a. Sonorous and Sibilant Sounds.—These consist of a prolonged tone, grave or shrill, or simply of a continuous hissing noise, accompanying the greater part of the act of inspiration or ex piration, or both together, but chiefly the latter : it is that noise which, when loud enough to be he,ard without applying the ear to the chest, is called wheezing. The depth or shrillness of the note indicates, within certain limits, whether the sound be pro, duced in larger or smaller tube,s.
b. Friction Soun4 caused by the rubbing together of two roughened surfaces of pleura, which, in their healthy state, glide noiselessly over each other. It has been already noticed that crepitation is sometimes so fine that it can scarcely be distinguished from friction, when the ear cannot discriminate the distinct explo sions of which the sound is composed. In a similar manner, friction may be so coarse as to be mistaken for crepitation. The chief differences are these: crepitation is more deep-seated, fric tion more superficial ; fine crepitation accompanies the act of inspiration only, is quite rhythmical with it, and terminates with it : friction may occur at any period of the respiratory act, is very commonly heard both with respiration and expiration, or is intermediate between them, and is not rhythmical with the inspi ration. The one is evidently part of the sound produced by the
movement of the air in the lung during inspiration ; the other has nothing to do with the respiratory sounds at all, but with the movements between the lung and the chest. Along with these distinctions must be taken the correlative evidence as to whether the affection be one of the lung or of the pleura.
c. Crumpling Sound.—It most nearly resembles the crumpling together of tissue-paper; it is heard most frequently at the apex of a tuberculous lung. Believed by some to be caused by the stretching of old bands of lymph, its rationale is not well under stood, and its value is not great.
d. Creaking: a very similar sound heard on deep inspiration, when the air first begins to penetrate a previously carnified lung. It has no interest but as a matter of curiosity after the subsidence of an attack of pleurisy. Not unlike to this is the creaking pro duced by old bands of lymph in the lower region of the thorax, or the rubbing together of roughened portions of pleura over tubercular deposit.
No attempt has been made to give detailed descriptions of these sounds, because they can only be learned by experience : good examples of each should be sought out, and carefully listened to, before making any attempt to discriminate them in obscure cases.