90 Our Inner Senses

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 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, hearing, 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 and thirst register in a part of the brain called the hypothalamus when a lack of food chemicals is 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 trigger changes in the brain. A sense called kinesthesia lets us know the relative positions of parts of our bodies. Even our breathing is triggered 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.

90 我们的内在感官

如果我们只能依靠五种感官生存，我们的状况确实会非常糟糕。我们分不清上下，不知道什么时候该吃饭或喝水，不知道自己的肌肉在做什么，也不知道四肢处于什么位置。我们不会知道自己的身体何时受到了损伤，因为我们不会感到疼痛。我们可能会冻僵而不打哆嗦，或者过热而不出一滴汗。五种感官——触觉、味觉、嗅觉、听觉和视觉——只对外部世界的刺激做出反应，但我们身体的内部世界也必须接收并回应重要的信息。

我们的内在感官让我们得以存活，并使我们能够使用外在感官。实际上，内在感官告诉我们的大脑如何运作我们的身体。当血液中检测到缺乏食物化学物质时，饥饿和口渴会在大脑中一个叫做下丘脑的区域显现出来。

另一种控制我们平衡的内在感官是由内耳中三个充满液体的环维持的。位置和重力的变化会影响这种液体的运动，并触发大脑的变化。一种叫做动觉的感官让我们知道身体各部分的相对位置。甚至我们的呼吸也是由一种感官触发的，这种感官能识别血液中二氧化碳过多和氧气不足的情况。

没有人能数清楚内在感官的数量。精细的内在感官的存在，恰恰表明我们人类是多么奇妙地复杂。