Skeletal System– Anatomy of a Long Bone

Long bones are longer than they are wide and include such specific bones as the femur (thigh bone), radius, and phalangeal bones (finger and toe bones).  All long bones have the same basic structure that we can look at with a simple drawing…

long-bone-drawing
Drawing of a long bone (unlabeled)
long-bone-drawing-labeled
Long bone drawing (labeled)

Here is a bit of information for each of the structures labeled above…

Articular cartilage:  Articular cartilage is usually found at the end of the long bone as part of a synovial joint (where the bone articulates with another bone).  Articular cartilage is usually made of hyaline cartilage (in the knee joint there is also fibrocartilage)

Proximal epiphysis:  The long bone is wider at its ends…  we call these ends epiphyses.  The epiphysis that is closer to the torso is called the proximal epiphysis.

Spongy bone (cancellous bone, trabecular bone):  Bone comes in two basic tissue types:  Spongy bone and Compact bone (more about this in “bone tissue”).  Where the bone tissue has many spaces in it (making it look a bit like a sponge) we call it spongy bone.  Spongy bone can also be called cancellous bone or trabecular bone (these terms refer to the small spicules or pieces of bone that make up the tissue).  The spaces here help make the bone lighter, while the arrangement of the spicules of bone keep the bone strong (kind of like the steel girders in a building are arranged to give the building strength).  The spaces in spongy bone usually contain red bone marrow (sight of blood cell formation).

Compact bone (cortical bone):  Compact bone is dense bone tissue and it is found along the diaphysis (shaft) of the long bone and the outside of the epiphyses of a long bone.  There are no grossly visible spaces in compact bone like we see in spongy bone.  Compact bone is also called “cortical bone” because it makes up the cortex (outside part) of all healthy bones.

Epiphyseal plate or Epiphyseal line: These two terms are not the same thing!!   The epiphyseal plate is made of actively growing hyaline cartilage and it is where the long bone grows in length in a child.  Once the child is done growing these epiphyseal plates fuse (turning completely into bone tissue) and become epiphyseal plates.  The epiphyseal plates fade with time as the bone remodels itself (bone is always remodeling itself–  it is an dynamic and ever changing tissue).

Diaphysis:  Also called the shaft of the bone the diaphysis is the part of the bone between the two epiphyses.  It is the long part of the long bone and most of it is made of compact bone.

Medullary cavity:  This is a cavity inside of the diaphysis.  The medullary cavity is also called the marrow cavity as it contains mostly yellow bone marrow.  Yellow bone marrow is adipose (fat) tissue.  So, bones store energy as well as support the body.

Distal epiphysis:  The distal epiphysis is the widened area of the long bone at the end that is furthest from the torso of the body.  The distal epiphysis also has articular cartilage and contains spongy bone (although on the drawing these were not included).

Up next:  Bone tissue

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