PERCUSSION, in medicine, is the method of eliciting sounds by tapping, or gently striking the surface of the body; its object being to determine by the nature of the sound the comparative density of the subjacent parts. This means of diagnosis was first employed by Avenbrugger in the middle of last century, and it was afterwards adopted by Corvisart in the investigation of heart diseases; but its value was not fully appreciated tiil Laennee made the diseases of the chest his peCuliar study; and since his time its application and various uses have been considerably extended by the labors of Piorry, Hughes Bennett, and other physicians.
Percussion is chiefly employed in the diagnosis of diseases of the lungs, heart, and abdominal 0171IDS. It may be direct (or, as some writers term it, immediate), or it may be mediate. In the former case, the part to be examined is struck with the ends of the three first fingers set close together on the same level, or with a small hammer tipped with India rubber; while in the litter, which is now almost universally adopted, a flat body is placed upon the chest, or other part to be examined, and is then struck by the fingers or hammer. The flat intervening body is termed a pleximeter (from the Gr. plexiF, a blow, and metron, a measure). The instrument usually sold as a pleximeter is a flat oval piece of ivory, but the left index or middle finger of the physician, with its flat surface fitted accurately to time part to be examined, acts equally well. The force of the stroke on the pleximeter—whether the stroke be made with the fingers or the hammer— must vary according as it is desired to elicit the sound from a superficial or a deep-seated part. The surface to be percussed should be exposed, or, at most, only covered with one layer of clothing; and the blow should fall perpendicularly on the pleximeter.
When percussion is made over a cavity filled with air—as the stomach or intestines—a hallow, drum-like, or (as it is usually termed by medical writers) a tym panitie sound is produced. When any part of the surface of the chest is struck below which there is a considerable depth of healthy lung-tissue, consisting of small cells filled with air, a clear sound, less loud and hollow than the tynipanitic sound, and termed the pulmonary percussion note, depending partly on the vibrations of air in the lung-cells, and partly on the vibrations of the walls of the chest, is evolved. When the subjacent substance is solid (as the heart, liver, or spleen) or fluid (as when there is effusion into a closed sac), the sound is dull in proportion to the density and want of elasticity of the part struck. The first that must be acquired, in order to make percussion useful in the diagnosis of disease, is an accurate knowledge of the sounds, elicited from the different parts in their nprmal condition. When, for example, the healthy pulmonary per cussion note is known, increased resonance of the walls of the chest will indicate a dilata tion of the air-cells (or pulmonary emphysema), while various degrees of dullness will afford evidence of such morbid changes as the effusion of fluid into the pleura (hyaro thorax). or inflammatory solidification of time lung-tissue (the hepatization of pneumonia), or tubercular deposition. The use of percussion in relation to diagnosis is further shown in the articles PERICARDITAS and PLEURISY.