"Welcome to the hypothalamus," I say, greeting the tour group of Mary's brain. "Watch your heads, but don't let the small size of the room fool you. The hypothalamus is one of, if not, the most important structures in the brain because it maintains homeostasis of the body including hunger, thirst, sexual behavior, and a wide variety of emotional behaviors.

"The walls of the hypothalamus are covered with meters, monitors, and switches all labeled with their specific function. On one wall, a huge clock with twenty-four hours on its face is marked Sleep/Wakefulness Cycles followed by at least one hundred different levers each identified with a distinct body function.

"The hypothalamus acts as a sort of biological clock. It helps time when Mary will be awake and when she will be asleep. Notice the darkened area on the clock from around 22:30 until 6:00. Mary will be getting a good night of sleep tonight. Plus, we plan to let her have a little nap this afternoon. It also controls the daily fluctuation in more than one hundred body functions. All we have to do is adjust each of the levers to keep the body homeostatic throughout the day. Well, all but blood pressure, heart rhythm, and breathing."

Laughing, I add, "We leave the basics up to the medulla."

"The hypothalamus also regulates internal body temperature," I state, pointing to a thermometer and switch on the next wall. "By adjusting that lever, we can either start the process that causes Mary to shiver when she gets too cold or make her perspire when she gets too warm.

"Finally, the hypothalamus plays an important role in the physical changes that occur with strong emotional responses. This button makes the heart pound, this one the palms sweaty. We are also responsible for butterflies in the stomach, lumps in the throat, and all the other uncomfortable physical changes that happen when Mary becomes emotional.

"You all had better move along and stop crowding us. We must continuously monitor Mary's systems. Why don't you head on over to the limbic system and see how the memories that sometimes spur intense emotional responses in Mary are formed?"

Wood, Samuel E. and Ellen Green Wood. The World of Psychology. Boston: Allyn & Bacon, 2002.