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 82 of which that known as "mediate" percussion, which alone is generally adopted, consists in applying the fore or middle finger of the left hand closely and firmly to the patient's chest, and then striking it with one or more fingers of the right. The object in view much resembles that of the careful housewife, who, in the changing tones of the stricken beer-barrel, traces but too surely the descending level of its fast-disappearing contents.

The lungs, in fact, when properly inflated with air, as is the case in their healthy condition at every inspiration, approach the walls of the chest and convert the latter, for all practical purposes, into a cavity filled with air, and comparable to the empty barrel aforesaid. Under these circumstances the act of percussion causes the production of a musical note of a certain tone, pitch, and quality, deviations from either of which properties indicate an unnatural state of things within. Such deviations are often sufficiently obvious, not only to the operator, but also to the patient himself; sometimes, however, they are very slight, and require much care for their just appreciation: the more so as the importance of their meaning is by no means necessarily in proportion to their slightness. Besides the nature of the note, percussion detects alterations in the elasticity of the walls of the chest, and other minute indications which tact and skill alone will draw from the nicely adjusted blow of the practised operator. His judgment also will regulate the force of the blow according to the circumstances of the case; and he will hardly ever cause it to produce pain.

To illustrate roughly what the “percussion note,” as it is termed, teaches, we may take two examples of disease which cause a modification of its sound, such as it occurs under natural and healthy circumstances. In one instance it may happen that fluid comes to be interposed between a greater or less portion of the lung, and a corresponding extent of the wall of the chest. It is obvious, as in the instance of the cask, that wherever this is the case the percussion note will have an entirely different character from that yielded by the remaining portion of the chest: it will be toneless and hard, where before it was musical and resonant. In the other instance, the lung itself may, in consequence of disease, lose the hollow, expansible nature which it owns in health, and become to a greater or less extent a solid body. Under these circumstances it is easy to understand, how the fingers of the operator, no longer impinging on a surface in contact with air, but on a hard substance, will draw a sound far removed from the healthy percussion note. These are but exceedingly simple examples of the principles of percussion. It is alike impossible and needless to enumerate here the many ways in which it may be applied, and the many conditions of the lungs, heart, and other contents of the chest, in the determination of which this process plays an important part. No one of its indications deserves to be disregarded, but each is capable of misleading, unless cleared up or confirmed by other means, of which, however, only the chief and all-important one, viz., auscultation, need be described here.

There are two methods, also, of practising this operation, both of which are adopted by profession: these are, the “immediate,” in which the ear is applied directly to the patient’s chest; and the “mediate,” where the much-maligned stethoscope comes into play. It is obvious, that apart from any acoustic advantages which the stethoscope affords, and which have been alike admitted and denied on its behalf, there are very many instances in which its employment for the purposes of auscultation becomes necessary from motives of convenience alone. But the principles of auscultation in no wise depend upon its employment.

The entry of the air into the innumerable minute bags or vesicles of which the lung essentially consists, causes the production of a sound which, in health, is of a characteristic and easily recognisable nature. It is generally compared to the soft sighing of the wind among the leaves of a tree; but it cannot really be described, and requires to be heard in order to be known. But before thus arriving at its final destination the air has to pass along the windpipe and its divisions, the bronchial or air tubes; these, becoming smaller and smaller as they divide and subdivide, ramify in every direction, until they terminate in the lung vesicles above mentioned. The passage of the air along these tubes is accompanied by a sound very different in its nature to the preceding, and which is not heard on listening over the healthy lung, the latter, on account of its non-density, not being a conductor of sound. Meanwhile the sighing murmur of the proper breath-sound finds an admirable conductor in the solid chest-wall, and comes clearly and distinctly to the ear of the listener.

Supposing, however, that there should take place in the condition of the lung a change similar to that mentioned when we were speaking of percussion, and that the soft, light, elastic tissue should become, in consequence of disease, dense and hard. Under these circumstances, the air no longer penetrates into the ultimate tissue of the lung, and the characteristic breath-sound is no longer heard; but now a good conductor has appeared in the shape of