§ 2. In cases in which the lung has become less dense than natural, it is immaterial whether the vesicles be distended, as in the common form of emphysema, or the air pass into the paren chyma of the lungs, as happens when its structure is torn : the natural elasticity which expels the air at each expiration is lost either case, the air stagnates, and the vesicular murmur is no longer audible; the sound of air moving in the large tubes would indeed be heard distinctly, were it not that the rarefied tissue has become a bad conductor; and hence it is only when superadded sounds indicate the motion of the air, or when the tubes, thick ened, roughened, or dilated, cause unusual vibration, as it passes to and fro, that the distant sounds reach the ear at all plainly: the voice meets with the same obstacle to its transmission, and is only heard when the tubes are thickened or dilated. The super added sounds, therefore, stand with many for the evidence of emphysema, while in reality they are so only secondarily : the thickening and dilatation of the tubes, and the increased voice and breath-sound which accompany them, are frequent sources of fallacy.
§ 3. No other condition of disease exactly simulates the dead, dull, inelastic sound of percussion, and the sense of resistance which is produced by the presence of fluid in the pleural cavity: occupying, as it always does, the inferior part, the intensity of the dulness gradually diminishes towards the apex, but of course it varies with the amount of fluid ; and inasmuch as pleurisy does not necessarily imply the presence of fluid, the dulness may be caused by effusion of lymph only. When this is the case, the
dull sound is mixed up with a 'certain degree of resonance, which has been compared to that of striking on wood ; one which in its greatest intensity is best heard when there is a thickened pleura with air in its cavity. If there be no pneumonia when fluid is effused, the lung is simply carnified from pressure ; it is pushed upwards, and hence the sounds belonging to this condition are most distinctly to be made out under the clavicle ; somewhere over the scapula sagophony is met with. When pneumonia is also present, the tubes remain more generally permeable to air, and the voice-sound is diffuse, somewhat metallic or brassy, but modified by the superstratum of fluid, so as to approach to sego phony; it differs essentially, however, in that it is diffuse and not local. If chronic thickening of the pleura exist, the breathing is weaker, but not otherwise modified by this circumstance.
The tympanitic percussion sound of air in the pleura is very rarely pure ; there is almost always a thickened membrane, which gives a wooden tone to the resonance, and fluid usually exists at the base. Percussion over a large cavity has an amphoric or cracked-pot resonance, which is somewhat analogous, but no mis take need occur from this cause if any degree of care be used ; because at the lower and back parts breathing, probably much altered and mixed with superadded sounds, can be detected where there is only a cavity at the apex, while none can be heard in pneumothorax, save where the air escaping from the lung causes the long amphoric blowing of expiration.