Paul Broca considered the limbic lobe to be the fifth lobe of the brain. It is on the medial surface of each hemisphere. It is actually a rim of cortex. This cortex is not well convoluted and is phylogenetically among the oldest structures of the brain. This circular area includes the septal region, hippocampal gyrus, cingulate gyrus and island of Reil. The limbic system includes the olfactory bulb and tract, hippocampus, fornix, amygdaloid body, and septal nuclei. The principal effectors of the limbic system are the reticular formation and the hypothalamus. This "old brain," especially the amygdala is quite involved in emotion and in reacting to danger. A recent functional MRI study done by faculty at Harvard University and New York University showed that the amygdala reacted to faces of people of a difference race. Two other areas: the cingulate gyrus and the anterior prefrontal cortex reacted to the alarm signals by accessing the danger and giving the all clear. The hippocampus is also involved in memory. Cells in that area die and regenerate constantly. One aspect of damage to the hippocampus is the inability to store new memories for more than a couple of minutes This is called antegrade amnesia. The hippocampus has connections with the cortex which store memories it generated. It is probable that one day science will be able to bypass the hippocampus and store memories in the cortex.