Since protoplasm is a semi-fluid substance, an organism consisting of many cells requires some kind of framework to give support to the body and to enable it to keep its shape. This function is carried out by the skeleton, which also acts as basis of attachment for the muscles and thus permits certain parts of the body to be moved. Parts of the skeleton, e.g. the skull and spinal column, have a protective function. Thus the term skeleton is given to all hard parts which support or protect an animal, or provide a basis of attachment for muscles. Such external features of the skeleton as hair, nails, hoofs, feathers, scales, etc., are known as the exoskeleton, while the hard parts found within the body constitute the endoskeleton.
Most joints of the endoskeleton are jointed together to permit movement, but some are joined tightly to each other by fibrous tissue so that they are immovable. The bones of the skull have irregular edges which interlock and may fuse together. The bones of some of the joints, e.g. vertebra?, are bound together by ligaments so that only slight movement can take place. But the bones of the limb joints have special large articulatory surfaces which permit great freedom of movement.
The bones of the upper arm and of the thigh are attached to the body skeleton by means of ball-and-socket joints. The head of the limb bone is rounded and fits into a cup-like hollow of the supporting framework, so that movement in every direction is possible. The surface of the head of the limb bone and that of the socket into which it fits are covered by a smooth layer of cartilage, the articular cartilage. The two parts of the joint are fixed to each other by a strong sheet of tissue in the form of a short tube, termed the capsular or elastic ligament. The ligament may completely enclose a small space between the head of the bone and the socket, and the space is lined by delicate fibrous tissue which closely fits the head and the socket and is termed the synovial capsule. The capsule contains a watery fluid, the synovia, which acts as a water cushion and reduces the friction between the two parts of the joint. Sometimes when the knee is injured the fluid in the joint capsule increases in quantity, causing the injury known as water on the knee. The knee and elbow are examples of another type of joint, termed the hinge joint, which allows movement in one plane only.
Movement of bones relative to one another, e.g. in the bending of the arm at the elbow is produced by the contraction and relaxation of muscles attached to the bones by means of tough, inelastic cords called tendons. The movement of any bone in one plane involves the operation of a pair of antagonistic muscles. One end of each muscle is attached to a fixed bone and the other to the movable bone. When one of the antagonistic muscles contracts, the other relaxes to cause the movement to take place. In the case of the arm, when the biceps muscle contracts, the triceps relaxes causing the arm to bend. The arm is straightened when the biceps relaxes and the triceps contracts. Muscles which produce bending movements of a limb are called flexors ; those which straighten the limb are called extensors.