The Skeletal System - Workings: how the skeletal system functions
The most obvious function of the skeletal system is to provide shape and form to the body. Like the steel girders of a building, the bones provide a framework around which the body is built. Delicate tissues and organs are attached to and protected by this bony framework. For example, the skull encloses the brain, the ribs protect the heart and lungs, and the vertebrae shield the spinal cord.
Bones are also storehouses for minerals, the most important of which are calcium and phosphorus. Both are used to build and maintain bones and teeth. Calcium serves other vital functions, too. Muscles need it in order to contract and blood needs it in order to clot. For these two bodily processes to occur, calcium must be present in the bloodstream at all times. When blood calcium levels drop below normal, hormones trigger osteoclasts to break down bone matrix, releasing calcium. When blood calcium levels rise above normal, hormones cause excess calcium to be deposited in the bones in the form of calcium salts.
As stated earlier, some bones serve as production sites for blood cells. Red blood cells and other blood elements are formed in the red bone marrow of certain flat and irregular bones.
Perhaps the most dramatic function of the skeletal system is to allow the human body to move through space. Walking, running, jumping, dancing, and even breathing are all possible because of the combined actions of the muscles and the bones. Skeletal muscles are attached to bones, and when a muscle contracts, it pulls on its attached bone and movement occurs at a specific joint.
Joints
Joints, also called articulations, are the places where two or more bones meet or articulate. Every bone in the body (with the exception of the hyoid bone in the neck) forms a joint with at least one other bone.
Joints allow a great variety of motion, and the function of each joint is closely related to its structure. When movement is not required, such as between the bones of the cranium, the joint is immobile and strong. The bones are locked together as if they were a single bone. Other joints, such as those between the vertebrae, allow limited movement while still maintaining a certain amount of strength. Finally, in areas where movement is more important than strength, joints can be freely movable. The shoulder joint, for example, allows the arm to move in a variety of ways. Because it allows such a range of motion, the shoulder joint is relatively weak and prone to injury.
As stated, ligaments fasten bone to bone at joints. To prevent friction, the ends of bones at joints are covered in smooth cartilage. In freely movable joints, extra protection is afforded by a joint capsule under the ligaments that surrounds the joint. Made of fibrous connective tissue, the capsule encloses the joint like a sleeve. Lining the capsule is the synovial membrane, which secretes a thick and slippery fluid into the joint cavity. This fluid, called synovial fluid, helps prevent further friction as the bones move.
In areas where tendons cross bone, small sacs filled with synovial fluid lie between the tendon and the joint. These sacs, called bursae (singular: bursa), help cushion the tendon as it slides across the bone.
The human species, or Homo sapiens , belongs to the hominid family tree. Hominid means "human types," and describes early creatures who split off from the apes and took to walking upright or on their hind legs.
The reasons that human ancestors started to walk upright are not known. Scientists believe it may have been a response to environmental changes: as tropical forests were beginning to shrink, walking might have been a better way to cross the grasslands to get to nearby patches of forest for food. Standing upright also may have been a means of defense that slowly evolved. By standing upright, animals appear bigger and more impressive in size than they normally are. In addition, the ability to stand up and get a wider view of the surroundings gives an animal an advantage in the tall grasses. Walking upright also frees up the hands to carry objects, such as tools.
The oldest known humanlike animals to have walked upright are believed to be Australopithecus afarensis , meaning the southern ape of the Afar region in Ethiopia, Africa, where the fossils were found. The most famous of these fossils, nicknamed Lucy, was found in 1974 near Hadar, Ethiopia, by a team of anthropologists led by American Donald Johanson (1943–). Lucy lived about 3.18 million years ago. Her skull, knees, and pelvis were more similar to humans than to apes. Her brain size was about one-third that of modern humans, yet larger than any apelike ancestor to have come before. She would have stood about 3.5 feet (1 meter) tall, with long arms, a V-shaped jaw, and a large projecting face.
Freely movable joints are the largest category of joints in the body, especially in the appendicular skeleton where mobility or movement is important. The five main types of freely movable joints are the ball-and-socket, saddle, hinge, pivot, and gliding.
BALL-AND-SOCKET JOINT. A ball-and-socket joint provides the most freedom of movement of any joint. In this type of joint, the round head of one bone (the ball) fits into a cup-shaped depression in another bone (the socket). The joint allows movement in all directions, including rotation. The shoulder and hip joints are examples of ball-and-socket joints.
SADDLE JOINT. A saddle joint allows the next greatest amount of movement. In a saddle joint, the bones are shaped like a horseback rider sitting in a saddle. All movement except rotation is possible with this type of joint. The joint at the base of the thumb is an example of a saddle joint.
HINGE JOINT. A hinge joint allows backward and forward movement in only one direction, much like a door opening and closing. In this type of joint, the convex surface of one bone fits in the concave surface of the other. The joints at the knees, elbows, and knuckles are hinge joints.
PIVOT JOINT. A pivot joint consists of a cylinder of one bone rotating within a ring formed by another bone. Movement occurs only around a single axis. The pivot joint between the atlas and the axis (first and second cervical vertebrae) allows the head to be turned from side to side.
GLIDING JOINT. A gliding or plane joint allows only a small amount of movement as the flattened or slightly curved surfaces of bones slide or glide over each other in various directions. The joints between the carpal bones in the wrist, the tarsal bones in the ankle, and the vertebrae in the spine are examples of gliding joints.
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