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Some animals have a hard, supporting skeleton within the tissues of their body. This type of skeleton is known as an endoskeleton. You might be surprised to learn that a sponge has an endoskeleton. Hard needlelike structures made of fibrous inorganic material help reinforce the sponge's shape. The sponge's endoskeleton is all that remains in the familiar bathroom sponge. Echinoderms, like sea urchins, have an endoskeleton composed of hard plates held together by proteins. The most familiar endoskeletons are those of the chordates, composed of bone and cartilage.

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Let's use the human skeleton as an example of a bony endoskeleton. Bone is a type of connective tissue that has many functions. Some bones provide support for the skeletal muscles. For example, the femur supports the muscles in the upper leg, which can cause the leg to move. The skull protects the brain, and the ribs protect the lungs, heart, and other organs.

Some bones contain red marrow and are sites for blood cell production. And bones provide stores of minerals, including calcium and phosphorous ions. As the body requires more calcium and phosphorus, these ions are taken from the bones. If the body has an excess of calcium and phosphorus ions, they are deposited in the bones.

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The role of bones in mineral storage is a factor in a disorder known as osteoporosis, or "porous bone". In normal bones, ions are removed from the bone at the same rate they are deposited. The mass and structure of the bone remains constant over time as long as calcium and phosphate are present in the diet. In people with osteoporosis, ions are removed faster than they are deposited. This causes the mass of the bone to decrease. The bones become more hollow, and they are likely to break much more easily than normal bones.

Women are more likely than men to get osteoporosis, partly because differences in sex hormone levels affect the rate the ions are released from the bones. Because we understand why osteoporosis occurs, we can help prevent or minimize the disease by promoting ion deposition through regular weight-bearing exercise, proper diet, and vitamin and hormone treatments.

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Now that we've introduced the functions of bone, let's look at the structure. Bones contain living cells, called osteocytes, found within a hard calcium-based matrix. Let's consider the thigh bone, the femur. The bone's outermost layer is composed of compact bone tissue. Compact bone tissue is made of blood vessels surrounded by dense cylinders of bone tissue. Blood vessels bring blood to the osteocytes found in the spaces between the cylinders.

At the ends of the thigh bone there is spongy bone tissue, which has less mass than compact bone tissue. While it appears spongy, the tissue is actually firm. Yellow marrow, composed mainly of fat, is found in the center of the bone. Yellow marrow is converted to red marrow, which produces red blood cells, in response to extreme blood loss.

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Now that we've introduced the structure of bones, let's see how they're put together to form a skeleton. We'll use the human skeleton as an example. The human skeleton is composed of an axial skeleton and an appendicular skeleton. The axial skeleton includes the skull bones, 26 vertebrae, the breastbone, and 12 pairs of ribs. It provides support and protection for the upright body. The appendicular skeleton includes the pectoral girdle, the pelvic girdle, and the arms, legs, hands, and feet. The bones of the appendicular skeleton support the arms and legs.

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Most land vertebrates evolved from a common ancestor, and share many common features. For example, while most land vertebrates are quadrupedal and don't walk upright, they have the same bones supporting their arms and legs as humans. The length and thickness of these homologous bones differ as their functions differ.

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The bones of the skeleton are connected at joints. How the bones are connected depends on the type of joint. The joints between the vertebrae, ribs, and breastbone allow only small movements, so the bones are connected by short pieces of cartilage. Synovial joints, like the elbow, have a large range of motion. Bones connected by synovial joints are held together by long cords of connective tissue called ligaments.

There are three types of synovial joints. The ball-and-socket joints of the hips and shoulders enable us to rotate our arms and legs, and move them in several planes. The hinge joint—at the elbow, for instance—enables movement only in one plane. The pivot joint, like the joint between the two bones in the forearm, lets us rotate our forearm at the elbow.

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The movement of the bones at a joint is caused by the contractions of muscles. A muscle is attached to a bone by connective tissue called a tendon. If you stretch or twist a joint too far, the ligaments and tendons can stretch and cause a strain. If the ligaments or tendons tear, it is called a sprain.

Let's look at how muscles cause the bones of an endoskeleton to move. We'll use the elbow as our example. When the flexor muscle, the biceps, contracts, the arm bends. When the extensor muscle, the triceps, contracts, the arm straightens. Click on each muscle to make it contract and move the arm.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education