Human beings are recognized by such external body parts as the head, torso, arms legs, and skin. On the inside, however, human bei ngs are marvelously complex Inside the human body are 206 bones and about 700 muscles; a brain of about 10 billion nerve cells a small intestine about 22 feet longa a heart that beats more than 3 billion times during a lifetimes and a network of blood vessels if laid end to that end, would encircle the earth's equator more than twice. These are just a few inter- nal parts of the human body. What's amazing is that a parts of the human body work together. To understand how, let's start with the basics.