Bone is living connective tissue.

A skeletal system is made mostly of two materials: bone and cartilage. Bone is, by definition, connective tissue--tissue that provides support or, as its name suggests, connects other tissues. It is porous, meaning it has spaces in it. These spaces give bones flexibility. Bone tissue is alive, and can grow and repair itself, despite being hard due to having minerals, especially calcium. Cartilage is also connective tissue, but it is made of proteins that make it more flexible than bone, and it does not contain the minerals that bones contain.

The human rib cage contains both bone (yellow) and cartilage (white).

Most vertebrates have skeletons that have both bone and cartilage--for example, humans have a skeleton made mostly of bone, with cartilage in their ears, nose, and parts of their rib cage. Some animals, like sharks, however, have skeletons made of cartilage, with a skull and jaws that contain a little calcium for extra support.

The axial skeleton is made of the skull, spinal column, and rib cage.

Whether made of cartilage or bone, a vertebrate skeleton can be divided into two major parts: the axial skeleton and the appendicular skeleton. The axial skeleton is made of the body's long axis. In humans, this long axis consists of the skull, spine, and rib cage. The axial skeleton has about 80 bones and is fused together; that is, it's all connected.

Based on what you recall about the functions of the skeleton, what do you suppose is the main purpose of the axial skeleton? Think about your answer, and then click the Show Me button to see if you are correct.

The primary function of the axial skeleton is protection. The skull protects the brain, the vertebral column protects the spinal cord, and the rib cage protects the heart and lungs.

The appendicular skeleton includes the arms and legs.

The appendicular skeleton consists of everything the axial skeleton does not include. Note the term "appendage" as part of the word "appendicular." In humans, the appendicular skeleton consists of the limbs--the arms and legs. This part of the skeleton is made of about 126 bones and it functions primarily to aid with movement.