Types of Bones

There are four major types of bones.  They are:  long bones, short bones, flat bones, and irregular bones.  The primary differences between the bones are their shape, and material they are made of.  The four types are listed below, in more detail, along with a diagram of each showing the primary differences.

Long Bones
The primary characteristic that differentiates long bones from other types of bones is that long bones have a shaft that is much longer than it is wide.  This shaft is called a diaphysis.  Long bones are made up primarily of compact bone, and have less marrow and spongy bone than some other types.  Most bones of the upper and lower extremities are classified as long bones, with the exception of the bones in the wrist, ankle, and the patella in the knee.

Short Bones
Short bones are shaped similarly to a cube.  Unlike long bones, short bones have a thin layer of compact bone, and are mostly filled with spongy bone.  Examples of short bones include the bones in the wrist and ankle, as well as sesamoid bones, including the patella.  Short bones are not as solidy and sturdy as long bones because they have less compact bone and more spongy bone making them more susceptible to compression, and even breaking.

Flat Bones
Flat bones have larger, flat surfaces.  The primary purpose of flat bones is to protect the body's organs, and as a site for muscle attachment.  Examples of flat bones include the ribs, cranial bones of the skull, and the bones of the shoulder girdle.

Irregular Bones
Bones that do not fall into the category of long, short, or flat are considered irregular bones.  The shape, size, and composition of irregular bones varies.  The vertebrae and some of the skull bones are irregular.