If you’ve been reading about space ships with human passengers, you’ve probably noticed that one of the problems that scientists will have to overcome is that of loss of equilibrium as a human being moves away from the pull of the earth’s gravity.
Equilibrium is the ability to adjust ourselves in relation to the earth, which constantly pulls at everything that is near it. Without equilibrium it would be impossible for a bird to fly, a man to walk, or for any living thing to have a sense of direction.
Man has two organs of equilibrium; they are located in the head near the ear. Each organ is like a sack and is filled with a liquid called lymph. Inside the organ, growing out from the bottom, is a kind of hair that springs out of a sensory, or recording cell. When the head is kept erect, the pressure on these hairs is distributed equally. If the head is inclined to one side or the other, the pressure on these hairs changes. The sensory cells are stimulated by this pressure and send nerve impulses to the brain. The brain sets certain muscles into action, and the body readjust itself and returns from an inclined to an upright position.
Our organs of equilibrium also enable us to know in which direction we are moving. Let’s imagine we got out for a drive in a motor car. As the car starts forward, our body sinks back against the seat. The liquid, or lymph, in the “canals” of the organs of equilibrium, bends the sensory hairs back. This causes certain nerves with which they are connected to send a message to the brain, and we feel that we are moving forward.
After a while, the lymph moves with our body, the sensory hairs are erect, and we no longer have that feeling of suddenly moving forward. Now suppose we step on the brake. Our body keeps going forward and so does the lymph in the organs. It therefore bends the hairs forward. The message goes to the brain and we feel ourselves slowing-up.
There are three “canals” in each organ of equilibrium, to correspond with the three planes of space. In this way, the sensory hairs tell us whether we’re going up or down, from side to side, or backwards or forwards.