| A joint is a place where 2
bones come together for movement. Joints are classified by how much the
bones they connect can move against one another. Immovable, or fibrous,
joints don't move. The dome of the skull, for example, is made of bony
plates, which must be immovable to protect the brain. Between the edges
of these plates are links, or joints, of fibrous tissue. Fibrous joints
also hold the teeth in the jawbone.
There are three kinds of freely moveable
joints that play a big part in voluntary movement: hinge, pivot, and
ball-and-socket. Hinge joints allow movement in one direction, as
seen in the knees and elbows. Pivot joints allow a rotating motion, like
that of the head moving from side to side. Ball-and-socket joints allow
the greatest freedom of movement. The hips and shoulders have this type
of joint, in which the spherical end of a long bone fits into the hollow
of another bone. Freely moveable, or synovial (most common), joints move
in many directions. The main joints of the body - found at the hip,
shoulders, elbows, knees, wrists, and ankles - are freely moveable.
Filled with synovial fluid, a lubricant.
The human body has more than 650
muscles,- connected to bones by tough, cordlike tissues called
tendons, which allow the muscles to pull on bones. Bones are
fastened to other bones by long, fibrous straps called
ligaments, which wrap around
the joints. Cartilage is a flexible, rubbery
substance that supports bones and protects
them when they rub against each other. Together, bones, muscles,
tendons, and ligaments form the musculoskeletal system.
Humans have three different kinds
of muscle - skeletal, smooth, and cardiac. Skeletal muscles are attached
to bone, mostly in the legs, arms, abdomen, chest, neck, and face. These
muscles are called striated because they are made up of fibers that have
horizontal stripes. They hold the skeleton together, give the body
shape, and help it with everyday movements.
Smooth, or involuntary, muscles are also made of fibers, but they look
smooth. The walls of the stomach and
intestines, which help break up food, contain smooth muscle.
Smooth muscle is also in the walls of blood vessels, which squeezes
blood through the vessels and helps maintain blood pressure. Smooth
muscles take longer to contract than skeletal muscles do, but they
can stay contracted for a long time because they don't tire easily.
Fascia is a smooth substance that separates one muscle from another so
the two can move independently. Bursae cushion moving parts such as the
prepatellar bursae found between the patella and the skin |
