The skeletal system, known more fondly as the skeleton, is made up of bones connected via cartilage and ligaments at joints. They are also connected to muscles, skeletal muscles to be exact, via tendons. The skeleton provides the frame for the body and supports the attachment points and joints on which the muscles act, allowing for movement, balance, and control. Bone is an important tissue for the storage of calcium, which is a necessary element in cell processes, and they contain marrow where red and white blood cells are produced.
A normal adult human has approximately 206 bones. They are classified based on their size and shape, which is also associated with their function. The main types of bones in the human body are long bones, short bones, flat bones, and irregular bones. All bones are covered by a blood vessel fed membrane called the periosteum. Bones are under a constant calcium flux, even when they are no longer growing. This leaves bones in the elderly, or those who lack calcium intake, brittle and easy to break.
Long bones are found in the extremities, or limbs. They have a shaft, called the diaphysis, made of compact bone. This is a dense scaffolding of calcified cartilage with Haversian canals, where blood vessels travel through the bone. The ends of the long bones are called the epiphyses. They are made of spongy bone and covered with articular cartilage for movement at the joints. Growth stops when the epiphyseal disc closes from calcification.
Short bones are small and somewhat cube-shaped. They consist of a thin layer of compact bone over spongy bone and are found in the wrists and ankles. Some short bones are embedded in tendons, including the patella (knee cap) These are sometimes referred to as sesamoid bones.
Flat bones are thin and sometimes slightly curved. They include the bones of the skull and the sternum. They contain red marrow and produce red blood cells. Irregular bones also contain marrow and produce blood cells. As their name implies, they have no defined shape. They are made of layers of compact and spongy bone and include the hips and vertebrae.
Joints are classified several ways, based on structure, function, biomechanics, and anatomy. Bones are held into place where they join by connective tissue (ligaments) and sometimes cartilage protects the ends of the bones from rubbing against one another. There are also joints in which fluid further protects movement.
At a young age, the bones of the skull are held together by fibrous sutures. This is known as a fibrous joint. This also includes where the teeth meet the jawbone and other immovable joints in the human body. Cartilaginous joints are those where bones are joined by cartilage. They include the spinal column and are known for limited flexibility.
The most well known joints are the synovial joints. A synovial joint is a structural classification for bones that are not directly joined and include most of the movable parts of the skeleton. They contain synovial fluid for lubrication against articular cartilage and a synovial membrane. They belong to various subtypes:
Gliding joints fingers
Hinge joints elbow (up and down) or knee
Pivot joint elbow (side to side)
Ball and socket - shoulder and hip
Scanlon - Essentials of Anatomy and Physiology, 2002.