Skeletal System 2.pdf

A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the hands (metacarpals, ph...

A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the hands (metacarpals, phalanges) and feet (metatarsals, phalanges). Long bones function as levers; they move when muscles contract. A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals, such as the lateral cuneiform, intermediate cuneiform, and medial cuneiform, of the ankles. Short bones provide stability and support as well as some limited motion. The term “flat bone” is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved. Examples include the cranial (skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs. An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones. While most irregular bones function to protect internal organs, because each is irregular in its own way, there is no one general function that fits this class of bones. A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular = patella) are the only sesamoid bones found in common with every person. Osteoporosis In osteoporosis the bone becomes less dense and as bone mass is lost. As seen in the SEM image, the trabeculae of the spongy bone develop pores that weakens their structure and the bone's ability to withstand external forces. Graph Showing Relationship Between Age and Bone Mass Bone density peaks at about 30 years of age. Females lose bone mass more rapidly than males. Bone Formation and Development Intramembranous ossification follows four steps. (1) Cells group into clusters and ossification centers form. (2) Secreted osteoid traps osteoblasts, which then become osteocytes. (3) Trabeculae of cancellous (spongy) form. (d) Compact bone develops superficial to the cancellous bone, and periosteum forms. Endochondral ossification follows five steps. (1) Mesenchymal cells differentiate into chondrocytes and form the cartilage model. (2) The cartilage model of the future bony skeleton and the perichondrium grows. (3) Capillaries penetrate cartilage. Periosteal bone collar develops, and primary ossification center develops. (4) Cartilage and chondrocytes continue to grow at ends of the bone; the medullary cavity forms. (5) Secondary ossification centers develop in the epiphyses; cartilage remains at epiphyseal (growth) plate. (6) Epiphyseal plate ossifies and becomes epiphyseal line; cartilage remains only at joint surface as articular cartilage. How Bones Grow in Length The epiphyseal plate is responsible form longitudinal bone growth, displaying five distinct zones of cartilage growth and ossification.  As a bone matures the epiphyseal plate progresses to an epiphyseal line. (A) Epiphyseal plates are visible in the growing bones of a 9-year-old. (B) Epiphyseal lines are the remnants of epiphyseal plates in a mature bone. How Bones Growth in the diameter of a bone is known as appositional growth. Both osteoblasts and Grow in osteoclasts are active during the process Diameter of remodeling. Group Activity Divisions of the Skeletal System