Hiccups (sometimes spelt hiccoughs, but always pronounced hiccups) are caused
by sudden contractions of the diaphragm, the main muscle of breathing. The
lungs hang inside the chest, a bit like sacks stretched up against the inside
of the chest wall. When we breathe in, we dont expand the lungs directly. The
lungs expand because we increase the volume of the chest cavity and the lungs,
because they are applied to the inside of the chest wall, expand with the rib
cage. The main muscle of breathing is the diaphragm, which forms the bottom of
the chest cavity and moves down, forcing the lungs to expand more. When you
hiccup, your diaphragm suddenly twitches, and this forces air into the lungs.
At the same time the glottis -a flappy bit at the top of your throat which is
part of your voice box - suddenly closes. This sudden closing blocks off the
air and causes the noise.
Hiccups dont start in the muscle of the diaphragm itself, but in the nerve
that supplies the diaphragm: the phrenic nerve. Its not uncommon for hiccups
to start during eating, because the nerves supplying the stomach have links
with the nerves of breathing, but hiccups can come on at any time. Most fits of
hiccups last only a few minutes, but very rarely they go on for days.
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