Classifications of Bones

Classifications of Bones

The bones of the skeleton are divided into the axial and appendicular groups. The axial skeleton makes up the body’s long axis and includes the bones that comprise the skull, vertebral column, and thoracic cage, which function to protect and support various body parts. The appendicular skeleton is formed by the bones of the upper and lower limbs and the girdles (shoulder and hip bones), which attach the limbs to the axial skeleton (FIGURE 7-1). The sizes of bones vary greatly in the human body, from the tiny inner ear bones to the femur or thighbone, which may be approximately two feet in length. Each unique bone shape serves a certain function. Every adult skeleton contains 206 major bones, which are classified based on their individual shapes:

■■ Flat bones: These resemble plates with broad surfaces and include the ribs, sternum or breastbone, scapulae or shoulder blades, and most skull bones. They provide protection for underlying soft tissues and may be thin and slightly curved.

■■ Irregular bones: Irregular bones have different and complex shapes and are often connected to other bones. They include many facial bones and those that make up the vertebrae in the spine and the pelvis.

■■ Short bones: Small and often cube-shaped, short bones include the carpal (wrist) bones and tarsal (ankle) bones.

■■ Long bones: These have long bone shafts with expanded ends and are much longer than they are wide and are named for their elongated shape instead of their actual size. For example, the three bones of each finger are long bones, even though they are relatively small. All bones of the limbs are long bones, except for the patella (kneecap) and the bones of the wrists and ankles. Long bones are located in the arms, legs, palms, soles, fingers, and toes (FIGURE 7-2). The humerus bone is an example of a long bone.

■■ Sutural bones: Also known as Wormian bones, these are the small, flat, and irregular bones between the flat bones of the skull. They range in size from as large as a quarter to as small as a grain of sand.

■■ Sesamoid bones: These are small, flat bones resembling sesame seeds that are most often located near joints within tendons of the hands, knees, and feet. The patellae are sesamoid bones. This type of bone may form in up to 26 locations in the body. Each individual has different num-bers of sesamoid bones. Some help to control the directions in which tendons pull and some have unknown functions.

1. Compare the structures and functions of the axial skeleton with the appendicular skeleton.

2. List the classifications of bones.

3. Give examples of long, short, flat, irregular, and sesamoid bones.