124 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. • 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. Surface Anatomy of Bones The external surfaces of bones are somewhat like the surface of the moon, scored with crater-like cavities, lined with ridges, and supporting hill-like bulbs. Though they may seem random, the locations of these various irregularities and markings on any given bone are consistent from person to person (Figure 5.4). This is because the surface features are functional, • Short bones are shaped roughly like a cube and are composed mainly of trabecular bone. The bones of the wrists and ankles are short bones. • 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 function to protect underlying organs and provide large areas for muscle attachments. The scapula, ribs, and the bones of the skull are considered flat bones. • Sesamoid bones are formed within tendons. The most prominent example of a sesamoid bone is the patella. Thoracic vertebra (lateral view) Scapula (posterior view) Femur (posterior view) Skull (lateral view) Inferior articular facet Inferior costal facet Spinous process Superior costal facet Transverse process Transverse costal facet Superior articular facet Lateral condyle Intercondylar fossa Lateral epicondyle Crest Sinuses Foramen Alveolus Meatus Foramen Medial condyle Medial epicondyle Adductor tubercle Linea aspera Gluteal tuberosity Intertrochanteric crest Neck Lesser trochanter Head Greater trochanter Acromion Supraspinous fossa Infraspinous fossa Spine © Body Scientific International Figure 5.4 Examples of bone markings and irregularities.