Chapter 5 - Appendicular Skeleton

Appendicular Skeleton
     The appendicular skeleton is composed of 126 bones of the limbs and the pectoral and pelvic girdles by which they connect to the axial skeleton.
Pectoral (Shoulder) Girdle
  The pectoral girdle consists of two bones:
  The clavicle is also known as the collarbone. The clavicle braces the shoulder joint against the sternum. Laterally, it articulates with the scapula.
  The scapulae are also known as the shoulder blades. The scapulae are flat, triangular bones. At the lateral angle of the scapulae, the glenoid cavity articulates with the head of the humerus to form the shoulder joint. 
  The pectoral girdle is very light and allows the upper limb to have considerable free movement. However, the greater freedom of movement makes the shoulder girdle less stable and shoulder dislocations more likely.
Upper Limbs
  There are 30 bones in each upper limb that are found in the following regions:
  The arm has only one long bone, the humerus. At the proximal end of the humerus there is a rounded head that articulates with the glenoid cavity of the scapula.
  There are two bones in the forearm:
  The radius is the lateral bone. The proximal part of the radius has a disc-like head that articulates with the capitulum of the humerus. The distal end of the radius widens and forms the primary articulation with the proximal carpal bones of the wrist. Because of the proximal articulation, the radius can cross over the front of the ulna when the hand is turned from one side to the other.
  The ulna is the medial bone of the forearm. The proximal end of the ulna forms the olecranon process which is the point of the elbow. 
  The wrist contains 8 short bones which are called the carpal bones. Each bone has a unique shape and name.
  The palm is supported by 5 long bones that are called metacarpals. Each bone is associated with a number starting with the thumb on the lateral side.
  The fingers, or digits, have longs bones that collectively are called the phalanges. The thumb has two phalanges and each finger has three.
Pelvic Girdle
  The pelvic girdle consists of two hip bones (a.k.a. coxal bones and ossa coxae) and the sacrum and coccyx. The pelvic girdle transfers the weight of the upper body to the legs and is designed for stability as it performs this role.
  Each hip bone results from the fusion of three bones:
  The ilium is a large, flaring bone that forms the superior part of the hip bone. The upper edge of the ilium is the iliac crest which is an important landmark. The crest ends anteriorly in the anterior superior iliac spine which can be easily felt and seen especially in thin people.
  The ischium forms the inferior part of the coxal bone. On the inferior surface of this bone there is a roughened part called the ischial tuberosity. The ischial tuberosities are the parts of the hip bones that press against any object that you sit on.
  The pubic bone, as it is also called, is the most anterior part of the hip bone. 
  The ilium, ischium and pubis together form a deep socket called the acetabulum which articulates with the head of the femur.
 Lower Limb
  The bones of the lower limb bear the body's weight and are larger and stronger that those of the upper limb.
Femur (thigh)
  The femur is the only bone of the thigh and it is the heaviest and strongest bone in the body. The proximal part of the femur has a ball-like head that fits into the acetabulum of the hip bone.
  The skeleton of the leg consists of two bones:
Tibia (shinbone)
  The tibia is the larger, medial bone of the leg. The anterior surface of the tibia forms an anterior border that can be easily felt. Distally there is a process called the medial malleolus that forms the inner bulge of the ankle.
  The fibula is a long, slender bone lateral to the tibia. The distal part of the fibula forms the lateral malleolus that forms the outer part of the ankle.
  The bones in the foot are arranged to form three arches. These arches are supported by ligaments and tendons. The weight of the body puts stresses on these arches that can be recovered as weight is taken off. In other words, these arches enable us to walk and run more efficiently.
  The foot is composed of:
  The tarsus consists of seven tarsal bones. Two notable tarsal bones are the calcaneus, or heel bone, and the talus which articulates with the tibia to form the hinge-like ankle joint.
  There are five metatarsals that form the sole of the foot.
  There are 14 phalanges arranged in a similar fashion to the phalanges in the hands. The great toe like the thumb of the hand has only two phalanges.