The adult human skeletal system consists of 206 bones, as well as a network of tendons, ligaments and cartilage that connects them. The skeletal system performs vital functions — support, movement, protection, blood cell production, calcium storage and endocrine regulation — that enable us to survive.

How does the Skeletal System help us?

Support
The main job of the skeleton is to provide support for our body. Without your skeleton your body would collapse into a heap. Your skeleton is strong but light. Without bones you'd be just a puddle of skin and guts on the floor.

Protection
Your skeleton also helps protect your internal organs and fragile body tissues. The brain, eyes, heart, lungs and spinal cord are all protected by your skeleton. Your cranium (skull) protects your brain and eyes, the ribs protect your heart and lungs and your vertebrae (spine, backbones) protect your spinal cord.

Movement
Bones provide the structure for muscles to attach so that our bodies are able to move. Tendons are tough inelastic bands that hold attach muscle to bone.

The muscular system is an organ system consisting of skeletal, smooth and cardiac muscles. It permits movement of the body, maintains posture, and circulates blood throughout the body.

The muscular system is responsible for the movement of the human body. Attached to the bones of the skeletal system are about 700 named muscles that make up roughly half of a person's body weight. Each of these muscles is a discrete organ constructed of skeletal muscle tissue, blood vessels, tendons, and nerves.

Types of Muscle Tissue

In humans and higher animals, there are three kinds of muscle cells, corresponding to the major categories of muscle: skeletal, cardiac, and smooth.

Skeletal muscles, also called striated muscles, are governed by voluntary commands, and allow a broad range of body movements. Cardiac muscles keep the heart beating, and are capable of uninterrupted activity without fatigue. Smooth muscles, like cardiac muscles, are subject to involuntary commands, and are regulated by the brainstem located at the base of the skull. These smooth muscles ensure that the internal organs function normally, such as the muscular contractions that move food through the digestive tract.

The three kinds of muscle tissue can be identified easily by their organizing structures, which are particularly visible under a microscope. Skeletal muscle tissue, the most common kind of muscle tissue in humans and other large animals, has striations, or grooves, that mark out each individual muscle cell. These cells, sometimes called muscle fibers, extend the length of the muscle. This is necessary for the cells to perform their function efficiently.

Smooth muscle tissue, as the name implies, has a uniform appearance, similar to that of non-muscular tissue. The cells do not need to be as elongated as skeletal muscle fibers, because the motion created by these muscles is more gradual and requires less energy.

Cardiac muscle tissue has striations like that of skeletal muscles, but the cells are smaller, like those of smooth muscles. They also have a distinctive branched structure that is better suited to the task of constantly pumping blood through the heart. Otherwise, the two types of muscle are very similar.