Here is the essential guide to the human brain, an authoritative reference book and timeline that examines the three pounds of matter inside our heads that does all our thinking for us. With 100 billion nerve cells joined by thousands more to every corner of the body, the brain is wired together with 100 trillion connections. That makes each and every human brain a contender for the most complex system in the universe, endowing us with an intellect that far outstrips any other creature. However, one difficult question remains: Are we intelligent enough to understand our own brains?