Classification of Bones
Bones are divided into four basic types or classifications (figure 2.1, a-b on page 20). Bone classification is based primarily on the shape of the bone. Long bones are longer than they are wide and are primarily found in the appendages (arms and legs).
Long bones include the femur, tibia, fibula, radius, ulna, and humerus. Long bones are slightly curved for strength and designed to absorb stress at several points. They consist of a long, thin part called the diaphysis, or shaft, and two bulbous-type ends called the epiphysis.
Short bones are basically cube shaped and are about as wide as they are long. Examples are the bones found in the wrist and ankle.
Flat bones are thin and generally flat. They offer considerable protection and a great deal of surface area for muscles to attach. Examples of flat bones are the cranial bones, which protect the brain, and the scapula (shoulder blade).
Irregular bones include many of the bones that do not fall into the other three categories. They have complex shapes and include bones like the vertebrae.
Structural Composition of Bone
A bone is made of many parts, with the proportions of each part depending on the size and shape of the bone. Generally, bones are composed of spongy and compact bone, periosteum and endosteum, and a medullary cavity (figure 2.2). The periosteum is a dense, white, fibrous sheath that covers the surface of the bone and is where muscles and tendons attach. The medullary cavity of the bone is a cavity in the center of the bone filled with yellow, fatty marrow. The endosteum is the layer of cells that line the medullary cavity. The rigid part of the bone is made of spongy and compact bone. The spongy bone is less dense and contains spaces so blood vessels and other nutrients can be supplied to the bone. The compact part of the bone contains few spaces and provides protection and strength.
How Bones Grow
The process through which bone grows in the body is called ossification. Bones, in particular long bones, have cartilaginous growth plates located at either end called epiphyseal plates. Initially, these plates are not completely hardened and are where growth occurs in the bone. These fragile growth plates can be damaged in growing children or teens and affect bone development. As a person matures, the epiphyseal plates harden, and growth stops between the ages of 21 and 25.
Children have large amounts of organic material in their bones, making their bones softer and more pliable. As we age, we have larger proportions of inorganic material, which causes bones to become brittle and more fragile. The structures in bones are in a continuous state of being built up and broken down. When exercise is combined with adequate rest and nutrition, healthy bones become thicker and stronger. Exercise helps build and promote healthy bone tissue and reduces the risk of bone disease such as osteoporosis.
The human skeleton consists of 206 bones and is divided into two parts: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones found around the axis (the imaginary midline of the body) and includes the skull, vertebral column, sternum, and ribs. The appendicular skeleton refers to the bones associated with the appendages and includes the bones in the arms, shoulders, legs, and hips (figure 2.3).
An articulation (joint) is the point of contact between bones or cartilage and bones. Joints are classified as immovable, slightly movable, or freely movable. The amount of movement possible at a joint depends on the way in which the bones fit together, the tightness of the tissue that surrounds the joints and the position of ligaments, muscles, and tendons. Ligaments are dense, regular, connective tissue that attach bone to bone at movable joints and help to protect the joint from dislocation.
The vertebral column, or backbone of the human skeleton, is typically made up of 26 bones called vertebrae (figure 2.4 on page 22). These vertebrae are divided into five sections. The part of the vertebral column found in the neck is called the cervical spine and contains seven smaller vertebrae. The part of the vertebral column found behind the rib cage is called the thoracic spine and consists of 12 midsized vertebrae. The lower back area of the vertebral column is called the lumbar spine and consists of five large vertebrae. Below the lumbar spine is the sacrum, which is one bone made up of five fused sacral vertebrae. The coccyx, or tailbone, is made up of four vertebrae fused into one or two bones.
A fibrocartilaginous tissue, called an intervertebral disc, is found between each vertebra. This tissue can be injured through trauma or overuse, especially in the weight-bearing lumbar area or in the delicate cervical area of the spine. Disc problems are not as common in the thoracic area because of the support and stability the ribs provide the vertebral column in this region.
This is an excerpt from Aquatic Fitness Professional Manual, Sixth Edition.