• Protection: protects the brain and internal organs
• Support: maintains upright posture
• Blood cell formation: hematopoiesis
• Mineral homeostasis
• Storage: stores fat and minerals.
• Leverage: A lever is a simple machine that magnifies speed of movement or force. The levers are mainly the long bones of the body and the axes (fulcrum) are the joints where the bones meet.

A specialized form of connective tissue, bone consists of both organic components (e.g. collagen) and inorganic minerals (calcium, phosphorus, magnesium, potassium, and sodium). The minerals calcium and phosphorus give bone its hardness, strength, and rigidity to resist compressive forces. The collagen fibers impart flexibility.  Magnesium, sodium, potassium,  and other trace elements act as "mortar" bonding the calcium and phosphorous. The bone cells themselves are embedded in a mineralized calcium "matrix" and collagen fibers.

Bone continuously remakes itself: New bone is produced and old bone is removed. Osteoblasts, the cells responsible for making bone, maintain the balance of calcium in the blood and bone. When this balance is disrupted, as in osteoporosis, the removal of bone exceeds its production, making bone thin and brittle, thus more easily fractured. The intestines, vitamin D, the kidney, parathyroid gland, and sex and adrenal hormones also play important roles in bone/calcium balance. In long bone, illustrated above, growth occurs at the diaphysis side (shaft) of the epiphyseal plate, thus increasing the length of the shaft. Long bone growth stops when the hyaline cartilage stops reproducing itself and fully converts to bone.

A joint, or articulation, is a union of two or more bones. Ligaments attach bone to bone, stabilizing and strengthening joints and determining the range of motion. Cartilage, a gel-like substance high in proteoglycans,  provides protective cushioning. There are three types of cartilage: 1) fibrocartilage (found in intervertebral discs), 2) elastic cartilage (found in the external ear and epiglottis), and 3) hyaline cartilage. Hyaline (or articular) cartilage is the most important cartilage: It serves as the "original" skeleton in the embryo from which bones develop; it spurs growth of long bones; and it lines and protects joints.

General Classifications of Bones

1. Long Bones -- "longer than they are wide:" clavicle, humerus, radius, ulna, femur, tibia, fibula, metatarsals, metacarpals. Purpose: provide support and serve as  the interconnected set of levers and linkages that allow us to create movement. (formed from hyaline/articular cartilage)
2. Short Bones: carpals and tarsals: consist mainly spongy bone covered with a thin layer of compact bone. Purpose: allow movement, provide elasticity, flexibility, & shock absorption.
3. Flat Bones:  ribs, sternum and scapula. Purpose: protect and provide attachment sites for muscles.
4. Irregular Bones:  skull, pelvis, and vertebrae. Purposes:  support weight, dissipate loads, protect the spinal cord, contribute to movement and provide sites for muscle attachment.
5. Sesamoid Bones: a short bone embedded within a tendon or joint capsule, i.e. patella. Purpose: alter the angle of insertion of the muscle.

A typical synovial joint, seen at right, has four main featues:

1. joint capsule -  the joint enclosure, reinforced by and strengthened with ligaments
2. synovial membrane -  a continuous sheet of connective tissue lining the capsule; its cells produce synovial fluid that lubricates the joint and prevents the two cartilage caps on the bones from rubbing together
3. synovial fluid - produced by the synovial membrane, the fluid lubricates the joint. In the normal joint, very little fluid (less than 5cc) exists in the cavity.
4. hyaline (articular) cartilage - where the bones actually "meet"

Click on the front or back views to see greater detail. Graphics modified from: The InnerBody: Anatomy Tutorials - Skeletal System
(Front View)	(Back View)

The skeleton has two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull, the spine, the ribs and the sternum (breastbone) and includes 80 bones. The appendicular skeleton, consisting of 126 bones,  includes two limb girdles (the shoulders and pelvis) and their attached limb bones. 

Axial Skeleton (80 bones)

• skull - consiting of 1) the cranium (which encloses and protects the brain) and 2) the facial skeleton. The upper teeth are embedded in the maxilla; the lower teeth, in the mandible.
• mandible (jaw) - the only freely movable bone of the skull
• ribs, sternum (breastbone) - comprising  the "thorax"/thoracic cage, protecting the heart and lungs
• vertebral column - the "spine" 

The vertebral body and the neural arch encircle the vertebral foramen. Stacked one on top of the other, these foramina form the vertebral canal, where the spinal cord resides.

Several structures strengthen the attachments between vertebrae: 1) anterior longitudinal ligaments in front of vertebral bodies and discs; and 2) posterior longitudinal ligaments behind bodies and discs; 3) the compact bone of the disc itself; 4) the interlocking hyaline cartilage surfaces of the neural arch joints; and 5) the ligaments attaching spinous processes to transverse processes.The intervertebral discs provide shock absorption.

Test yourself: Name the bones of the skull and the  vertebral column.   Anterior Skull Lateral  Skull Vertebral Column

The orientation of the neural arch joints determines allowable motions: 1) the cervical spine () to rotate, flex forward, flex sideways, and extend backward; 2) the thoracic spine () to rotate; and 3) the lumbar spine () to flex forward, flex sideways, and extend backward. The sacrum () has a dual character, being part of both the vertebral column and pelvis. As such, it transmits the upper body weight to the lower exterminites.

Appendicular skeleton (126 bones, 64 in the shoulders and upper limbs and 62 in the pelvis and lower limbs)

• Upper Extremity - The arms (humerus - upper arm bone) are ultimately attached to the thorax,  via synovial joints, at the collarbone (clavicle) and shoulder bone (scapula) (shoulder joint). The scapula is attached to the thoracic cage only by muscles.  The elbow joint unites the humerus with the two lower arm bones - the ulna and radius. Three sets of joints connect the radius and ulna to the bones of the palm (metacarpals), via the eight small wrist carpals. Further, the knuckles (metacarpophalangeal, or MCP, joints) connect the metacarpals to the proximal phalanx of the fingers. Each finger has 3 phalanges (proximal, middle, distal), except the thumb which has only two.
• shoulder/ scapula
• arm and forearm, elbow
• hand
• Lower Extremity - The pelvis transmits the upper body weight from the sacrum (at the sacroiliac joint) to the legs. It begins as 3 hip bones (ilium, ischium, and pubis) which fuse together when growth is completed. The hip joint unites the pelvis to the thigh bone (femur); the knee joint, which includes the knee cap (patella), links the femur to the lower leg bones - the tibia and fibula. The ankle joint links the lower leg bones to the talus. The body weight is then transmitted to the heel (calcaneous) and to the balls of the feet via the tarsal and metatarsal foot bones. The toes have a phalangeal structure like the fingers.
• pelvic girdle
• thigh and leg. knee, 
• foot/ankle/toe

Web resources: The InnerBody: Anatomy Tutorials - Skeletal System Maricopa: Muscular and Skeletal Systems Visible Human Project  MayoHealth - Interactive Tour of the Skeleton eSkeletons Project - Human skeleton DAVID - Online Atlas of Human Anatomy - Joints, Bones Cornell University Medical Center PathNotes: Bone Pathology See related: PATTS "Significant Body Systems" - Muscular/ Musculoskeletal System

Acknowledgments:  Joel DeLisa and Walter C. Stolov, "Significant Body Systems," in: Handbook of Severe Disability, edited by Walter C. Stolov and Michael R. Clowers. US Department of Education, Rehabilitation Services Administration, 1981, pages 30-36. Catherine Parker Anthony and Gary A. Thibodeau, Textbook of Anatomy & Physiology. St. Louis: Mosby, 1983, pages 88-166. J. Crimando, Anatomy and Physiology Tutorials (Maricopa)