The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few invertebrates such as sponges, jellyfish, adult sea squirts and starfish do not have one, even if diffuse neural tissue is present. It is located in the head, usually close to the primary sensory organs for such senses as vision, hearing, balance, taste, and smell. The brain of a vertebrate is the most complex organ of its body. In a typical human the cerebral cortex (the largest part) is estimated to contain 15–33 billion neurons, each connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.

From an evolutionary-biological point of view, the function of the brain is to exert centralized control over the other organs of the body. The brain acts on the rest of the body either by generating patterns of muscle activity or by driving secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information-integrating capabilities of a centralized brain.

From a philosophical point of view, what makes the brain special in comparison to other organs is that it forms the physical structure that generates the mind. As Hippocrates put it: "Men ought to know that from nothing else but the brain come joys, delights, laughter and sports, and sorrows, griefs, despondency, and lamentations." In the early part of psychology, the mind was thought to be separate from the brain. However, after early scientists conducted experiments it was determined that the mind was a component of a functioning brain that expressed certain behaviours based on the external environment and the development of the organism. The mechanisms by which brain activity gives rise to consciousness and thought have been very challenging to understand: despite rapid scientific progress, much about how the brain works remains a mystery. The operations of individual brain cells are now understood in considerable detail, but the way they cooperate in ensembles of millions has been very difficult to decipher. The most promising approaches treat the brain as a biological computer, very different in mechanism from electronic computers, but similar in the sense that it acquires information from the surrounding world, stores it, and processes it in a variety of ways.

This article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar as it shares the properties of other brains. The ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context. The most important is brain disease and the effects of brain damage, covered in the human brain article because the most common diseases of the human brain either do not show up in other species, or else manifest themselves in different ways.

Noun

Wikipedia brain (plural brains)

1. The control center of the central nervous system of an animal located in the skull which is responsible for perception, cognition, attention, memory, emotion, and action.
2. An intelligent person.

   He was a total brain.

3. (English pluralia tantum) A person who provides the intelligence required for something.

   He is the brains behind the scheme.

4. (in the plural) Intellect.

   He has a lot of brains.

   • 2008 Quaker Action (magazine) Rights trampled in rush to deport immigrant workers, Fall 2008, Vol. 89, No. 3, p. 8:

     "We provided a lot of brains and a lot of heart to the response when it was needed," says Sandra Sanchez, director of AFSC's Immigrants' Voice Program in Des Moines.

5. By analogy with a human brain, the part of a machine or computer that performs calculations.

   The computer's brain is capable of millions of calculations a second.

• harns
• See also Wikisaurus:brain
• See also Wikisaurus:genius

Quotes about the brain.

Sourced

• Aristotle taught that the brain exists merely to cool the blood and is not involved in the process of thinking. This is true only of certain persons.
  • Will Cuppy, The Decline and Fall of Practically Everybody, 1950

• Brain, n. An apparatus with which we think we think.
  • Ambrose Bierce, The Devil's Dictionary

• Do we trivialize a sublime feeling if we appreciate its dependence on the brain? Not in the least. Its significance does not depend on its being a soul state or a brain state...Humility bids us to take ourselves as we are; we do not have to be cosmically significant to be genuinely significant.
  • Brain-Wise by Patricia Churchland

• "We do have an organ for understanding and recognizing moral facts. It is called the brain.
  • A Neurocomputational Perspective by Paul Churchland

• "If the human brain were so simple that we could understand it, we would be so simple that we couldn't."
  • Emerson M. Pugh, As quoted in The Biological Origin of Human Values