Comparative Neuroscience/Overview of Mammalian Neuroanatomy
The mammalian brain can be viewed as three brains in one--the reptilian brain, the limbic system, and the neocortex. This controversial division of the brain in such a fashion was originally proposed by Paul MacLean in the 1950s.
The reptilian brain includes the brain stem and cerebellum. This term came from the fact that a reptile's brain is dominated by these areas, which control instinctive behavior and thinking. In humans, this part of the brain controls the muscles, balance and autonomic functions such as breathing and heartbeat.
The limbic system, also called the mammalian brain, is the source of our emotions and instincts. When this part of the brain is accessed, emotions are produced. MacLean observed that everything in the limbic system is either agreeable or disagreeable. This means that survival is based on the avoidance of pain (disagreeable) and the recurrence of pleasure (agreeable).
The neocortex, forms part of the cerebral cortex, and is known to be the center for cognition controling higher-order thinking, reasoning and speech.
While there is much evidence against this model in fact, it is still considered a useful way of conceptualising the brain.
The corpus callosum controls the transfer of signals between the left and right hemispheres of the brain. In the 1950s, Roger Sperry discovered the function of the left and right hemispheres by studying brain function in people whose corpus callosa had been severed.