Passage Two

If we had to rely on only five senses for survival, we would be in very sad shape indeed. We wouldn’t know up from down. We wouldn’t know when to eat or drink. We wouldn’t know what our muscles were doing or what position our limbs were in. We wouldn’t know when our body was damaged because we wouldn’t feel pain. We might freeze to death without even a shiver, or overheat without a drop of sweat. The five senses--touch, taste, smell, bearing and sight-respond only to stimulation from the outside world, but the inside world of our bodies must also receive and respond to important messages. Our internal senses keep us alive and enable us to use our external senses. In effect, the internal senses tell our brains how to run our bodies. Hunger or thirst register in a part of the brain called the hypothalamus(丘脑下部) , when a lack of food chemicals are detected in the blood. Another internal sense that controls our balance is maintained by three fluid-filled loops in the inner ear. Changes in position and gravity affect the motion of this liquid and cause changes in the brain. A sense called kinesthesia(肌肉运动知觉) lets us know the relative positions of parts of our bodies. Even our breathing is caused by a sense that identifies an overabundance of carbon dioxide and a lack of oxygen in the blood. No one has been able to count the number of internal senses. The presence of delicate internal senses shows just how marvelously complex we human beings are.

Hunger is caused by （）.