All our consciousness depends upon work done by the brain. When we think, our brain is at work, or when we see, or when we feel pain. A person who has breathed a sufficient quantity of chloroform of ether, or who has had a large enough dose of opium of alcohol, cannot feel pain even when the skin is cut, because pain is really felt in the brain, and the brain of such a person is prevented from Working.

The question, then, is: How do anesthetics, as these things are called, stop the working of the brain? Such an anesthetic as chloroform is made up of certain chemical molecules. When chloroform is breathed these molecules pass into the substance of the brain itself. There the chloroform molecules combine with the molecules of the brain, probably with the result that the brain itself can no longer use up the oxygen in the blood, and so has to stop Working. But as soon as the person stops breathing chloroform, and the blood going to the brain becomes free of it, the chloroform passes back from his brain into the blood, is breathed away by the lungs, going back just by the way it came, and the person becomes conscious again.