Knowledge you can walk. Read the idea below, then step up to the general, across to peers or down to the specific.
Physical
Long Bones
Long bones are bones that are longer than they are wide, such as the femur and humerus, and they carry much of the body's load.
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The idea
Long bones are bones that are longer than they are wide. They are one of five types of bones: long, short, flat, irregular and sesamoid. Long bones include the femur, tibia and fibula in the legs; the humerus, radius and ulna in the arms; the metacarpals and metatarsals in the hands and feet; the phalanges of the fingers and toes; and the clavicles, or collarbones. The long bones of the leg make up nearly half of an adult's height.
A long bone has a central shaft, called the diaphysis, with a wider end called an epiphysis at each end. The ends are covered with smooth hyaline cartilage, which lets the bones glide against each other at a joint. The outside of the bone is wrapped in a layer of connective tissue called the periosteum. Beneath it lies hard compact bone, then a lighter layer of spongy bone. The hollow center, the medullary cavity, contains the bone marrow.
Long bones carry much of the body's weight. The femur and tibia take most of the load during daily activities, and long bones are crucial for moving the skeleton. In children and teenagers, long bones grow longer at the epiphyseal plate, often called the growth plate. There, a process called endochondral ossification slowly replaces cartilage with bone. Growth hormone, made by the pituitary gland, stimulates this growth in length.
Check yourself
Question 1 of 3
What is the central shaft of a long bone called?
Composition
what it's made of
- Diaphysis — The long central shaft of the bone.
- Epiphyses — The wider ends, one at each end of the shaft.
- Articular cartilage — Smooth hyaline cartilage covering the ends where bones meet at joints.
- Periosteum — The layer of connective tissue that covers the outside of the bone.
- Compact and spongy bone — A hard outer shell of compact bone over a lighter inner layer of spongy bone.
- Bone marrow — Soft tissue held in the hollow medullary cavity.
Connection
how it relates
Under Skeletal System
Change
how and why it evolves
- Past
- Before birth, most of these bones begin as cartilage models that are gradually replaced by bone.
- Now
- Their marrow changes with age: it is red marrow in a child and yellow marrow in an adult. Today a surgical procedure called distraction osteogenesis can be used to stretch a leg or arm bone.
- Next
- Research on bone health will likely keep improving ways to prevent fractures and weak bones in older people.