Percussion is not equally applicable over all parts of the chest. (1) In front its variations are readily perceived, but it is only over the upper third that the indications are of much value with reference to the lungs. In the middle third the heart on the left side prevents a correct comparison with the right ; and lower down, while enlargement of the liver may be the cause of dulness on the right side, distension of the stomach with gas may give rise to unusual resonance on the left. Applied over the region of the heart, it teaches us whether a larger portion of lung-tissue than usual be displaced by disease of this organ. or, on the contrary, whether the lung have encroached on the ordinary space of precordial dulness. (2) At either side the upward pressure of the abdominal viscera tends to invalidate any results of percussion below, and those only are the trustworthy which are obtained from the region bordering on the axillte ; and even here stomach resonance in rare cases makes itself heard. (3) Over the back the thickness of the walls of the chest limits us in a very great measure to the inner border and lower angle of the scapula, as it requires considerable tact to make the difference perceptible even in the supra-spinal region, where, notwithstanding, it is much more readily applicable than upon or just below the spine of the scapula. In a downward and out ward direction we are met by the same difficulties, which tend to invalidate the effects of percussion in front and on either side : for practical purposes, however, the information derived from the region on either side of the spine, when the scapuls3 are drawn aside by the arms being crossed in front, is quite sufficient.
c. Auscultation.—In this term we include all the sounds pro duced by the movement of the air; whether in ordinary breath ing, in forced inspiration in the act of coughing, or in the reso nance of the voice. We have to observe the sound caused by its simple motion backwards and forwards in the air-tubes and vesi cles, to take note of the force with which the voice formed at the larynx is tran.smitted through the tissue of the lung, and to listen for anything unusual or abnormal, which we may call superadded sounds.
The vesicular murmur, as it is called, heard loudest and often only during inspiration, is that which characterizes healthy lung: it is distinguished from unhealthy breath-sounds of all kinds by its great softness, but in loudness and distinctness, perhaps, no two chests are exactly alike. The resonance of the voice also differs extremely in different persons, and even in different parts of the same lung in perfect health ; in disease its chief value is derived from a want of correspondence between those in which its intensity is usually equal. The characters of superadded sounds will be discussed in Chapter XIX. Knowledge on these points, while most essential to correct diagnosis, cannot be con veyed by any written description.; it can only be acquired by repeated examinations of the chest in health as well as in disease. The student must in the first instance exercise much patience and attention, and especially not restrict himself, to persons laboring under some form of pulmonary disease. If on first applying his ear to the stethoscope he should hear nothing, he may cause the patient to inspire deeply, to talk, or to cough, when some sound will be produced ; and if that sound be peculiar, he ought to listen to it till it can be recollected and recognized again, and if possible he should get some more experienced auscultator to ex plain it. By this means, in a wonderfully short time, he will
i find himself quite competent to say what is healthy and what unhealthy breathing, what is natural and what superadded sound.
In the detailed treatises on auscultation descriptions of all possible sounds are given. and names are too often employed which have tended rather to perplex than to instruct. The nomenclature has unfortunately been derived from the morbid condition with which the sounds have been supposed to be associated ; and in well-marked examples, no doubt, the name and the associa tion are correct; but as it necessarily happens that such morbid states are not separated from each other by any distinct line of demarcation, and that the actual character of the sound cannot be very clearly defined, it seems unwise to employ a name which suggests a theory of disease, while prosecuting an inquiry which is only ultimately to lead to its discovery. It is better, therefore, to confine ourselves as much as possible to terms which convey ideas of sound rather than ideas of disease.
Auscultation is best performed in front, by means of the stethoscope. Over the back the ear more readily takes cognizance of the condition of extensive tracts of lung-tissue, when applied directly, with only the intervention of a fold of linen ; here the sounds have to pass through much thicker parietes, and therefore it is unwise still further to deaden them by the intervention of an instrument; when it becomes important to localize a sound, the stethoscope may be used.
In conditions of disease we meet with modifications of the breath and voice-sounds, and with snperadded sounds. There can be no absolute standard of health to which the breath or voice-sound can be at all times referred ; and hence, as in percussion, our judgment in regard to them must be in great measure formed by comparison of different parts of the same chest. The student must place no reliance on what he may consider deviations from the ideal standard, but confine himself to discovering a want of consistency between the two sides, and it will often require the exercise of his clearest judgment and most correct reasoning to deduce from this want of consistency the exact nature of the deviation. It is to be observed that difference in the intensity of the voice-sound is most liable to mislead, and is least to be relied on as indicating the condition of the lungs ; difference in the loudness and quality of breath-sound affords more direct and more satisfactory evidence ; difference in the resonance on percussion is unmistakable proof of different degrees of density of the lung, if the parietes be free from disease, while superadded sound is of necessity connected with something abnormal; and we have only to determine what that sound exactly is, and what physical elements can give rise to it. The combination of the evidence derived from these sources, with the history of the case, and the other symptoms of disease, forms the basis upon which our judgment c oncerning the pathological condition of the lungs ought to rest; it is most important to remember that no one of these facts, taken singly, is sufficient to warrant any deduction regarding its nature ; and that the larger the number of facts which coincide, the more will this deduction partake of the nature of certainty.