128 Introduction to Anatomy & Physiology a microscopic level inside the mineralized part of bone. Blood vessels and nerves course through these tiny tunnels (Figure 4.7). Major passageways running lengthwise through the bone are called Haversian (ha-VER-zhen) canals. Tiny cavities called lacunae (la-KOO-nee) are covering of articular cartilage over the ends of the bone in chicken drumsticks. How does living bone receive nourishment and get rid of waste products? Bone has what you might think of as its own subway system. An intricate array of passageways exists at At the same time that the osteoblasts are doing their work, the osteoclasts resorb layers of bone inside the medullary cavity, causing the diameter of the cavity to be progressively enlarged. This beautifully engineered process occurs in such a way that a healthy bone remains optimally functional, lightweight, and strong enough to resist daily stresses. Adult Bone Development As people age, there is a progressive loss of collagen and a subsequent increase in bone brittleness. This means that children are often able to sustain falls and other accidents without harm, while older adults tend to be increasingly vulnerable to bone fractures. Bone mineral normally peaks in women at about 25 to 28 years of age and in men at about 30 to 35 years of age. Thereafter, bone mass is progressively lost. Because women tend to have smaller bones than men, the loss of bone mass and bone mineral density is generally more problematic for women. Life Span Review 1. During which parts of life are osteoblasts and osteoclasts active? 2. How and why is the medullary cavity in bone enlarged? 3. What is the importance of the epiphyseal plate? 4. Why are elderly individuals more prone to bone fractures? © Body Scientific International Figure 4.7 A—A microscopic view of the inside of bone tissue. B—An even more magnified view. Why might someone compare the inside of living bone to a subway system? Periosteum Cortical bone Blood vessel Haversian canal Perforating (Volkmann’s) canal B Magnified view A Pie-shaped section Spongy bone Osteons (Haversian system) Blood vessel continues into medullary cavity containing marrow Lamellae Lamella Osteocyte Canaliculus Lacuna Haversian canal Copyright Goodheart-Willcox Co., Inc.