The commonest form of muscle that we see is the red, lean meat of beef, mutton, or pork; and this will give us a good idea of how our own muscles look.

All muscles, whatever their size or shape, are made up of little spindle-shaped or strap-shaped cells, or wriggling "body-cells" arranged in bands or strings.

The size of a given muscle depends upon the number of cells that it contains.
The astonishing variety of movements which muscles can make is due to the fact that they have the power when stirred up, or stimulated, of changing their shape.

As most of the muscle substance is arranged in bands, this change of shape on the part of the tiny cells that make up the band means that the band grows thicker and at the same time shorter,--just as a stretched rubber band does when it slackens,--so that it pulls nearer together the bones or other structures to which it is fastened at each end by fibrous cords called _tendons_, or sinews.
This shortening of the muscle band is known as _contraction_.
When you wish, for instance, to lift your hand toward your face, you unconsciously send a message from your brain down the nerve cables in your spinal cord, out through the nerve-wires of your neck and shoulder, to the big _biceps_ muscle on the front of your upper arm.

This muscle then contracts, or shortens, and pulls up the forearm and hand, by bending the elbow joint.