To retain their shapes, all organisms need some type of structural

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1 36 1 The Skeletal System To retain their shapes, all organisms need some type of structural support. Unicellular organisms have a cytoskeleton that provides structural support. In multicellular animals, support is provided by some form of skeleton, including the external exoskeletons of arthropods and the internal endoskeletons of vertebrates. The human skeleton is composed of a type of connective tissue called bone. Bones and other connective tissues, such as cartilage and ligaments, form the skeletal system. Scientists can infer a lot about the behavior of extinct species by studying fossil bones and reconstructing skeletons. The human skeleton also contains important clues. The shape of your hip bones shows that you walk upright on two legs. The structure of the bones in your hands, especially your opposable thumbs, indicates that you have the ability to grasp objects. The size and shape of your skull is a clue that you have a welldeveloped brain. The Skeleton The skeletal system has many important functions. The skeleton supports the body, protects internal organs, provides for movement, stores mineral reserves, and provides a site for blood cell formation. The bones that make up the skeletal system support and shape the body much like an internal wooden frame supports a house. Just as a house could not stand without its wooden frame, the human body would collapse without its bony skeleton. Bones protect the delicate internal organs of the body. For example, the skull forms a protective shell around the brain, and the ribs form a basketlike cage that protects the heart and lungs. Bones provide a system of levers on which muscles act to produce movement. Levers are rigid rods that can be moved about a fixed point. In addition, bones contain reserves of minerals, mainly calcium salts, that are important to many body processes. Finally, bones are the site of blood cell formation. Blood cells are produced in the soft marrow tissue that fills the internal cavities in some bones. There are 206 bones in the adult human skeleton. As shown in Figure 36 2 on page 922, these bones can be divided into two parts the axial skeleton and the appendicular skeleton. The axial skeleton supports the central axis of the body. It consists of the skull, the vertebral column, and the rib cage. The bones of the arms and legs, along with the bones of the pelvis and shoulder area, form the appendicular skeleton. SECTION RESOURCES Print: Laboratory Manual A, Chapter 36 Lab Teaching Resources, Lesson Plan 36 1, Adapted Section Summary 36 1, Adapted Worksheets 36 1, Section Summary 36 1, Worksheets 36 1, Section Review 36 1 Reading and Study Workbook A, Adapted Reading and Study Workbook B, Time Saver Key Concepts What are the functions of the skeletal system? What is the structure of a typical bone? What are the three different kinds of s? Vocabulary periosteum Haversian canal bone marrow cartilage ossification ligament Reading Strategy: Asking Questions Before you read, rewrite the headings in this section as how, why, or what questions about the skeletal system. As you read, write brief answers to those heading questions. Figure 36 1 Bones provide a system of levers on which muscles act to produce movement. Without this coordination, movement would not be possible. Issues and Decision Making, Issues and Decisions 38 Technology: itext, Transparencies Plus, 1 FOCUS Objectives State the functions of the skeletal system Describe the structure of a typical bone Explain how bones develop Identify the three different kinds of s. Vocabulary Preview Tell students that words beginning with os, the Latin word for bone, have something to do with bone. For example, the word ossification means the process of bone formation. Challenge students to find other words beginning with os, such as osteocyte, and explain each word s connection with bone. Reading Strategy Have students preview the material in the section by studying the figures and reading the captions. They should make note of any words they do not know and find the definitions as they read the section. 2 INSTRUCT The Skeleton Show students a three-dimensional model of the human skeleton. Challenge them to identify the bones of the axial skeleton (skull, vertebral column, rib cage) and the appendicular skeleton (arms, legs, pelvic girdle, pectoral girdle). Allow students to manipulate the bones so that they have a better understanding of how the axial skeleton supports the body and the appendicular skeleton allows movement. Skeletal, Muscular, and Integumentary Systems 921

2 36 1 (continued) Structure of Bones Address Misconceptions Students may have difficulty conceiving of bone as living tissue. Ask: Which do you think is a better model of a bone, a stick of chalk or a piece of sponge? (Some students may say that a stick of chalk is a better model.) Point out that a stick of chalk may look more like a bone, but a piece of sponge is more like a bone in its structure. Both the sponge and the bone contain a network of tubes or spaces through which things can pass. Ask: What passes through the tubes and spaces inside bone? (Blood vessels and nerves) Use Visuals Figure 36 3 Make sure students understand how the two parts of the figure are related. Point out how the drawing on the right shows a cross section of a tiny piece of the bone on the left. Guide students in using the figure to distinguish between compact and spongy bone tissue. Ask: What structures are found in compact bone? (Haversian canals, veins, arteries, and osteocytes) Where is spongy bone found? (Beneath compact bone at the ends of long bones and in the middle of short, flat bones) Development of Bones Build Science Skills Comparing and Contrasting Work with the class to create a table comparing and contrasting bone and cartilage. Have a volunteer record the information in a chart on the chalkboard as the class brainstorms the similarities and differences between the two types of tissue. Axial Skeleton Skull Sternum Ribs Vertebral column Appendicular Skeleton Clavicle Scapula Radius Pelvis Ulna Carpals Metacarpals Phalanges Tarsals Metatarsals Phalanges Figure 36 2 The skeleton supports the body. The human skeleton is divided into two parts: the axial skeleton and the appendicular skeleton. NSTA For: Links on bones and s Visit: Web Code: cbn-0361 SUPPORT FOR ENGLISH LANGUAGE LEARNERS Structure of Bones It is easy to think of bones as nonliving. After all, most of the mass of bone is mineral salts mainly calcium and phosphorus. However, bones are living tissue. Bones are a solid network of living cells and protein fibers that are surrounded by deposits of calcium salts. Figure 36 3 shows the structure of a typical bone. The bone is surrounded by a tough layer of connective tissue called the periosteum (pehr-ee- AHS-tee-um). Blood vessels that pass through the periosteum carry oxygen and nutrients to the bone. Beneath the periosteum is a thick layer of compact bone. Although compact bone is dense, it is far from being solid. Running through compact bone is a network of tubes called Haversian (huh-vur-zhun) canals that contain blood vessels and nerves. A less dense tissue known as spongy bone is found inside the outer layer of compact bone. It is found in the ends of long bones and in the middle part of short, flat bones. Despite its name, spongy bone is not soft and spongy; it is actually quite strong. Near the ends of bones where force is applied, spongy bone is organized into structures that resemble the supporting girders in a bridge. This latticework structure of spongy bone helps to add strength to bone without adding mass. Osteocytes, which are mature bone cells, are embedded in the bone matrix. Two other kinds of bone cells osteoclasts (AHS-tee-oh-klasts) and osteoblasts line the Haversian canals and the surfaces of compact and spongy bone. Osteoclasts break down bone. Osteoblasts produce bone. Although we stop growing in our late teens, our bones are continuously remodeled through the activity of osteoclasts and osteoblasts. Within bones are cavities that contain a soft tissue called bone marrow. There are two types of bone marrow: yellow and red. Yellow marrow is made up primarily of fat cells. Red marrow produces red blood cells, some kinds of white blood cells, and cell fragments called platelets. Development of Bones The skeleton of an embryo is composed almost entirely of a type of connective tissue called cartilage. The cells that make up cartilage are scattered in a network of protein fibers including both tough collagen and flexible elastin. NSTA Download a worksheet on bones and s for students to complete, and find additional teacher support from NSTA SciLinks. Comprehension: Key Concept Beginning On the board, rewrite the boldface sentence on page 921 as individual sentences that each express one function of the skeletal system. Then, pair ESL students with students who are proficient in English. Have the student pairs construct a concept circle (cluster diagram) with Functions of the Skeletal System in the center and the five functions connected to the center by lines. Intermediate Read aloud the boldface sentence on page 921, and write it on the board. Ask individual students, including some ESL students, to identify and describe the functions of the skeletal system. Give students copies of Figure Work with students to identify parts of the skeleton that perform specific functions, e.g., the skull protects, the femur supports. Have students label the appropriate parts with the functions. 922 Chapter 36

3 FIGURE 36 3 STRUCTURE OF A BONE Bones are a solid network of living cells and protein fibers that are supported by deposits of calcium salts. A typical long bone such as the femur contains spongy bone and compact bone. Within compact bone are Haversian canals, which contain blood vessels. Spongy bone Compact bone Periosteum Spongy bone Haversian canal Compact bone Haversian Canal (magnification: 200 ) Make Connections Health Science Inform students that force must be placed on bone for ossification to occur, because it is force that stimulates the osteoblasts to secrete the minerals that replace cartilage. Ask: What effect do you think an exercise such as walking would have on the bones of the legs? (It would stimulate ossification, so the bones would contain more minerals and be stronger.) Ask: What do you think might happen to bones that are not exposed to force, such as the bones of astronauts in zero gravity? (The bones would lose minerals because of lack of force exerted on them, so they would become weaker.) Bone marrow Unlike bone, cartilage does not contain blood vessels. Cartilage cells must rely on the diffusion of nutrients from the tiny blood vessels in surrounding tissues. Because cartilage is dense and fibrous, it can support weight, despite its extreme flexibility. Cartilage is replaced by bone during the process of bone formation called ossification (ahs-uh-fih-kay-shun). Ossification begins to take place up to seven months before birth. Bone tissue forms as osteoblasts secrete mineral deposits that replace the cartilage in developing bones. When the osteoblasts become surrounded by bone tissue, they mature into osteocytes. Many long bones, including those of the arms and legs, have growth plates at either end. The growth of cartilage at these plates causes the bones to lengthen. Gradually, this new growth of cartilage is replaced by bone tissue, and the bones become larger and stronger. During late adolescence or early adulthood, the cartilage in the growth plates is replaced by bone, the bones become completely ossified, and the person stops growing. In adults, cartilage is found in those parts of the body that are flexible, such as the tip of the nose and the external ears. Cartilage also is found where the ribs are attached to the sternum, which allows the rib cage to move during breathing. What is ossification? Osteocyte Artery Vein Periosteum Demonstrate to students that even ossified bones contain a framework of collagen. Bring a clean chicken bone to class and, after pointing out how relatively hard and inflexible it is, place it in a beaker of vinegar to soak. After a few days, remove the bone and invite students to inspect it. They will observe that the bone has become rubbery and flexible. Explain that the vinegar dissolved the calcium (mineral) in the bone, leaving behind the collagen (protein). Ask: What role does collagen play in an ossified bone? (It provides a framework for the minerals in the bone and gives the bone some flexibility.) TEACHER TO TEACHER When introducing the skeletal system, I go to a local health clinic and obtain a series of X-rays of the s and bones that will be discussed. Technicians from local clinics are sometimes willing to visit the class and discuss the different X-rays, as well as explain the various aspects of X-ray medicine. As a class, we have also built an X-ray skeleton by mounting the various X-rays on a bulletin board. This skeleton is also a very good tool to use in reviewing the chapter. Bob Sprang Biology Teacher Mitchell Senior High School Mitchell, South Dakota Answer to... Ossification is the process of bone formation in which cartilage is replaced by bone. Skeletal, Muscular, and Integumentary Systems 923

4 36 1 (continued) Types of Joints Build Science Skills Using Models Provide students with materials such as craft sticks, toothpicks, pipe cleaners, modeling clay, tacks, and glue. Then, challenge them to create models of one or more types of s shown in Figure Invite students to demonstrate their completed models to the class. Ask: What type of and what range of motion does your model illustrate? (Models should illustrate the range of motion of one of the four types of s shown in the figure.) Call on other students to name examples of that type of. Ask a volunteer to model the movement of several different s. As you name each, have the student demonstrate the range of motion permitted by the. In each case, challenge the rest of the class to name other s that have the same range of motion. Structure of Joints Use Community Resources Invite a professional from the medical community to speak to the class about s and problems. Possible speakers might include a radiology technician, physical therapist, chiropractor, or physician s assistant in sports medicine, rheumatology, or orthopedics. Encourage students to prepare questions for the speaker in advance. Afterward, have them write a summary of what they learned. Radius Saddle Ball-and-Socket Joint Ulna Hinge Clavicle Ball-andsocket Scapula Hinge Joint Pivot Joint Pivot Saddle Joint Metacarpals Carpals Figure 36 4 Freely movable s are classified by the type of movement they permit. The s illustrated are in the shoulder, knee, elbow, and hand. Types of Joints A place where one bone attaches to another bone is called a. Joints permit bones to move without damaging each other. Some s, such as those of the shoulder, allow extensive movement. Others, like the s of the fully developed skull, allow no movement at all. Depending on its type of movement, a is classified as immovable, slightly movable, or freely movable. SE page reduction 80% Immovable Joints Immovable s, often called fixed s, allow no movement. The bones at an immovable are interlocked and held together by connective tissue, or they are fused. The places where the bones in the skull meet are examples of immovable s. Slightly Movable Joints Slightly movable s permit a small amount of restricted movement. Unlike the bones of immovable s, the bones of slightly movable s are separated from each other. The s between the two bones of the lower leg and the s between adjacent vertebrae are examples of slightly movable s. Freely Movable Joints Freely movable s permit movement in one or more directions. Freely movable s are grouped according to the shapes of the surfaces of the adjacent bones. The most common types of freely movable s are shown in Figure Ball-and-socket s permit movement in many directions. They allow the widest range of movement of any. Hinge s permit backand-forth motion, like the opening and closing of a door. Pivot s allow one bone to rotate around another. Saddle s permit one bone to slide in two directions. What are the four common types of freely movable s? For: Joint Movement activity Visit: PHSchool.com Web Code: cbp-0361 For: Joint Movement activity Visit: PHSchool.com Web Code: cbe-0361 Students explore the skeletal and muscular systems through various movements. 924 Chapter 36 PROGRAM BIOLOGY RESOURCES UPDATE Bionic s Osteoarthritis plagues many older adults, causing them to have stiff, aching s and keeping them from being as active as they would like. Replacing arthritic s, especially the hip and knee, with artificial s made of metal and plastic is an increasingly common solution to this problem. The major drawback has been that artificial s tend to wear out in just 10 to 15 years. Now, a new type of polyethylene is being used to make artificial s that last much longer. Machines that test artificial s by putting them through a million movements a week have confirmed that the polyethylene s should last for at least 27 years. Scientists are also researching ways to rebuild aging s so they will not need to be replaced. For example, they are testing a type of cell that replaces damaged cartilage and a protein paste that helps repair damaged s.

5 Structure of Joints In freely movable s, cartilage covers the surfaces where two bones come together. This protects the bones as they move against each other. The s are also surrounded by a fibrous capsule that helps hold the bones together while still allowing them to move. The capsule consists of two layers. One layer forms strips of tough connective tissue called ligaments. Ligaments, which hold bones together in a, are attached to the membranes that surround bones. Cells in the other layer of the capsule produce a substance called synovial (sin- OH-vee-ul) fluid. Synovial fluid enables the surfaces of the to slide over each other smoothly. In some freely movable s, such as the knee in Figure 36 5, small sacs of synovial fluid called bursae (BUR-see; singular: bursa) form. A bursa reduces the friction between the bones of a and also acts as a tiny shock absorber. Skeletal System Disorders Bones and s can be damaged, just like any other tissue. Excessive strain on a may produce inflammation, a response in which excess fluid causes swelling, pain, heat, and redness. Inflammation of a bursa is called bursitis. A more serious disorder is arthritis, which involves inflammation of the itself. In older people, especially women, loss of calcium in the bones can lead to a condition known as osteoporosis. Osteoporosis is a weakening of the bones that can cause serious fractures. Sound nutrition, including plenty of calcium in the diet, and weight-bearing exercise are among the best ways to prevent this serious problem. Muscle Tendon Bursa Ligament Synovial fluid Cartilage Fat Figure 36 5 The knee is protected by cartilage and bursae. The ligaments hold the bones composing the knee femur, patella, tibia, and fibula together. Inferring How do the cartilage and bursae help reduce friction? Skeletal System Disorders Make Connections Health Science Point out that there are several different types of arthritis, including rheumatoid arthritis and osteoarthritis, also called degenerative disease. Challenge students to identify ways in which the two types differ. (Possible answers: rheumatoid arthritis is caused by the immune system attacking s and usually occurs by young adulthood; osteoarthritis is caused by wear and tear on s and usually occurs after middle age.) 3 ASSESS Evaluate Understanding Read each of the Vocabulary words in the section. As you read, call on students at random to define the terms without referring to their books. Reteach Work with students to make a table summarizing the similarities and differences among the different types of s. Label the columns: Type of Joint, Range of Motion, Examples Section Assessment 1. Key Concept List the different functions of the skeletal system. 2. Key Concept Describe the structure of a typical bone. 3. Key Concept What is a? List the three types of s. 4. How does compact bone differ from spongy bone? 5. Critical Thinking Inferring Why do you think the amount of cartilage decreases and the amount of bone increases as a person develops? Creative Writing Use library or Internet resources to find out more about osteoporosis. Then, develop an advertising campaign for the dairy industry based on the relationship between milk and healthy bone development. Students advertising campaigns should make a convincing argument for milk consumption that includes information regarding the high incidence of osteoporosis at older ages, especially in women; the serious potential health consequences of osteoporosis, including broken bones; the role of dietary calcium in preventing osteoporosis; and the high calcium content of milk Section Assessment 1. The skeletal system supports the body, protects internal organs, allows movement, stores mineral reserves, and provides a site for blood cell formation. 2. A typical bone has a thick layer of compact bone covered by periosteum. Haversian canals contain the blood vessels and nerves. At the ends of long bones, there is a layer of spongy bone beneath the compact bone layer. 3. A is a place where one bone attaches to another. Three types of s are immovable, slightly movable, and freely movable. 4. Compact bone is denser than spongy bone. Spongy bone is found in the ends of long bones and in the middle of short, flat bones. 5. The cartilage decreases because minerals replace cartilage during ossification. If your class subscribes to the itext, use it to review the Key Concepts in. Answers to... Ball-and-socket, hinge, pivot, and saddle Figure 36 5 They help reduce friction by providing a smooth, flexible surface between bones in s. Skeletal, Muscular, and Integumentary Systems 925