6.3 Human 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.

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.

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

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.

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

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.

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.

The appendicular skeleton consists of the pectoral girdle with the arms and the pelvic girdle with the legs. The pectoral girdle and arms and pelvic girdle and legs will be explored in greater detail in the following section.

The pectoral girdle and arms

The pectoral girdle consists of two clavicles (collar bones) and two scapulae (shoulder blades). Each clavicle is attached to the sternum in the front and the scapulae at the sides and they help to support the shoulders. The clavicle is the most frequently broken bone in the body as it often takes the full impact of falls on outstretched arms or of blows to the shoulder. The pectoral girdle is connected by muscles to the back of the thorax to enable it to have a supporting structure as well as giving the shoulders greater freedom of movement which in turn allows greater mobility of the arms. Any limit to movement is provided by the clavicle.

Each upper arm has a single bone called the humerus which fits into the Glenoid cavity on the scapula to form a ball and socket joint. This cavity is very shallow which allows the arms to move in almost any direction. The forearm consists of two bones namely the ulna in line with the little finger and the radius in line with the thumb. The joint at the elbow is a hinge joint. The wrist consists of eight small carpal bones arranged in two rows of four. The palm of the hand consists of five metacarpal bones. There are 14 digits (short bones) or phalanges in each hand, two in each thumb and three in each of the fingers.

Functions of pectoral girdle

• Forms a strong support structure for the attachments of the arms.
• Provides large area of bone for the attachment of muscles.
• Forms ball-and-socket joints with the arms which allows the arms to move freely.

Pelvic girdle and the legs

The pelvic girdle consists of hip bones joined at the front by cartilage called the pubic symphysis and they are attached to the sacrum at the back. Each hip bone consists of three fused bones (ilium, ischium and pubis). Portions of all three bones contribute to the formation of the acetabulum, a deep socket into which the head of the femur (thigh bone) joins to form the hip joint.

The femur in the leg is the largest and strongest bone in the body. The upper end forms a ball and socket joint with the hip bone while the lower end articulates with the tibia to form the hinge joint of the knee. The patella or kneecap is a flat triangular bone which is embedded in the tendon of the thigh muscle and attached by a ligament to the tibia.

There are two bones in the lower leg: the tibia (shin bone) which is the larger of the two and supports most of the mass. The upper end articulates with the femur while the lower end articulates with one of the tarsal bones to form the ankle joint. The fibula (calf bone) is thinner than the tibia and serves mainly for the attachment of muscles. It is attached to the femur and is articulated to the top and bottom of the tibia.

The structure of the foot is similar to that of the hand. However, the foot supports the weight of the body, so it is stronger and less mobile than the hand. There are seven tarsals or ankle bones, only one of which, the talus, articulates with the tibia. The talus us also know as the ankle bone. The heel bone (calcaneum) is the largest of the tarsal bones and is the bone to which the calf muscle is attached. The heel bone presses firmly on the ground when one stands, walks or runs.