36-1 the Skeletal System

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Summary

36-1 The Skeletal System The skeletal system supports the body, pro- Immovable joints, such as the joints in tects internal organs, provides for move- the skull, allow no movement. Slightly ment, stores mineral reserves, and provides movable joints, such as the joints in the a site for blood cell formation. The skeleton spine, allow a small amount of restricted is divided into two parts: the axial skeleton movement. Freely movable joints permit and the appendicular skeleton. The axial movement in one or more directions. Freely skeleton includes the skull, ribs, and spine. movable joints are classified by the type of The appendicular skeleton includes all the movement they permit. bones associated with the arms and legs, Ball-and-socket joints, such as the shoul- including bones of the shoulders, hips, der, allow the widest range of movement of hands, and feet. any joint. Hinge joints, such as the knee, The bones that make up the skeletal permit only back-and-forth movement. system are living tissue. Bones are a solid Pivot joints, such as the elbow, allow one network of living cells and protein fibers bone to rotate around another. Saddle joints, that are surrounded by deposits of calcium such as those in the hand, allow one bone to salts. A typical bone is surrounded by a slide in two directions. tough layer of connective tissue called the Strips of tough connective tissue, called periosteum. Beneath the periosteum is a ligaments, hold bones together in a joint. thick layer of compact bone. Running The bony surfaces of the joint are covered through compact bone is a network of tubes with cartilage. A substance called synovial called Haversian canals. These canals con- fluid forms a thin film on the cartilage and tain blood vessels and nerves. Inside the makes the joint surfaces slippery. layer of compact bone is spongy bone. Bones and joints can be damaged by Spongy bone is quite strong and adds excessive strain or disease. Arthritis is a dis- strength to bones without adding mass. order that involves inflammation of the Within bones are cavities that contain a soft joints. Osteoporosis is a condition in which tissue called bone marrow. Bone marrow bones weaken. Weak bones are likely to can be yellow or red. Yellow marrow is fracture, or break. made up of fat. Red marrow produces blood cells. The skeleton of an embryo is composed 36-2 The Muscular System almost entirely of cartilage. Cartilage is a Muscle tissue is found everywhere in the type of connective tissue that is tough but body. There are three different types of flexible. Cartilage is replaced by bone dur- muscle tissue: skeletal, smooth, and cardiac. ing the process of bone formation, or ossifi- Skeletal muscles are usually attached to cation. Ossification starts before birth and bones. They appear to be striped, so they continues until adulthood. are also called striated muscles. Skeletal A place where one bone attaches to muscles are responsible for voluntary another bone is called a joint. Joints permit movements such as dancing. bones to move without damaging each Smooth muscles line blood vessels and other. Depending on its type of movement, the digestive tract. They are not striated or a joint is classified as immovable, slightly under conscious control. Smooth muscles movable, or freely movable. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 64 Name Class Date move food through the digestive tract and integumentary system. The integumentary control the flow of blood through the circu- system has many functions. It serves as a latory system. Cardiac muscle is found only barrier against infection and injury, helps to in the heart. Like smooth muscle, it is not regulate body temperature, removes waste under conscious control. products from the body, and provides pro- Skeletal muscle cells are called muscle tection against ultraviolet radiation from fibers. Muscle fibers are composed of the sun. smaller structures called myofibrils. Each The skin is made up of two main layers: myofibril is made up of even smaller struc- the epidermis and the dermis. The epider- tures called filaments. Filaments can be mis is the outer layer of the skin. Cells of thick or thin. Thick filaments are made of a the epidermis produce keratin. Keratin is a protein called myosin. Thin filaments are tough, fibrous protein that helps keep the made of a protein called actin. A muscle epidermis flexible and waterproof. The epi- contracts when the thin filaments in the dermis also contains cells, called melano- muscle fiber slide over the thick filaments. cytes, that produce melanin. Melanin is a Impulses from motor neurons control dark drown pigment that helps protect the the contraction of skeletal muscles. The skin from ultraviolet rays. point of contact between a motor neuron The dermis is the inner layer of skin. It and a muscle fiber is called a neuromuscu- contains nerves, blood vessels, glands, and lar junction. A neurotransmitter named other structures not found in the epidermis. acetylcholine is released by the motor neu- The dermis works with other organs to ron into the synapse. Acetylcholine trans- maintain homeostasis. It helps to regulate mits the impulse across the synapse to the body temperature. Sweat glands in the der- skeletal muscle cell. The more muscle cells mis produce sweat when the body gets too that are stimulated to contract, the stronger hot. When the sweat evaporates from the the contraction. skin, it cools the body. Skeletal muscles are joined to bones by Too much sunlight can produce skin can- tough connective tissues called tendons. cer. You can protect against skin cancer by Tendons pull on bones and make them wearing a hat, sunglasses, and protective work like levers. Muscles provide the force clothing. You also should use sunscreen with to move the bones. Most skeletal muscles a sun protection factor (SPF) of at least 15. work in opposing pairs. When one muscle In addition to the skin, the integumen- contracts, the other relaxes. tary system includes the hair and nails. Regular exercise is important in main- Both hair and nails are composed mainly of taining the strength and flexibility of mus- keratin. Hair on the head protects the scalp cles. Regular exercise also strengthens from sunlight and cold. Hair in the nostrils bones. Strong bones and muscles are less and around the eyes prevents dirt from likely to become injured. entering the body. Hair is produced by structures called hair follicles. Hair follicles 36-3 The Integumentary System are located in the dermis. Nails grow from The skin is the single largest organ of the an area called the nail root Nails protect the body. It is also the largest component of the tips of the fingers and toes.