In animals, the brain, or encephalon, is the control center of the central nervous system. In most animals, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, taste and olfaction. In humans, it is an organ of thought. While all vertebrates have a brain, invertebrates have either a centralized brain or collections of individual ganglia. Brains can be extremely complex. For example, the human brain contains more than 100 billion neurons, each linked to as many as 10,000 others. Most brains exhibit a substantial distinction between grey matter and white matter. Gray matter consists of the cell bodies of the neurons, while white matter consists of the fibers (axons) that connect neurons. Many functions are controlled by coordinated activity of the brain and spinal cord. Moreover, some behaviors such as simple reflexes and basic locomotion, can be executed under spinal cord control alone.