The skeletal system provides the body with an overall structural framework and has two divisions, the axial skeleton and the appendicular skeleton. The axial skeleton is found around the vertical axis of the body and includes the skull, vertebral column, sternum (breastbone), and ribs. The appendicular skeleton consists of the upper and lower limbs and the girdles that attach the limbs to the axial skeleton. The pectoral girdle (or shoulder) is associated with the upper limbs (arms), and the pelvic girdle (or pelvis) is associated with the lower limbs (legs). The bones of the appendicular skeletal system are arranged in a manner that allows people to perform basic functions such as locomotion, gathering food and other objects, and using tools. These bones are also involved in protection, mineral storage, and production of blood cells and platelets.
There are four types of bones found in the appendicular skeleton: long bones, short bones, flat bones, and irregular bones. A long bone is longer than it is wide and has a narrow shaft in the middle with knobs on each end. Long bones include the collarbones and the bones in the upper and lower leg, the upper and lower arm, the hand and foot, and the fingers and toes. A short bone is approximately cube-shaped and includes the kneecap and each bone in the wrist and ankle. The shoulder blade is an example of a flat bone, which is flat and thin. An irregular bone does not have a clearly defined shape based on the other categories and includes the bones that make up the hip. Because bones interact with other bones and are the site of attachment for the muscle tendons and ligaments that hold bones together, each bone has unique bone markings. Bony projections include processes, condyles, epicondyles, tuberosities, and tubercles. Processes are outgrowths on the bone. Condyles and epicondyles articulate with other bones, and tuberosities and tubercles are sites of tendon and ligament attachment. When describing bones and bone markings on the limbs, the terms proximal and distal are frequently used. Proximal refers to structures closer to the torso of the body, and distal refers to structures farther away. For example, the wrist is proximal to the hand, and the ankle is distal to the lower leg.
The pectoral girdle is made up of two pairs of bones, the clavicles and the scapulae. These provide the supportive framework for and the attachment site of the upper limb. The arm, forearm, wrist, and hand are four distinct regions of the appendicular skeleton that make up the upper limb. The humerus makes up the upper arm, which meets the forearm at the elbow. The two long bones of the forearm are the ulna and radius that run the entire length of the forearm. The posterior protrusion at the elbow is the most proximal part of the ulna. The radius runs lateral to the ulna and meets with wrist bones, eight small bones called carpals. The hand contains five bones in the palm called metacarpals and several small bones called phalanges that make up the fingers. The skeleton of the upper limb allows the body to perform many functions, including tool use, food gathering, and caring for self and others.
The lower limb joins the axial skeleton at the pelvic girdle, made up of large irregular bones called coxal bones. The thigh, leg, and foot are the three regions that make up the lower limb of the human body. The thigh contains a single bone, the femur. The lower leg is between the knee and the foot and is made up of two bones, the tibia and fibula. The femur articulates with the tibia, and at this joint there is a small bone called the patella. There are seven tarsal bones that form the ankle and proximal foot, five metatarsals that form the distal foot, and several phalanges that make up the toes. Unlike the arm, which has a wide range of motion, the leg has a more limited range and is more stable during locomotor activities.
There are 126 bones that make up the appendicular skeleton of an adult human. Each limb contains 30 bones, the pectoral girdle contains four bones, and the pelvic girdle contains two bones. Articulations, or joints, connect these bones together. For example, the upper limb consists of articulations between the humerus and ulna to form the elbow joint. Bones are held together by strong sheets of tissues called ligaments. To prevent bones from wearing down from direct contact, bones are protected at the ends by cartilage, a connective tissue with a rubbery consistency.