The appendicular skeleton consists of the girdles and the skeleton of the limbs. The upper (anterior) limbs are attached to the pectoral (shoulder) girdle and the lower (posterior) limbs are attached to the pelvic (hip) girdle.
The Pectoral (Shoulder) Girdle
The Pectoral girdle consists of two shoulder blades (scapulae) and two collar bones (clavicles). These bones articulate with one another, allowing some degree of movement.
Scapulae (shoulder blades)
The shoulder blade is a flat triangular bone which stretches from the shoulder to the vertebral column, posteriorly. The bony ridge forms a prominent projection, the acromion, above the shoulder joint. Beneath the clavicle and just on the inside of the shoulder joint, is another bony projection of the scapulae, the coracoid process, which also serves for the attachment of muscles. The upper outer corner of the shoulder blade ends in the glenoid cavity into which fits the head of the humerus forming a ball and socket joint.
Clavicles (collar bones)
Each collar bone is rod-shaped and roughly S-shaped. It lies horizontally and articulates with manubrium of the sternum at the sternoclavicular joint and forms the acromioclavicular joint with the acromion process of the scapula.
The Upper Limb
The skeleton of the upper limbs or arm may be divided into five main regions: an upper arm bone, the forearm (radius and ulna), the wrist, the palm of the hand and the fingers.
Humerus (The upper arm)
The upper arm is a single long bone. The upper end consists of a hemi-spherical ball which fits into the socket of the shoulder blade to form the shoulder joint. The lower end of the humerus forms a shallow ball and socket joint with the radius and a hinge joint with the ulna in the elbow.
Radius and ulna (The forearm)
The two long bones of the forearm are known as the radius and the ulna. The ulna is the larger of the two bones and is situated on the inner side (i.e. the little finger side) of the forearm. The proximal end of the ulna articulates with the distal end of the humerus forming a strong hinge joint in the elbow region. The distal end of the ulna is slender and plays a minor role in the formation of the wrist joint. The radius is situated on the thumb side of the forearm. Its proximal end articulates with both the humerus and the ulna. The broad, distal end of the radius forms a major part of the wrist joint, where it articulates with the wrist bones (carpals).
The radius also allows the forearm to be rotated. The radio-ulnar joints are pivot joints in which the moving bone is the radius. As the head of the radius pivots at these joints, the lower end of the radius moves round the lower head of the ulna.
Carpal (wrist) bones
There are eight carpal bones. These are small, short bones that are arranged in two rows of four.
Proximal row: scaphoid, lunate, triquetrum, pisiform
Distal row: Trapezium, trapezoid, capitate, hamate
These bones are closely fitted together and held in position by ligaments that allow a limited amount of movement between them.
Metacarpal bones (bones of the hand)
The palm is supported by five long metacarpals. The metacarpals articulate with carpals at one end and with the phalanges at the other end.
Phalanges (fingers bones)
The fingers are made up of 14 phalanges. There are three phalanges in each finger but only two in the thumb. They articulate with the metacarpal bones and with each other, by hinge joints.
The Pelvic (Hip) Girdle
The pelvic girdle consists of two large, sturdy hip bones. The pelvis is the term given to the basin-shaped structure formed by the pelvic girdle and the sacrum.
Each hip bone consists of three fused bones namely the ilium, ischium and the pubis.
The ilium is the largest of the three and forms the upper part of the hip bones. The sacrum fits like a wedge posteriorly between the two hip bones.
The sacrum has a large, flat articular surface on each side for articulation with the ilia.
The ischium forms the inferior part of the hip bone.
The two pubic bones are attached centrally and anteriorly by a symphysis which consists of fibrocartilage and ligaments, called the pubic symphysis.
The two hip bones and the sacrum form a complete bony ring, the pelvis . On the lateral side of the point where the fused bones meet, there is a deep depression into which the head of the femur fits, the acetabulum.
The Lower Limb
The pelvic girdle forms a strong support for the attachment of the limbs. Strong muscles of the back, the legs and the buttocks are attached to it. It protects some of the internal organs.
The skeleton of the lower limb may be divided into five main regions: the upper leg (thigh), the lower leg, the ankle, the arch of the foot and the toes.
The upper leg (thigh)
The upper leg has a single long bone, the femur and is the longest bone in the body. The head of the femur is turned slightly inwards and has a large, rounded portion which articulates in the acetabulum, forming a ball-and-socket joint. At its distal end, the femur widens to form two large condyles which form the hinged knee joint with the main long bone (tibia) of the lower leg. On the anterior side of these two condyles, there is an articular surface against which the kneecap (patella) slides.
The patella is a small, triangular-shaped sesamoid bone associated with the knee joint. Its posterior surface articulates with the patellar surface of the femur and its anterior surface is in the patellar tendon.
The Lower Leg
Tibia (shin bone)
The two bones of the lower leg are the tibia (shinbone) medially and the fibula laterally. The tibia is the larger of the two and extends from the knee to the ankle. The proximal end of the tibia has two articulating facets into which the condyles of the femur fit to form the knee joint. The head of the fibula articulates with the inferior aspect of the lateral condyle, forming the proximal tibiofibular joint.
The distal end of the tibia forms the ankle joint with the talus and the fibula. The medial malleolus is a downward projection of the bone, medial to the ankle joint.
The fibula is smaller than the tibia and is situated on the outside and slightly behind it. The proximal end articulates with the tibia but does not form part of the knee joint (tibiofibular joint). The distal end forms the lateral malleolus, which helps stabilise the ankle joint.
Tarsal (ankle) bones
There are seven short, thick tarsal bones, the largest of which is the calcaneus (heel bone), which presses firmly onto the ground when one stands, walks or runs. The remaining bones are the talus, navicular, cuboid and three cuneiform bones. The bones articulate with each other and with the metatarsal bones.
Metatarsals (bones of the foot)
The arch is formed partly by some of the tarsals but mainly by the five long metatarsals, which extends from the tarsals to the toes. At their proximal ends they are articulate with the tarsal bones and at their distal ends articulate with the phalanges.
Phalanges (toes bones)
There are 14 short phalanges arranged in a similar manner to those in the finger, i.e. two in the hallux (big toe) and three in each of the other toes.