The human brain is the center of the human nervous system. It has the same general structure as the brains of other mammals, but is larger than expected on the basis of body size among other primates. Estimates for the number of neurons (nerve cells) in the human brain range from 80 to 120 billion. Most of the expansion comes from the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the cerebral cortex devoted to vision is also greatly enlarged in human beings, and several cortical areas play specific roles in language, a skill that is unique to humans.

Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the human brain is susceptible to many types of damage and disease. The most common forms of physical damage are closed head injuries such as a blow to the head, a stroke, or poisoning by a variety of chemicals that can act as neurotoxins. Infection of the brain, though serious, is rare due to the biological barriers which protect it. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, multiple sclerosis, and Alzheimer's disease. A number of psychiatric conditions, such as schizophrenia and depression, are thought to be associated with brain dysfunctions, although the nature of such brain anomalies is not well understood.

Sean Ronald Hill (born February 14, 1970) is an American former professional ice hockey defenseman who played 17 seasons in the National Hockey League (NHL) for eight different teams. He won the Stanley Cup in 1993 with the Montreal Canadiens.

Hill was drafted in the 8th round, 167th overall by the Montreal Canadiens in the 1988 NHL Entry Draft from Wisconsin Badgers and was a member of the United States 1992 Winter Olympic Team. On October 8, 1993, Hill scored the first goal in the history of the Mighty Ducks of Anaheim in a 2-7 loss to the Detroit Red Wings. In August 2006 he signed a one year contract with the Islanders.

On April 20, 2007, Hill became the first player to be suspended for abusing the performance enhancing substance policy the NHL and NHLPA have put in place. Hill began the mandatory 20 game suspension in Game 5 of the Islanders' Eastern Conference Quarter-Final versus the Buffalo Sabres. The Islanders lost the game and were eliminated from the playoffs. Islanders General Manager Garth Snow said that he supported the league's decision to suspend Hill. Hill, who was signed by the Minnesota Wild as a free agent in July 2007, would be required to sit out the first 19 games of the 2007/08 season without pay. In July, Hill passed both a lie detector test and independent drug test regarding his substance abuse.^{[citation needed]} In a statement, the Wild said, "We believe Sean did not knowingly take any banned performance-enhancing substance." Hill returned to the lineup on November 21, 2007, after completing the 20-game suspension.