Biology » Introducing Animal Systems » Human Skeleton

Axial Skeleton

Axial Skeleton

The axial skeleton forms the central axis of the body and consists of the skull, vertebral column and rib cage and sternum.

The Skull

The skull consists of the cranium and facial bones.

The cranium consists of eight flat bones joined together by immovable joints called sutures. The cranium surrounds and protects the brain. There is a large opening at the base of the skull called the foramen magnum through which the spinal cord passes. On either side of the foramen magnum is a projection which articulates with the first vertebra (called the atlas) to allow for the nodding movement of the head.

Axial Skeleton

The cranium.

There are \(\text{15}\) facial bones. These are irregular bones that include the cheek bones, nasal bones, temples, upper jaw bone (maxilla)) and lower jaw bones (mandible). The only movable bone is the lower jaw. The upper and lower jaws bear the sockets for the \(\text{32}\) permanent teeth.

The number, type and arrangement of the teeth in an animal is indicated by a dental formula. The human dental formula is: 2.1.2.3/2.1.2.3. This formula represents the numbers of each type of teeth in half of the upper jaw and half of the lower jaw. This formula tells us that in both the upper and lower halves there are 2 incisors, 1 canine, 2 premolars and 3 molars.

Axial Skeleton

Dental formula in a human adult.

The human vertebral column

The vertebral column typically consists of 24 articulating vertebrae and 9 fused vertebrae in the sacrum and the coccyx. Between the vertebrae are discs of fibrocartilage, which prevent friction between vertebrae, and act as shock absorbers during walking, running and jumping. Spinal nerves are able to enter and leave the spinal cord through gaps between adjacent vertebrae. Strong ligaments and muscles around the spine stabilise the vertebrae and help to control movement. The vertebrae join up to each other in such a way that there is a continuous spinal canal which runs from the base of the skull to the pelvic girdle. This canal contains the spinal cord. The entire vertebral canal can be divided into five regions.

  • Cervical region
  • Thoracic region
  • Lumbar region
  • Sacral region
  • Coccyx

Did You Know?

A few interesting facts about the human skeleton:

  • Humans have seven neck bones – the same as giraffes.
  • The strongest and longest bone is the thigh bone (the femur).
  • One out of 20 people have an extra rib.
Axial Skeleton

Human vertebral column.

The cervical (neck) region consists of seven vertebrae. The first cervical vertebra, called the atlas, supports the skull and allows for the nodding movement of the head. The second vertebra, called the axis, has a projection on which the atlas pivots to give the side to side movement of the head.

Axial Skeleton

Cervical region of the skeleton is highlighted in red.

The thoracic region (chest) consists of 12 vertebrae, which each bear a pair of ribs.

Axial Skeleton

The thoracic vertebrae give rise to 12 pairs of ribs.

The lumbar region (lower back) consists of five vertebrae. This region has the largest vertebrae as it carries the weight of the body.

Axial Skeleton

The lumbar region is highlighted by the red circle.

The sacral region consists of five fused vertebrae, forming a bone called the sacrum. the sacrum forms part of the pelvic girdle which provides surfaces for the attachment of muscles and the legs.

The coccyx is made up of four fused bones. These bones form the tail in those mammals that have tails.

Fact:

In a newborn baby the entire vertebral column curves backwards probably because of the confines of the uterus. Initially a baby cannot support the weight of its head. When after about 3 months it is able to support its head, the cervical forward curve is complete. The lumbar forward curve is complete when the baby is able to stand on its own and ready to learn to walk.

Functions of the vertebral column

  • Supports the skull
  • Surrounds and protects the spinal cord
  • Provides attachment for ribs, girdles, and back muscles
  • Separate vertebrae and S-shaped curvature provide flexibility allowing humans to bend backwards, forwards and sideways
  • Fibrocartilage discs between the vertebrae act as shock absorbers

The rib cage and sternum

The rib cage is a bony and cartilaginous structure. A typical rib cage consists of 24 ribs (12 pairs), the sternum (an inverted T-shaped structure connecting the rib bones), costal cartilages and the 12 thoracic vertebrae shown in the diagram below. The first seven pairs of ribs connect directly to the sternum and are referred to as true ribs. The remaining five pairs of ribs do not connect directly to the sternum and are referred to as false ribs. The rib cage aids in the protection of the heart and lungs. With the help of the diaphragm and the intercostal muscles, they increase and decrease the volume of the thoracic cavity thereby allowing inhalation and exhalation to take place.

Axial Skeleton

The human rib cage.

[Attributions and Licenses]


This is a lesson from the tutorial, Introducing Animal Systems and you are encouraged to log in or register, so that you can track your progress.

Log In

Ask Question, Post Comment, Tip or Contribution

Do NOT follow this link or you will be banned from the site!