126 Introduction to Anatomy & Physiology The bulbous endings of the long bones are known as epiphyses (eh-PIF-i-seez). These regions are composed of trabecular bone that contains red marrow, which participates in the formation of red blood cells, or erythrocytes (e-RITH-roh-sights), and some white blood cells, or leukocytes (LOO-koh-sights). Each epiphysis is surrounded by a protective covering of articular (ar-TIK-yoo-lar) cartilage. You may have noticed the shiny white as well as nerves. It is involved in bone growth, repair, and nutrition. The hollow center of the diaphysis is the medullary cavity, or canal. Beginning when a person is about five years old, this cavity is filled with yellow bone marrow, which has a rich supply of blood vessels and is a storehouse for fat. The medullary cavity is lined by a membrane known as the endosteum (ehn-DAHS-tee-um). © Body Scientific International Figure 4.4 The anatomical structure of a long bone. A—Anterior view of the humerus with the interior of the top half exposed. B—Cortical and trabecular bone of the epiphysis. C—Enlargement of the diaphysis. Periosteum Periosteum C Diaphysis A Humerus B Cortical and trabecular bone Epiphysis Diaphysis Nutrient arteries Cortical bone Cortical bone Epiphysis Articular cartilage Articular cartilage Articular cartilage Endosteum Endosteum Blood vessel Yellow bone marrow Yellow bone marrow (in medullary cavity) Medullary canal Cortical bone Trabecular bone Red marrow cavities Trabecular bone Copyright Goodheart-Willcox Co., Inc.