The human skeletal system is the support structure of the human body and is made up of all the bones in the human skeleton. The skeletal system serves to give the body support and to protect internal organs and components of the nervous system, including the brain and the spinal column. Tiny bones in the human ear are key to our sense of hearing. The human skeletal system also provides attachment points for muscles, which allow the movement of various parts of the body, and the marrow, which fills the cavities in the larger bones and produces most of the body's blood cells.
The number of bones in the human body can vary from individual to individual, but on the average, most mature humans have 206 bones. In infants, who have as many as 270 total bones when born, certain bones are still less than completely formed, particularly those of the skull, and many bones grow together as children age grow to adulthood. The connective tissue between bones, which cushions the joints, is called cartilage. This firm, tough, and flexible material, while not bone, is also considered part of the human skeletal system.
The bones of the human body are divided into groups for purposes of classification. The core or central group of bones are the skull, vertebrae, sternum, and ribs. All the other bones are grouped together as appendicular bones. Physicians and those who study human anatomy often group other bones in small subgroups as well. The thoracic bones, for example, are simply the vertebrae, ribs, and sternum, the vertebral column consists of the vertebrae, and the upper limb bones are the bones of the arms, including the shoulder blades and collar bones.
Besides the primary functions of support, protection and blood cell production, the human skeletal system has other functions as well. It produces lymphocytes and helps to regulate the flow of lymphatic fluids. The tissues of the marrow also play a role in the production of certain hormones that influence the way our bodies use insulin. They also store and help regulate the body's usage of other hormones as well, particularly those relating to human growth. Marrow tissues, besides creating blood cells, can also store excess fatty acids.