146 Introduction to Anatomy & Physiology the reproductive organs, bladder, and part of the large intestine. It is formed by two large, strong coxal (KAHK-sal) bones (hip bones, also known as the os coxa) and the sacrum (Figure 4.24). These three bones, with the addition of the coccyx, comprise the pelvis. The female pelvis is wider than the male pelvis to enable pregnancy and childbirth. The pelvis is one of the hallmark differences that allows classification of a skeleton as male or female. Each coxal bone is formed by the fusion of the ilium, ischium, and pubis. During childhood these are three separate bones. The ilium (IL-ee-um) comprises most of each coxal bone, connecting posteriorly to the sacrum at the sacroiliac (sa-kroh-IL-ee-ak) joint. The prominent upper edge of the ilium, which can usually be palpated (examined or felt by touch), is called the iliac crest. The ischium (IS-kee-um), which forms the inferior portion of each coxal bone, is the bone that supports the weight of the upper body during sitting. Within each coxal bone lies an acetabulum (as-eh-TAB-yoo-lum): a deep, bony socket that receives the head of the thigh bone. The pubis (PYOO-bis) is the anterior portion of each coxal bone. The two pubic bones fuse in the center front of the body at the pubic symphysis (PYOO-bik SIM-fi-sis), where the bones are joined by a disc of hyaline cartilage. Whereas the joints of the fingers permit motion in only one plane, the thumb has the ability to freely rotate and to stretch across the palm of the hand. This capability, known as an opposable thumb, is shared only by humans and other primates. Check Your Understanding 1. Which bones make up the appendicular skeleton? 2. Which bones are included in the shoulder complex? 3. Which forearm bone enables rotation of the hand around the longitudinal axis of the arm? 4. How many bones are in each wrist and hand? The Lower Extremity The lower extremity is well designed for its functions of weight bearing and gait, including walking and running. During sporting activities, the muscles, bones, and joints of the lower extremity also enable movements involved in jumping, skating, surfing, skiing, and dancing, for example. The Pelvic Girdle As the name suggests, the pelvic girdle is a bony encasement of the pelvic region that shelters © Body Scientific International Figure 4.23 Anterior view of the bones of the right hand. Proximal phalanx 1 2 1 3 4 5 Phalanges First metacarpal Hamate Pisiform Triquetrum Lunate Trapezoid Trapezium Capitate Scaphoid Metacarpal bones Middle phalanx 2 Carpal bones Distal phalanx 2 Proximal phalanx 2 Distal phalanx 1 Copyright Goodheart-Willcox Co., Inc.