The skeletal system consists of bone, cartilage, tendons, and ligaments.

Functions of the Skeletal System

• Bones support the body, protect internal organs, provide levers on which muscles act, store minerals, and produce blood cells.

• Cartilage provides a model for bone formation and growth, provides a smooth cushion between adjacent bones, and provides firm, flexible support.

• Tendons attach muscles to bones, and ligaments attach bones to bones.

Connective Tissue

• Connective tissue consists of matrix and the cells that produce matrix.

• Varying amounts of collagen, proteoglycan, and mineral in the matrix determine the characteristics of the connective tissue.

General Features of Bone

• Long bones consist of a diaphysis (shaft), epiphyses (ends), and epiphyseal (growth) plates. The diaphysis contains a medullary cavity, and the end of the epiphysis is covered by articular cartilage.

Compact Bone

• Compact bone tissue consists of osteons.

• Osteons consist of osteocytes organized into lamellae surrounding central canals.

Cancellous Bone

• Cancellous bone tissue consists of trabeculae without central canals.

Bone Ossification

• Bone ossification is either intramembranous or endochondral.

• Intramembranous ossification occurs within connective tissue membranes.

• Endochondral ossification occurs within cartilage.

Bone Growth

• Bone growth occurs by apposition. Bone elongation occurs at the epiphyseal plate as chondrocytes proliferate, hypertrophy, die, and are replaced by bone.

Bone Remodeling

• Bone remodeling consists of removal of existing bone by osteoclasts and deposition of new bone by osteoblasts.

Bone Repair

• During bone repair, cells move into the damaged area and form a callus, which is replaced by bone.

General Considerations of Bone Anatomy

There are 206 bones.

Axial Skeleton

• The axial skeleton includes the skull, vertebral column, and thoracic cage.

Skull

• The skull consists of 28 bones: 8 cranial vault bones, 14 facial bones, and 6 auditory ossicles.

• From a lateral view, the parietal, temporal, and sphenoid bones can be seen.

• From a frontal view, the orbits and nasal cavity can be seen, as well as associated bones and structures, such as the frontal bone, zygomatic bone, maxilla, and mandible.

• The interior of the cranial vault contains three fossae with several foramina.

• Seen from below, the base of the skull reveals numerous foramina and other structures, such as processes for muscle attachment.

Vertebral Column

• The vertebral column contains 7 cervical, 12 thoracic, and 5 lumbar vertebrae, plus 1 sacral and 1 coccygeal bone.

• Each vertebra consists of a body, an arch, and processes.

• Regional differences in vertebrae are as follows: cervical vertebrae have transverse foramina; thoracic vertebrae have long spinous processes and attachment sites for the ribs; lumbar vertebrae have rectangular transverse and spinous processes, and the position of their facets limit rotation; the sacrum is a single, fused bone; the coccyx is four or fewer fused vertebrae.

Thoracic Cage

• The thoracic cage consists of thoracic vertebrae, ribs, and sternum.

• There are 12 pairs of ribs: 7 true and 5 false (two of the false ribs are also called floating ribs).

• The sternum consists of the manubrium, body, and xiphoid process.

Appendicular Skeleton

• The appendicular skeleton consists of the bones of the upper and lower limbs and their girdles.

Pectoral Girdle

• The pectoral girdle includes the scapula and clavicle.

Upper Limb

• The upper limb consists of the arm (humerus), forearm (ulna and radius), wrist (eight carpal bones), and hand (five metacarpals, three phalanges in each finger, and two phalanges in the thumb).

Pelvic Girdle

• The pelvic girdle is made up of the sacrum and two coxae. Each coxa consists of an ilium, ischium, and pubis.

Lower Limb

• The lower limb includes the thigh (femur), leg (tibia and fibula), ankle (seven tarsals), and foot (metatarsals and phalanges, similar to the bones in the hand).

Articulations

• An articulation is a place where bones come together.

Fibrous Joints

• Fibrous joints consist of bones united by fibrous connective tissue. They allow little or no movement.

Cartilaginous Joints

• Cartilaginous joints consist of bones united by cartilage, and they exhibit slight movement.

Synovial Joints

• Synovial joints consist of articular cartilage over the uniting bones, a joint cavity lined by a synovial membrane and containing synovial fluid, and a joint capsule. They are highly movable joints.

• Synovial joints can be classified as plane, saddle, hinge, pivot, ball-and-socket, or ellipsoid.

Types of Movement

• The major types of movement include flexion/extension, abduction/adduction, pronation/supination, eversion/inversion, rotation, protraction/retraction, elevation/depression, excursion, opposition/reposition, and circumduction.>>