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either by pressure from without, as in pleuritic effusion, or by deposit within, as in pneumonia or pulmonary apoplexy, then, in consequence of the parts below resonating only very imperfectly with the parietes, from defective compressibility and elasticity preventing their free vibration, a dull, heavy, or dead sound is produced—as in a cask filled with water, or a drum with dough. When the portion of the parietes which covers the heart is similarly struck, the resulting sound is also dull, in consequence of the slight amount of compressibility and elasticity possessed by that organ and its contained fluid. When, therefore, the heart is considerably enlarged, when its investing membrane is distended with effused pus or serum, or when its principal vessels are greatly enlarged by aneurism, the extent of dulness in the præcordial region is increased. These few examples will suffice to illustrate the great importance of percussion in the diagnosis of diseases of the chest. The practice of this mode of exploration, apparently so simple, requires great nicety, tact, and delicacy, for its efficient application. Great care is also required, in its exercise, that the examiner be not misled and deceived by trifling circumstances connected with the manner in which it is performed or with the parietes of the patient.

1. Different Modes of Percussion.

Percussion may be either mediate or immediate. It is immediate, when the striking body, whether it be the hand of the examiner, or any other instrument, falls directly upon the parietes of the chest, or when some article of clothing only intervenes between them and the percussing body. It is mediate, when some solid material, as a disc of wood or ivory, a piece of leather, or the finger of the left hand, is interposed between the parietes and the striking body for the purpose of increasing or modifying the sound.

1. Immediate Percussion may be performed with any suitable instrument, as the end of a stethoscope, or with the knuckle of the contracted forefinger of the right hand, but is most efficiently practised with the points of the appressed fingers. The extremities of the fore, the middle, and the ring finger, should be pressed close together, and brought as nearly as possible to a level surface, beyond which the nails should not extend, and to which they should scarcely reach. The patient being in the position pointed out in a preceding chapter, and the parietes being rendered tense, the knuckle of the fore-finger, or the points of the appressed fingers arranged as just explained, should be smartly struck, in a direction perpendicular to the surface, successively upon the

sternum, clavicle, the ribs, and scapulæ. The corresponding regions of the two sides should be percussed immediately after each other, so that the resulting sounds may be accurately compared, as dulness, not absolute only, but comparative, often affords most important indications. It may be well to state that the tips, the actual bony extremities, of the fingers should be the parts coming in contact with the chest, and not the soft bulbous sentient palmar surfaces of the last phalanges, otherwise the sound will not be so clear, as the percussing instrument will not be so firm, and therefore not so well calculated to elicit it. The examiner should also be especially careful that the fingers, or other percussor employed, strike the chest at a right angle with the surface struck; as, if they fall upon the parietes in a direction only slightly oblique, the effect will be greatly diminished.

2. Mediate Percussion.-Direct or immediate percussion may be sometimes employed with advantage, particularly when great nicety of discrimination is not required, and when it is not necessary that the examination should be prolonged. It is, however, when extensively employed, open to some and not slight objections. When, for example, the patient is an irritable or nervous person, each stroke of the fingers or percussor causes excitement, and the frequent repetition of

the slight tappings upon the unprotected chest induces a state of the circulation and respiration entirely opposed to the calm and quiet condition, which it is so desirable should be preserved during the future stages of the examination. When the patient is thin, or the skin delicate, the direct percussion of the slightly covered osseous structures causes inconvenience, sometimes amounting to pain. When the individual, on the other hand, is fat, or very muscular, when the integuments are oedematous, or when in the female the mammary glands are large and full, or loose and pendulous, direct percussion cannot be employed with advantage, in consequence of the dense and non-elastic soft tissues preventing the stroke of the finger reaching the bony parietes with sufficient force and sharpness to induce effectual vibrations. There are also some parts, in which, from the absence of bony covering, immediate percussion is entirely useless.

On these accounts, and perhaps some others, mediate percussion has been employed, and a variety of instruments have been proposed for its performance. Thin discs of wood, ivory, cork, and India rubber, with and without different kinds of tongues or lips to be used as handles, have been introduced and advocated under the name of Pleximeters, for the purpose of receiving and conveying the stroke of the finger, or of a variety of little hammers, under the title of

Plessors. Each of these may possess some advantages, and each has had something and some one to recommend it for general employment, but each participates more or less in the objection of itself giving rise when struck to a sound, which interferes with that dependent on the vibration of the thoracic parietes.

If any substance intervene between the striking instrument and the chest, if any pleximeter be employed, it is clear that its vibratory power should be as similar as possible to that of the vibrating parietes; that it should be composed of materials identical with them,-should in fact be made of bone covered with skin. Such materials we have already prepared. A pleximeter of bone covered, with skin admirably adapted for the purpose, and always ready for use, we possess in the fingers of the left hand. This pleximeter possesses many advantages in addition to those already mentioned.

It is not open to the objection of sometimes causing pain by firm pressure upon fat and thin persons, as are the wood or ivory discs which are occasionally employed. It is capable of being much more nicely adjusted to uneven surfaces than either these rigid substances, or stiff leather, and can be much more easily maintained in the required situation than flexible but elastic India rubber. In thin persons the intercostal spaces form hollows which cannot be filled by unyielding pleximeters; air consequently

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