The loudness of these sounds in the same individual. at different periods, or at different parts of the chest, depends on three circumstances—the size and form of the spaces over which • we listen, the force with which the air moves or the voice is produced, and the power of conducting sound possessed by the superficial parte. We may exclude the second of those, as being in great measure under our control, with this remark, that now and then it hap pens that over an entire lung the breathing may be unnaturally loud in conse quence of its having a vicarious duty to perform in supplying the defect of its fellow; the air simply moves faster and more freely—its sound is exagge rated, and not otherwise changed. In regard to the size and form of the spaces over which we listen, it must be remembered that not only are these changed by disease, increased or diminished in size, but at any given spot we encounter vesicles, small bronchi, and large bronchi, at different depths from the surface ; and that if the breathing in the vesicles be stopped, we shall hear the sounds in the larger spaces more or less loudly, according to the conducting power of the lung-tissue and the degree of noise the air produces in them. For example, consolidation will produce all of these effects in vary ing degrees : let, it gives rise to more or less difference in percussion resonance ; 2d. it impedes or suppresses vesicular breathing ; 3d, it increases the conduct ing power of the tissue; 4th. it makes the large tubes more rigid, and the breath-sound in them more noisy. Or, again, unnatural spaces or cavities, of varying size, existing along with more or less of consolidation, will give rise to a similar series of phenomena. On .the other hand, unusual expansion of the lung, while it causes a stoppage of the vesicular breathing, is attended with opposite effects in increpe of resonance, in diminution of the conducting power, and in lessening the noise of movement in the large tubes. Two fur
ther considerations must be borne in mind with relation to these charges in the breath and voice-sounds—viz., that the rhythm and quality of the breathing (the ratio of the inspiration to the expiration. and the softness or harshness of the breathing) vary with different sizes and forms of spaces, and conse quently become the measure of their capacity ; while, by the power of the voice, we are best able to judge of the quality of the superficial structures as a medium for conducting sound. and consequently, of their degree of solidity. By some it has been alleged that when the tubes or parietes of a cavity are more rigid, the air is more easily thrown into sonorous vibrations, and that this cause is more powerful in producing vocal resonance than the sound-con ducting property; the conclusion is the same in either case. Some allowance, however, is to be made for the size of the space, and it would appear to have something to do with the intensity of the vocal vibration of the air.
Superadded sounds have reference to the presence of some extraneous matter which, in consequence of the movement of the air, or that of the lung tissue. gives rise to sounds which have no resemblance at all to thosepro duced by healthy breathing. They may be caused by the two surfaces of the pleura moving on each other with a rubbing sound, or by consolidation of the lung giving rule to crackling noises as it expands when air enters, or by air coming into contact with fluid, whether serous, purulent, or inspiseated; and the sound in each of these cases may give very direct evidence of the physical facts which combine for its production; but standing alone-as a symptom of disease, it would be of comparatively small value in determining the condition of the patient.