The structure and function of the human skeleton
Adrian Whyte and Joanna Johnson, Contributor

Claudia Collins-Jackson (right), The Gleaner Western Bureau circulation supervisor, explains The Gleaner's Rubix Cube Challenge to two Herbert Morrison High students during day one of MoBay Expo 2005 at the Montego Bay Cruise Shipping Terminal, on October 7 last year. - Claudine Housen Photo

THE MAIN functions of the skeleton are for :

* Support: The vertebrae support the back, the girdles support the limbs.

* Movement: Muscle attachments allow movement as levers.

* Shape: The skull, for example, gives shape to the face.

* Protection: Body organs are protected; cranium protects brain; ribs protect heart and lungs; the vertebral column protects the spinal cord.

* Production of red blood cells: These are made in the red bone marrow of long bones.

WHAT IS THE SKELETON MADE OF?

Most of the human skeleton is made up of bone. Although bone looks dead, it is really alive. It is a living tissue made up of a ground substance called matrix and bone cells and many blood vessels through it (as the rate of diffusion would be too low to keep the cells alive). The matrix is like that of cartilage but it contains a complex calcium phosphate mineral that makes the bone hard and strong. It is prevented from being brittle by collagen.

The central portion of a bone contains marrow, this may be yellow (a fat store) or red (the site of red blood cell and some white cell production). The marrow is surrounded by compact bone that withstands forces in a downward direction.

The head of the bone may have spongy bone between the compact bone and the marrow. Spongy bone also runs down the length of most bones other than the long bones of the arms and legs. This can withstand forces from many directions. This arrangement allows bones to be strong yet light.

THE HUMAN SKELETON

The human skeleton is made up of two main parts. The axial skeleton consists of the skull and the vertebral column (backbone). The appendicular skeleton consists of the girdles (shoulder and hip) to which the limbs (arms and legs) are attached.

THE VERTEBRAL COLUMN

Three types of vertebrae:

• Cervical (7)
• Thoracic (12)
• Lumbar (5)

Two types of modified vertebrae:

• Sacrum - fusion of four vertebrae
• Coccyx - fusion of five caudal vertebrae (tail-bone)

The first seven vertebrae are called the cervical vertebrae. Located at the top of the spinal column, these bones form a flexible framework for the neck and support the head. The first cervical vertebrae is called the atlas and the second is called the axis. The atlas' shape allows the head to nod 'yes' and the axis' shape allows the head to shake 'no'.

The next12 vertebrae are called the thoracic vertebrae. These bones move with the ribs to form the rear anchor of the rib cage. Thoracic vertebrae are larger than cervical vertebrae and increase in size from top to bottom.

After the thoracic vertebrae come the lumbar vertebrae. These five bones are the largest vertebrae in the spinal column. These vertebrae support most of the body's weight and are attached to many of the back muscles.

The sacrum is a triangular bone located just below the lumbar vertebrae. It consists of four sacral vertebrae which become fused into a single bone. The sacrum forms the back wall of the pelvic girdle and moves with it.

The bottom of the spinal column is called the coccyx or tailbone. It consists of five bones that are fused together in an adult.

The lower part of each vertebra consists of a tiny bone called the centrum. There are projections at the anterior (front) and posterior (back) of each vertebra, which allow each one to fit into the adjacent one. The centra of the bones fits over one another forming a long column that supports the weight of the body.

In the middle of each vertebra is the neural canal. When the bones of the vertebral column are all joined together, a long channel is formed through which the spinal cord runs.

The bones of the vertebral column are held together by ligaments. They are held firmly together, but the elastic property in the ligaments allows some amount of flexibility. In between the vertebrae are intervertebral discs made of fibrous cartilage that act as shock absorbers, allowing the back to move and preventing injury, which might occur while jumping or even walking.

THE RIB CAGE

The function of the rib cage is to protect the heart and the lungs. It also helps in the breathing movements. It consists of 12 pairs of ribs that are joined to the thoracic vertebrae in the back. Ten pairs of these ribs are joined to the sternum or breastbone in the front by cartilage. The last two pairs are known as floating ribs.

THE GIRDLES

The shoulder girdle is known as the pectoral girdle. It consists of two bones. The clavicle is called the collarbone, and is joined to the sternum in front. The shoulder blade is known as the scapula.

The hip or pelvic girdle is made up of three bones fused together. It is broader in females than in males. This allows for easier passage of a baby at birth. This girdle protects the female reproductive organs and the lower part of the abdomen.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School
masterbio@gmail.com.