Understanding Bones

The Skeletal System

An adult human skeleton consists of 206 named bones. Many of these bones are paired with one on either side of the body (left and right). Infants and children have more than 206 bones because some bones fuse later in life. An example of this would be the sternum which consists of the manubrium, sternum and zyphoid which fuse to become the sternum.

There are two prinicipal divisions of the human skeleton: the axial skeleton and the appendicular skeleton (126 bones).
The axial skeleton is made up of the 80 bones that lie around the longitudinal axis of the body: skull bones, auditory ossicles (ear bones), hyoid bone, ribs, sternum and vertebrae.
The appendicular skeleton includes bones of the upper and lower limbs as well as the bones that connect the limbs to the axial skeleton. Functionally, the auditory ossicles in the ear are not part of either the axial or appendicular skeleton but are grouped with the axial skeleton because the are inside the skull. (3 left and 3 right)

Bone classification is based on their shape and location. Almost all bones can be classified into 5 types based on shape; long, short, flat, irregular and sesamoid.

1. Long bones have a greater length than width, have a long shaft and are slightly curved for strength in order to absorb the stress of the body’s weight Ie. the femur
2. Short bones are smaller and cube shaped being almost equal in length and width Ie. carpal bones of the wrist
3. Flat bones are thinner and consist of 2 nearly parallel plates Ie. the sternum or shoulder blade
4. Irregular bones have complex shapes and can’t be grouped into any specific category Ie. vertebrae, hip bones and several facial bones
5. Sesamoid bones are shaped like sesame seeds and develop/grow in tendons where there is more tension or physical stress (such as the palms) in order to protect tendons from excessive wear and tear. The number varies from person to person and they are not always completely ossified and are usually only a few millimetres in diameter with the exception of the patella (knee cap) which grows in the quadriceps tendon.
6. Just for Fun I thought I would include an additional type of classification of bone which are sutural bones and classified by their location. They are simply bones that are located in sutures in between cranial bones. The sutures are a thin fibrous layer of periosteum. These are usually flat bones which is why I didn’t include them as a 6th classification. The number of them that each person has also varies from person to person.

Bone Features

Bones have surface markings and structural features for specific functions. Most of these develop as a person grows depending on the forces exerted on them as the person grows. These forces can come from tendons, ligaments, aponeuroses, and fasciae. When forces are significant enough new bone is deposited resulting in roughened or raised areas. There are two main types of surface markings;

• Depressions and openings which allow for the passage of nerves and blood vessels or form joints.
• Processes or projections that either help form joints or are attachment sites for connective tissue such as; ligaments and tendons

Bone Surface Markings

Bones have surface markings and structural features adapted for specific functions which happen over time in response to various stresses and tension placed on them by tendons, ligaments, aponeurosis and fasciae. As the body grows new bone is placed where it is needed as a result of the tension resulting in raised or roughened areas. Compression of a bone surface results in a depression.
There are two major types of surface markings;

1. Depressions and Openings – These allow for the passage of blood vessels and nerves, or to form joints
2. Processes, Projections and Outgrowths – These help form joints or serve as attachments sites for connective tissue such as; ligaments and tendons.

Depressions and openings: Sites allowing the passage of soft tissue (nerves, blood vessels, ligaments, tendons) or formation of joints

Processes: Projections or outgrowths on bone that form joints or attachment sites for connective tissue such as ligaments and tendons