KOCH'S LAWS. 23 



Since these laws were formulated another proce- 

 dure has been evolved which may give valuable 

 evidence as to etiology. This pertains to the ag- 

 glutination test, or, as we speak of it in connection 

 with typhoid fever, the Gruber-Widal reaction. 

 This principle, that in the acquiring of immunity 

 to a microbic infection the serum of an individual 

 gains in agglutinating power for the micro-organ- 

 ism,, has been found to hold true in many infec- 

 tions. Consequently, if one has in hand the speci- 

 fic micro-organism for a disease, he would expect 

 the serum of a patient sick of this disease to have a 

 stronger agglutinating power for this micro-or- 

 ganism than , for others which were accidentally 

 present ; and this power would also be greater than 

 that possessed by the serum of one who had not 

 had this particular disease. -In spite of some pos- 

 sibilities of error the agglutination test has been 

 of distinct value in the recognition of the specific 

 micro-organisms in certain diseases, as in the case 

 of the germ of epidemic dysentery (Shiga). 



Also the more recent development of the opso- 

 nins and, particularly,, of the phenomenon of fixa- 

 tion of complement, promise to be of value in the 

 recognition of specific micro-organisms. 



All Koch's laws have not been complied with in obstacles 

 certain cases, because of various difficulties 

 which have been encountered. First, the patho- 

 genic protozoa can not be cultivated on artificial 

 media (we must except the success of Novy and 

 McNeal with certain trypanosomes, and of Mus- 

 grave and Clegg with the Amoeba coli under sym- 

 biotic conditions) ; second, certain bacteria which 

 may be found constantly in a given disease have 

 not been cultivated artificially (spirillum of recur- 



