The skeletal system is versatile in that it serves multiple structural purposes. It protects the soft organs of the body, such as the lungs, heart, and brain. It gives shape to the entire body, connecting arms and legs to a central trunk, which allows an organism to make different types of bodily movement. It serves as the anchor for skeletal muscle, which pulls and pushes on the bones to generate movement. Lastly, different parts of the skeleton connect to form joints, which allow different parts of the skeleton to move independently of each other.
Protection for Soft Organs
One of the main structural roles of the skeletal system is to protect the soft organs. This is arguably the most important role of the skeleton, since soft tissue such as the brain controls bodily movement. The brain is also the control center for hormones that regulate bone growth and strength. Other soft organs that need protecting are the heart and lungs. An organism can survive a broken arm bone, but an injury to the heart or a lung can be quickly fatal.
Humans are bipedal organisms, meaning we walk on two legs. Part of what makes bipedalism possible is a spinal column that supports a central trunk region that is connected to and anchors our leg bones. Our legs move by rotating at the hip joint. In addition to allowing bipedal motion (and other varieties of motion), the skeletal system provides the overall framework of the body. Bones are able to grow and thicken, which allows an organism to grow taller or longer, and to support the extra body weight.
Support for Muscles
The skeletal system provides a solid structure or frame, but cannot create movement. Skeletal muscle is what creates movement by contractions and relaxations. However, skeletal muscle needs to be anchored to the skeleton in order to produce coordinated and efficient motion, not just random spasms. Muscles anchor themselves to bones by tissue called tendons, which is not to be confused with ligaments, which connect bones to bones. Tendons are a type of tough connective tissue, able to withstand tremendous amounts of pressure.
Joints for Movement
A crucial feature of the human skeletal system is that it is not a rigid system, but contains different rigid parts that can move in relation to each other. This type of movement is made possible by joints, which are where two bones are held together by muscle, tendons and ligaments. The human body has six types of joints. Gliding (or plane) joints allow for gliding movements, as in the connection between your finger bones and your hand bone. Hinge joints are for bending and straightening, as in your knees and elbows. Pivot (or rotary) joints allow for rotational movement, like that of your skull on top of your spinal column. Condylar (or ellipsoid) joints allow for movement in two directions, as in your wrist. Saddle joints are similar to condylar joints, but enable movement in more directions -- as in where your thumb connects to your hand. Lastly, ball and socket joints are your shoulders and hips.
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