The lacu-nae communicate with other adjacent lacunae by tiny canals (canaliculi) that ramify throughout the bone connecting adjacent cells (see Figure 3.2) and the haversian canals. The osteocytes, therefore, obtain nu-trients from the blood vessels in the haversian canals.