The second segment appears as a slight swelling in lower vertebrates and enlarges in the higher primates and ourselves into the midbrain . The latter is part of the limbic system . The limbic system , essentially alike in all mammals, lies above the brain stem and under the cortex and consists of a number of interconnected structures. Researchers have linked these structures to hormones, drives, temperature control, emotion, and one part, the hippocampus to memory formation. Neurons affecting heart rate and respiration appear concentrated in the hypothalamus and direct most of the physiological changes that accompany strong emotion.
Aggressive behavior is linked to the action of the amygdala , which lies next to the hippocampus.
The latter plays a crucial role in processing various forms of information as part of our long term memory. Damage to the hippocampus will produce global retrograde amnesia , or the inability to lay down new stores of information. In a sense, the human brain is like an archeological site with the outer layer composed of the most recent brain structure, and the deeper layers consisting of structures from our shared evolutionary history with the reptiles and mammals. Finally, the third section, the forebrain appears as a mere bump in the brain of the frog but balloons into the cerebrum of higher life forms and covers the brain stem like the head of a mushroom. It has further evolved in humans into the walnut-like configuration of left and right hemispheres.