PERCUSSION (Lat, percussio, from percu tere, to strike through, from prr, through quatere, to strike). In medicine, the method of eliciting sounds by tapping or gently striking the surface of the body, its object being to deter mine by the nature of the sound the comparative density of the subjacent parts. This method of diagnosis is not of recent date, for we find it mentioned by Hippocrates. It was employed by Auenbrugger in the middle of the eighteenth cen tury and later by Corvissart in the investigation of heart disease, and by Lannee in diseases of the chest. But as the only way of praetieing per cussion was by striking the surface itself with the tips of the fingers or knuckles, teehnieallv known as direct or immediate percussion, its value was limited, and it was not until Piorry introduced the mediate method—the stroke being made not upon the surface itself, but upon some intervening substance applied closely to it—that the practice became useful. This flat intervening body, made of wood, ivory, or gutta-percha, is called a pleximeter. It is struck either with a small hammer or picssor, or with one or more finger-tips. Instead of an ivory or rubber plexi meter the left index or middle finger of the ex aminer may lie used, with its fiat surface fitted accurately to the part under investigation. The force of the stroke on the pleximeter—whetlicr the stroke be made with the fingers or the ham mer—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 per pendicularly on the pleximeter. When percus sion is made over a considerable cavity filled with air—as the stomach or intestines—a hollow, drum-like, or tympanitic 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 tympanitie sound, and termed the pulmonary percussion note, depending partly on the vibra tions 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 strnek.
Auscultatory percussion is practiced when, in stead of listening to the percussion sounds as transmitted through the air, the stethoscope is placed upon the chest near the point of percus sion and the sound conveyed through it to the examiner's ear. Sec AUSCULTATION.