ABAPTISTON, in surgery, the perforating part of the instrument called a trepan. The word is from the negative a, and σπύνα to sink under. This instrument hath had various contrivances to prevent its sinking suddenly upon the membranes of the brain when the operator was sawing the skull: whence its name. But the present practice proves all precautions needless, unless the operator is attentive and careful when he uses this instrument.