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.
Did You Know?
The female pelvic girdle is wider and lighter than the male. This is an adaptation to allow for pregnancy and childbirth.
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 taluses also known 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.
There are 5 metatarsal bones which form the ball and arch of the foot. The 14 phalanges of the toes are the counterparts of those in the fingers, with the big toe having two phalanges and the other 4 having 3 phalanges each. Together these structures consist of the bones form the lower limb shown in the figure below.