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It is fascinating to know that billions of neurons may control behavior, and that the nervous system is so large that it is subdivided into regions based on location and function. For example the autonomic nervous system, made up of spinal and cranial nerves, sends and receives information related actions of the internal organs associated with sympathetic or parasympathetic stimulation. It is difficult to comprehend that without this antagonistic relationship between sympathetic and parasympathetic responses my pupils would not allow light to enter the eye or my blood flow to skeletal muscles would be restricted. Although many behaviors in the body are unconscious it is comforting that some behaviors, such as breathing, are partially controlled by the conscious mind.