Human Brain
The human brain is a 3-pound (1.4-kilogram) mass of jelly-like fats and tissues—yet it's the most complex of all known living structures. Up to one trillion nerve cells work together and coordinate the physical actions and mental processes that set humans apart from other species.
Photograph by Fred Hossler/Getty Images

Brain

See pictures of the brain, brain tumors, and other ailments from National Geographic.