The skeleton is made up of all the bones in one’s body. Your skeleton has five major functions. It provides shape and support, enables you to move,
protects your organs, produces blood cells, and stores minerals and other materials until your body needs them. The backbone, or vertebral column,
is the center of the skeleton. The backbone is made up of 26 small bones, or vertebrae (singular vertebra). If your backbone were just one bone, you would not be able to bend or twist.
A joint is a place in the body where two bones come together. Joints allow bones to move in different ways. Immovable joints connect bones in a way that allows little or no movement. Movable joints allow the body to make a wide range of movements. The bones in movable joints are held together by a strong connective tissue called a ligament. Cartilage is a
connective tissue that is more flexible than bone.
Bones are complex living structures that undergo growth and development. A thin, tough membrane covers all of a bone except the ends. Blood vessels and nerves enter and leave the bone through the membrane.
Beneath the membrane is a layer of compact bone, which is hard and dense, but not solid. Small canals run through the compact bone, carrying blood
vessels and nerves from the bone’s surface to the living cells within the bone.
Just inside the compact bone is a layer of spongy bone, which has many small spaces within it. Spongy bone is also found at the ends of the bone. The
spaces in bone contain a soft connective tissue called marrow. There are two types of marrow—red and yellow. Red bone marrow produces blood cells.
Yellow marrow stores fat that serves as an energy reserve.
The bones of your skeleton are both strong and lightweight. Bones are hard because they are made up of two minerals—phosphorus and calcium.
New bone tissue forms continually throughout your life.
A combination of a balanced diet and regular exercise are important for a lifetime of healthy bones. As people become older, their bones begin to lose some minerals. Mineral loss can lead to osteoporosis, a condition in which the body’s bones become weak and break easily. Regular exercise and a diet rich in calcium can help prevent osteoporosis.
The Muscular System
There are about 600 muscles in your body. The muscles that are not under your conscious control are called involuntary muscles. Involuntary muscles are responsible for activities such as breathing and digesting food. The muscles that are under your control are called voluntary muscles. Smiling
and turning the pages in a book are actions of voluntary muscles.
Your body has three types of muscle tissue—skeletal muscle, smooth muscle, and cardiac muscle. Some of these muscle tissues are involuntary, and some are voluntary. Skeletal muscles are attached to the bones of your skeleton. At the end of a skeletal muscle is a tendon. A tendon is a strong connective tissue that attaches muscle to bone. Because you have conscious
control of skeletal muscles, they are classified as voluntary muscles. These muscles provide the force that moves your bones. Skeletal muscles react
quickly and tire quickly. Skeletal muscle cells appear banded, or striated. For this reason, they are sometimes called striated muscles.
Smooth muscles are called involuntary muscles because they work automatically. They are inside many internal organs of the body, and control many types of movements inside your body, such as those involved in the process of digestion. Smooth muscles react more slowly and tire more slowly than skeletal muscles. Cardiac muscles are involuntary muscles found only
in the heart. Cardiac muscles do not get tired.
Muscles work by contracting, or becoming shorter and thicker. Because muscle cells can only contract, not extend, skeletal muscles must work in pairs. While one muscle contracts, the other muscle in the pair relaxes to its original length. For example, in order to move the lower arm, the biceps muscle on the front of the upper arm contracts to bend the elbow. This lifts
the forearm and hand. As the biceps contracts, the triceps on the back of the upper arm returns to its original length. To straighten the elbow, the triceps
muscle contracts while the biceps returns to its original length.
Exercise is important for maintaining both muscular strength and flexibility. Exercise makes individual muscle cells grow wider, thicker, and stronger. Sometimes, muscle injuries such as strains and cramps, can occur. Resting the injured area can help it heal.
7th
The Rock Cycle
Earth’s rocks are not as unchanging as they seem. Forces deep inside Earth and at the surface produce a slow cycle that builds, destroys, and changes
the rocks in the crust. The rock cycle is a series of processes on Earth’s surface and in the crust and mantle that slowly change rocks from one kind
to another. One possible pathway through the rock cycle began with the formation of the igneous rock granite beneath the surface. The forces of mountain
building slowly pushed the granite upward, forming a mountain. Slowly, water and wind wore away the granite. The resulting sand was carried by
streams to the ocean. Over millions of years, layers of sediment piled up on the ocean floor and changed to sandstone, a sedimentary rock. Over time,
the sandstone became deeply buried. Heat and pressure changed the sandstone to the metamorphic rock quartzite. The changes of the rock cycle are closely related to plate tectonics.
Plate movements start the rock cycle by helping to form magma, the source of igneous rocks. Plate movements also cause faulting, folding, and other motions of the crust that help to form sedimentary and metamorphic rocks.
Where oceanic plates move apart, magma formed from melted mantle rock moves upward and fills the gap with new igneous rock. A collision of continental plates may push rocks so deep that they melt and form magma.
This magma slowly cools and hardens to form igneous rock.
Sedimentary rock is formed when mountains are worn away by erosion. The mountains were formed by the collision of continental plates that produced faults, folds, and uplift of the crust. Metamorphic rock forms from heat and pressure on rocks pushed down deep in the mantle by a collision between continental plates.
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