The Skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the body. Exoskeletons may undergo periodic moulting as the animal grows. Endoskeletons are internal, as is typical of many vertebrates; they are usually surrounded by skin and musculature, though they often enclose vital organs. Endoskeletons are attachment points for musculature and act as leverage for movement, and in many animals contain marrow, which produces blood cells. Skeletons may or may not be mineralized - human skeletons are calcified, while shark skeletons are cartilaginous - and may be jointed for flexibility and motility or rigid for structural strength.
The average adult human skeleton has around 206 bones. These bones meet at joints, the majority of which are freely movable. The skeleton also contains cartilage for elasticity. Ligaments are strong strips of fibrous connective tissue that hold bones together at joints, thereby stabilizing the skeleton during movement.
The Skeletal System serves many important functions; it provides the shape and form for our bodies in addition to supporting, protecting, allowing bodily movement, producing blood for the body, and storing minerals.
Its 206 bones form a rigid framework to which the softer tissues and organs of the body are attached.
Vital organs are protected by the skeletal system. The brain is protected by the surrounding skull as the heart and lungs are encased by the sternum and rib cage.
Bodily movement is carried out by the interaction of the muscular and skeletal systems. For this reason, they are often grouped together as the musculo-skeletal system. Muscles are connected to bones by tendons. Bones are connected to each other by ligaments. Where bones meet one another is typically called a joint. Muscles which cause movement of a joint are connected to two different bones and contract to pull them together. An example would be the contraction of the biceps and a relaxation of the triceps. This produces a bend at the elbow. The contraction of the triceps and relaxation of the biceps produces the effect of straightening the arm.
Blood cells are produced by the marrow located in some bones. An average of 2.6 million red blood cells are produced each second by the bone marrow to replace those worn out and destroyed by the liver.
Bones serve as a storage area for minerals such as calcium and phosphorus. When an excess is present in the blood, buildup will occur within the bones. When the supply of these minerals within the blood is low, it will be withdrawn from the bones to replenish the supply.