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.
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;
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;
|Fissure||Narrow slit between adjacent parts of bones that blood vessels or nerves pass through||Superior orbital fissure of the sphenoid bone|
|Foramen||Opening that blood vessels or nerves pass through||Optic foremen of the sphenoid bone|
|Fossa||Shallow depression||Coronoid fossa of the humerus|
|Sulcus||Furrow along a bone surface that accommodates a blood vessel, nerve, or tendon||Intertubercular sulcus of the humerus|
|Meatus||Tube-like opening||External auditory meatus of the temporal bone|
|Processes that form joints|
|Condyle||Large round protuberance at the end of a bone||Lateral condyle of the femur|
|Facet||Smooth, flat articular surface||Superior articular facet of a vertebra|
|Head||Rounded articular projection supported on the neck of a bone||Head of the femur|
|Processes that form attachment sites for connective tissue|
|Crest||Prominent ridge or elongated projection||Iliac rest of the hip bone|
|Epicondyle||Projection above a condyle||Medial epicondyle of the femur|
|Line (linea)||Long, narrow ridge or border (less prominent than a crest)||Linea aspera of the femur|
|Spinous process||Sharp, slender projection||Spinous process of a vertebra|
|Trochanter||Very large projection||Greater trochanter of the femur|
|Tubercle||Small, rounded projection||Greater tubercle of the humerus|
|Tuberosity||Large, rounded, usually roughened projection||Ischial tuberosity of the hip bone|