ETIOLOGY. 25 



teriologists, possessing diflFerent degrees of skill, and often by differ- 

 ent methods of staining, with microscopes of all kinds, good and bad, 

 have sought for this bacillus, and it is not strange that now and then 

 some man fails to find the organism in a genuine case of tuberculosis. 



Another most important point in this connection lies in the fact 

 that the clinical and the bacteriological diagnoses do not always agree. 

 The most skilful clinicians may differ concerning a case of membra- 

 nous sore throat. One is sure that it is diphtheria ; a second is in 

 doubt and reports it as a suspicious or doubtful case ; and a third is 

 sure that it is not diphtheria. A bacteriological examination may 

 reveal or fail to reveal the presence of the Loeffler bacillus. Again, 

 it may be that any number of the most competent clinicians agree 

 in saying that the case is or is not one of diphtheria, and yet a bac- 

 teriological examination may result in a contradictory diagnosis. 

 This is exactly what has happened in the study of diphtheria. From 

 statistics gathered by Novy, it appears that of 8,186 cases of clinical 

 diphtheria, diagnosed as such by different men in Europe and Amer- 

 ica (to May, 1895), the Loeffler bacillus was found in 5,943, the 

 bacteriological examinations also being made by different men. The 

 clinical diagnosis was confirmed bacteriologically in 72.6 per cent, of 

 these cases. On the other hand, of 333 cases diagnosed as diphtheria 

 by Baginsky, 332 furnished the bacUlus, and of 117 seen by Kossel, 

 all were confirmed by the bacteriological examination. These figures 

 are given to illustrate the factors of variation that may arise in the 

 application of the first of Koch's rules. 



Shall we accept the clinical or the bacteriological classification of 

 disease ? There can be no doubt that the latter is the more exact, 

 and its adoption will lead to a more accurate and scientific study of 

 disease. An etiological classification of the infectious diseases is one 

 of the great desiderata of scientific medicine. Whether it will ulti- 

 mately be made upon the morphological characters of the bacteria or 

 on their poisonous products cannot yet be determined. There are 

 certain objections to making the first of these the basis that seem 

 well-nigh insuperable, and some of these will be discussed later. 



The importance of the first of Koch's rules is self-evident ; how- 

 ever, the invariable presence of any germ in a certain disease does 

 not prove that the former is the cause of the latter. Indeed, so long 

 as the investigation goes no further than this, we are justified in say- 

 ing that the microorganism may be an accompaniment or a conse- 

 quence of the disease ; therefore, additional evidence is wanting, and 

 is furnished by complying with the other rules of Koch. 



The second rule is complied with by means of plate and other cul- 

 tures, a description of which would be out of place here. 



The third and fourth rules are difficult of application, because the 

 lower animals are often immune to many of flie diseases to which 

 man is susceptible. 



