The appendicular skeleton joins with the axial skeleton at the shoulders and hips. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Two bones, the clavicle (collar bone) and scapula (shoulder blade) form one shoulder. The scapula rest on top of the ribs in the back of the body. It connects to the clavicle, the bone which attaches the entire shoulder structure to the skeleton at the sternum. The clavicle is a slender bone that is easily broken. Because the scapula is so loosely attached, it is easily dislocated from the clavicle, hence the dislocated shoulder injuries commonly suffered by persons playing sports. The major advantage to the loose attachment of the pectoral girdle is that it allows for a wide range of shoulder motions and greater overall freedom of movement.
Unlike the pectoral girdle, the pelvic girdle, or hips, is strong and dense. Each hip, left and right, consists of three fused bones, the ilium, ischium and pubic. Collectively, these three bones are known as the innominate bone. The innominates fuse with the sacrum to form the pelvic girdle. Specifically, the iliums shape the hips and the two ischial bones support the body when a person sits. The two pubic bones meet anteriorly at a cartilaginous joint. The pelvic girdle is bowl-shaped, with an opening at the bottom. In a pregnant woman, this bony opening is a passageway through which her baby must pass during birth. To facilitate the baby's passage, the body secretes a hormone called relaxin which loosens the joint between the pubic bones. In addition, the pelvic girdle of women is generally wider than that of men. This also helps to facilitate birth, but is a slight impediment for walking and running. Hence, men, with their narrower hips, are better adapted for such activities. The pelvic girdle protects the lower abdominal organs, such as the intestines, and helps supports the weight of the body above it.
The arms and legs, appendages of the body, are very similar in form. Each attaches to the girdle, pectoral or pelvic, via a ball and socket joint, a special type of synovial joint. In the shoulder, the socket, called the glenoid cavity, is shallow. The shallowness of the glenoid cavity allows for great freedom of movement. The hip socket, or acetabulum, is larger and deeper. This deep socket, combined with the rigid and massive structure of the hips, give the legs much less mobility and flexibility than the arms.
The humerus, or upper arm bone, is the long bone between the elbow and the shoulder. It connects the arm to the pectoral girdle. In the leg the femur, or thigh bone, is the long bone between the knee and hip which connects the leg to the pelvic girdle. The humerus and femur are sturdy bones, especially the femur, which is a weight bearing bone. Since the arms and legs are jointed, the humerus and femur are connected to other bones at the end opposite the ball and socket joint. In the elbow, this second joint is a type of synovial joint called a hinge joint. Two types of synovial joints occur in the knee region, a condylar joint (like the condylar joint in the first vertebra) which connects the leg bones, and a plane, or gliding joint, between the patella (knee cap) and femur.
At the elbow the humerus attaches to a set of parallel bones, the ulna and radius, bones of the forearm. The radius is the bone below the thumb that rotates when the hand is turned over and back. The ulna and radius then attach to the carpel bones of the wrist. Eight small carpel bones make up the wrist and connect to the hand. The hand is made up of five long, slender metacarpal bones (the palms) and 14 phalanges of the hand (fingers and thumb). Some phalanges form joints with each other, giving the human hand great dexterity.
Similarly, in the leg, the femur forms a joint with the patella and with the fibula and tibia bones of the lower leg. The tibia, or shin bone, is larger than the fibula and forms the joint behind the patella with the femur. Like the femur, the tibia is also a weight bearing bone. At the ankle joint, the fibula and tibia connect to the tarsals of the upper foot. There are seven tarsals of the upper foot, forming the ankle and the heel. The tarsals in turn connect to five long, slender metatarsals of the lower foot. The metatarsals form the foot's arch and sole and connect to the phalanges of the feet (toes). The 14 foot phalanges are shorter and less agile than the hand phalanges. Several types of synovial joints occur in the hands and feet, including plane, ellipsoid and saddle. Plane joints occur between toe bones, allowing limited movement. Ellipsoid joints between the finger and palm bones give the fingers circular mobility, unlike the toes. The saddle joint at the base of the thumb helps make the hands the most important part of the body in terms of dexterity and manipulation. A saddle joint also occurs at the ankles.