Posted in Biology on Friday, 27 April 2012
This edited article about human biology originally appeared in Look and Learn issue number 696 published on 17 May 1975.
Being able to keep upright is called “balance” or, as scientists say, “maintaining equilibrium.” We can do this because nature has provided us with two built-in “spirit levels.”
The ordinary spirit level consists of a glass tube containing a liquid. The tube is not quite full, so that a bubble of air floats in it.
When the spirit level lies flat, the air bubble is in the centre of the tube. Tip the tube to the right, the bubble moves to the left; tip it to the left, the bubble moves to the right.
But once the tube has been tipped, it must be moved by someone before the bubble is level.
The “spirit levels” in our ears do much better than that. If we tip backwards or forwards or to the right or left, they automatically bring us level again.
Each “spirit level” consists of three semicircular canals, containing a fluid, and an hour-glass shaped organ. The upper part of the “hour-glass” is called the utriculus and the lower part is the sacculus.
The fluid in the canals tends to stay still, but if we move our head it moves and bumps against nerves at the end of each canal. This causes a nerve message to be sent to the brain, telling us how much and in what direction our head has moved.
Inside the utriculus and sacculus are tiny pendulums of crystal suspended from very fine hairs.
If our body gets into a position from which we are in danger of losing our balance and so toppling over, the pendulums swing with it and bump against nerves. These instantly send a message to the brain warning of the danger.
The brain then causes return messages to be sent to the nerves and muscles controlling the movements to restore our balance. In this way the combined actions of the pendulums and of the fluid in the canals enable us to keep our balance.
If you keep twisting your body round and round you will feel giddy and lose your sense of balance. This is because you have churned up the fluid in the canals and put your “spirit levels” out of action.
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