Thus guarded, however, diagnosis is to the student the best, only legitimate introduction to the wards of an hospital; because, while its simplicity delights and its approach to certainty encourages him, it also best prepares him for understanding the uses of remedies; it teaches him what medicine can, as well as what medicine cannot accomplish; it teaches him the vanity of hunting after specifics; it saves him from becoming utterly sceptic.
If it were true that the symptoms by which a disease is recog nized were exactly analogous in all cases, it would be enough that the student should commit to memory the summary contained in systematic treatises, when he would be at once in a condition to pronounce an opinion upon any case put before him. But this is far from being the case: the idiosyncrasy of the individual, including in this term all the differences exhibited by various persons in their susceptibility to the influence of the same noxious substance or emanation; not less than this, the varying power of the causes of diseas, which, though unproved, and perhaps incapable of proof, we cannot deny, exerting an influence now more potent, now weaker ; the combination of these two circum stances leads to an almost endless variety in the outward mani festations of their operation on the human frame. The perplexity thus produced has led men to seek for some symptom which may of itself determine the nature of the malady, which may be con sidered in the common phrase " pathognomonic" of the disease. Such simple indications would be invaluable if they were attain able, but unfortunately the proof they are supposed to afford is based upon false induction. Some of the greatest minds have fallen into this error, and none more than they who have culti vated the physical aids to diagnosis, first introduced by the great Laennec.
By means of auscultation and percussion we reach a class of phenomena much simpler, and more nearly related to the diseased action, than those evidences which come to us through the cir cuitous channel of disturbed function, reacting as every function does upon other organs, itself again altered or modified by them. They are, in fact, the necessary consequences of the morbid con dition of the parts, but they are not the direct exponents of that state; it is only by inference that we deduce from the acoustic signs the nature of the pathological change. With reference to
the lungs, for example, we learn by percussion the relative density of the parts struck, but the cause of that density must be proved by other circumstances. By auscultation we discover that the air enters more or less freely into one portion of the lung as com pared with another; that it meets with obstacles which produce certain sounds; that the acoustic properties of the lung are changed but the. causes of these phenomena must be sought than in the phenomena themselves. Crepitation is often spoken of as pathognomonic of pneumonia. Now it is quite true that clinical observation has shown, in a vast number of cases, that when, after death, fibrin is found effused into the parenchyma, such an obstruction to the admission of air at one period exists, that it enters the lung with a puff of crackling noise called crepi tation; but until it can be shown that the noise stands to the fibrin in the relation of effect to cause, it is a false induction to assume that it is a certain evidence of its presence. And when we consider how possible it must be that some other cause of obstruction may produce the same effect, or one so nearly alike that it e,annot be distinguished from it, how possible that some other sound altogether may be mistaken for it; when we further know that both these events do continually happen in practice, and that cases of pneumonia do frequently present themselves in which crepitation is not heard at all, it will at once be conceded that, though an important auxiliary, it is not an essential point in diagnosis.
In another class of diseases, the changes of structure are before our eyes; and here, if at all, the symptom might be regarded as pathognomonic--the pustules of smallpox, for example. But what shall we say of a ease when death ensues before the pustule is formed ? I have seen differences of opinion prevail regarding such an one only a few hours before the patient expired. And similar BOUTCA38 of fallacy might be adduced of all csorresponding instances of the visible, tangible results of disease.