The appendicular skeleton is the portion of human and many animal skeletons that includes the limbs, the pelvis, and the pectoral girdle — in other words, most things outside of the skull, the spine, and the ribs. For adult humans, this includes approximately 126 of the roughly 206 fully formed bones. Their main function is movement, balance, and general support, and they are often credited with allowing humans to move in distinct and precise ways, including writing and drawing. Mineral storage and blood cell production also happen here.
Understanding the Human Skeleton
Medical professionals and biologists often refer to the human skeleton by dividing it into two parts, though most agree that there is really only one core skeleton. The division is primarily done out of convenience, and as a way to distinguish between how the bones work together and why. Most of a person’s bones are part of the appendicular portion. This includes the arms, the legs, and the pelvis; basically everything that is directly involved with movement and pivot power.
The outlying bones are usually known as the “axial” skeleton, and include the skull, the spinal column, the sternum, and the ribcage. These are usually more concerned with protection of things like the heart and the brain and, particularly in the case of the spine, manage overall bodily support and balance. Movement plays a role, certainly, but it isn’t the primary function the way it is for the bones in the appendicular region.
Intersection of Appendicular and Axial Skeletons
Two girdles, the pectoral girdle and the pelvis, serve as anchors to attach the appendicular portion to the axial skeleton. The pectoral girdle, consisting of the collarbone and shoulder blades, connects the upper limbs to the sternum. Each shoulder blade sits at rest over the ribs of the back, and the collarbone attaches in the front of the body with the sternum. The upper arm bone fits into a cuff of muscles that sits between the shoulder blade and collarbone. This is the only point of attachment of the upper limbs to the axial skeleton, and as a result the shoulder joint is allowed a wide range of motion compared to other joints. It also holds increased potential for injury, however, so care must be taken by athletes to avoid shoulder dislocations.
Lower limbs attach at the hips where the femur — the largest bone in the human body — fits into the pelvis. The pelvis is generally very sturdy, though it is actually comprised of several distinct bones joined by five different cartilaginous joints. In most people, the largest and most prominent areas of the pelvis are the hips. Each hip is made up of three fused bones: the pubic bone, the ilium, and the ischium. At the back of the body, the hips come together at the sacrum, which connects the lower appendicular skeleton to the axial skeleton by fusing with the tailbone.
Mechanics of Attachment
Both the upper and lower limbs attach to the girdles using ball and socket joints, in which the rounded end of a bone fits into a cup-shaped socket of muscles. This arrangement allows for a maximum degree of flexibility, allowing the limbs to move freely in a rotating motion. By comparison, the hinge joints of the knees and elbows allow a relatively restricted motion along a single plane.
Hands and Feet
Perhaps the most complex structures of the human skeleton, the hands and feet together make up more than half of the bones in the human body, with 27 bones in each hand and 26 in each foot. The wrists and ankles are a particular type of joint known as a condyloid. These joints allow movement along two planes but with less freedom of rotation than the ball and socket joints. The delicate structure and flexibility of the hands and feet are remarkable in that they allow people to do many of the things that are considered to be uniquely human, such as writing, playing music, and walking upright.