Chapter 4 The Skeletal System 125 Check Your Understanding 1. What percentage of bone mass comes from its mineral content? 2. What is collagen? 3. Where is cortical bone typically found? 4. Where is trabecular bone typically found? 5. List the five shape categories of bone. Anatomical Structure of Long Bones The diaphysis (digh-AF-i-sis) of a long bone is the hollow shaft of the bone composed of cortical bone (Figure 4.4). A fibrous connective tissue membrane called the periosteum (per-ee-AHS- tee-um) surrounds and protects the diaphysis. The periosteum contains blood and lymph vessels, • Flat bones are thin, relatively large in surface area, and generally curved to some extent. Structurally, they consist of two thin layers of cortical bone with a layer of trabecular bone in between. These bones protect underlying organs and also provide large areas for muscle attachments. The scapula and the bones of the skull are considered flat bones. • Sesamoid bones are bones that are formed within tendons. The patella (at the front of the knee) is an example of a sesamoid bone. • Irregular bones are all those bones that do not fit into one of the preceding categories. They have individualized shapes to fulfill specific functions. The bones of the spinal column and hip girdle are in this category. © Body Scientific International Figure 4.3 The five categories of bones. Short bones Long bone Irregular bone Sesamoid bone Flat bone Copyright Goodheart-Willcox Co., Inc.