THE APPENDICULAR SKELETON

| Pectoral Girdle| Upper limb/Arm | Pelvic Girdle| Lower limb/Leg|

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.

The shoulder blade is a flat triangular bone which stretches from the shoulder to the vertebral column at the back. On the back side it has a bony ridge for the attachment of the muscles. The bony ridge forms a prominent projection, the acromion, above the shoulder joint. Beneath the collar bone and just on the inside of the shoulder joint, is another bony projection of the shoulder blade, 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 upper arm bone, forming a ball and socket joint.

Each collar bone is rod-shaped and roughly S-shaped. It lies horizontally and articulates with the upper end of the breastbone, right in the middle and front, just above the first rib. The lateral end articulates with the acromium. Collar bones serve as a support for the shoulder blades in front and keep the shoulder blades back so that the arms can hang freely at the sides of the body. They prevent the pectoral girdles from getting out of joint easily and ample movement of the shoulders.

The Pectoral Girdle.

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.

The Upper Arm (Humerus)

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.

The Forearm (Radius and Ulna)

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 upper end of the ulna articulates with the lower end of the humerus forming a strong hinge joint in the elbow region. The lower 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 upper end articulates with both the humerus and the ulna. The broad, lower 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.

The Wrist

The wrist consists of eight carpal bones. These are small, short bones that are arranged in two rows of four. They have articulating facets which allow them to slide over one another.

The Palm 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.

The Fingers

The fingers are made up of fourteen phalanges. There are three phalanges in each finger but only two in the thumb.


The Pelvic (Hip) Girdle.

The pelvic girdle consists of two large, sturdy hip bones. 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 and the pubis the central in front. The two pubic bones are attached in the middle, on the front side by a symphysis which consists of fibrocartilage and ligaments, the pubic symphysis. The two hip bones and the sacrum form a complete bony ring, the pelvis . On the outer side of the point where the fused bones meet, there is a deep hip socket into which the head of the femur fits.

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. In females it forms a strong basin-like structure for supporting and protecting the developing foetus during child-bearing.

The Pelvic Girdle.

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 or 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 acetubulum, forming a ball-and-socket joint. At its distal end, the femur widens to form two large knobs (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, flat bone which develops on the tendon of the thigh muscle and is attached by ligaments to the tibia. This enables movement in the knee joint.

The Lower Leg

The two bones of the lower leg are the tibia (shinbone) in front and the fibula behind. The tibia is the larger of the two and extends from the knee to the ankle. The upper end of the tibia has two articulating facets into which the condyles of the femur fit to form the knee joint. The lower end of the tibia articulates with one of the tarsals to form the ankle joint. The fibula is smaller than the tibia and is situated on the outside and slightly behind it. The upper end articulates with the tibia but does not form part of the knee joint. The lower end forms part of the ankle joint.

The Ankle

There are seven short, thick tarsal bones, the largest of which is the heel bone (calcaneum), which presses firmly onto the ground when one stands, walks or runs. The calf muscles are attached to the calcenum, allowing the heel to be lifted during locomotion.

The Arch 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. The arch is modified for receiving the weight of the body.

The Toes

There are fourteen short phalanges in the toes of each foot. The big toe has two phalanges and the other toes have three in each.