The skeletal system defines the body structure of animals. The bones that constitute it protect the internal organs and act as a reserve of the mineral calcium. The bones undergo a continuous remodeling process by which the older tissue gets replaced by newer tissue to maintain their health and vitality.
Bones, joints and cartilage
Bones are attached to each other by joints. There are different types of joints, such as ball-and-socket joints and hinge joints, serving different purposes. The direction and range of motion of the bones are determined by the joint type. While ball-and-socket joints facilitate the rotation of the bones about the joint, only bending movements in a single direction are allowed by the hinge joints. There are immovable joints too.
The tips of the bones are protected by a soft, flexible tissue called cartilage which helps in reducing friction between the bones during joint movements.
Types of muscles
Muscles are of different types, such as skeletal muscles attached to bones, and smooth muscles that occur in the different organs. Cardiac muscles are another type of muscles found only in the wall of the heart. Movement of the joints is brought about by the skeletal muscles. The peristaltic movement, which pushes the food along the digestive tract, is the result of the contraction and relaxation of the smooth muscles on the walls of the stomach and the intestines. While cardiac muscles pump out the blood from the heart, circulation of the blood along the arteries is also helped by their muscular walls.
Tendons and ligaments
Connective tissue is a tough material consisting of collagen, a special type of protein. Bands of this tissue attaching the muscles to the bone are called tendons. They are enclosed in flexible sheaths which allow the free movement of the joints. The ligaments connecting the bones are also made up of the tough connective tissue, but unlike tendons, they are slightly stretchy. Ligaments occur around joints, offering support to them.