Under normal circumstances the stomach is invisible; but if its walls are thickened from hypertrophy of its muscular coat, and the abdominal parietes are attenuated, each contraction of the viscus can be seen as an undulating swelling, which slowly traverses the surface of the abdomen from left to right. The movements occur spontaneously and in rhythmic sequence after every meal, and may be excited at any time by rubbing the abdominal wall or by the application of ice to the skin of the epigastrium. The frequency and force of these contractions vary under different conditions, being feeble and intermittent when the stomach is empty or its tissues abnormally thin, but extremely conspicuous and practically incessant when the hypertrophied organ is endeavouring to force its contents through an obstructed pylorus. It may be observed that, unlike visible peristalsis of the colon, the movements of the stomach are not accompanied by pain.