Hiccups are caused by a spasm, or sudden, involuntary contraction, of the diaphragm (the muscular partition separating the chest and abdominal
cavities).

Normally during breathing the diaphragm contracts and flattens, drawing air into the lungs and then forcing air out as it relaxes. A spasm of the diaphragm causes air to be inhaled suddenly. As the air rushes through the airway on its way to the lungs, the opening into the windpipe, called the glottis, closes abruptly. As a result, the flow of air is halted and the vocal cords close rapidly, producing the characteristic hic sound.

You can find out more here.
Source(s):
http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761573251


Permalink | Report