TUBERCULOSIS. 233 



tion of the weakened cells. In the immediate neighbour- 

 hood of the young tuberculous tissue, z>., in those cells 

 that have been slightly affected by the products of the 

 tubercle bacillus it sets up a further reaction ; it stimulates 

 these cells, and at the same time causes a dilatation of the 

 vessels and probably also an increase in their number (this 

 latter may be only apparent) ; a larger amount of food 

 material is brought up for the nutrition of these cells, 

 excreted matter is more readily carried away both by vessels 

 and lymphatics, and in consequence of this, cells that have 

 been too far stimulated by the tubercle poison to recover, 

 die off, whilst those that are still capable of living, even 

 under the excessive stimulation, proliferate and help to form 

 the barrier between the dead mass and the surrounding 

 normal tissues. In this way it would appear that in certain 

 cases of tuberculous disease, Koch has produced an imitation 

 of the natural process of cure. By a process of combined 

 reasoning and experimentation, and basing his method of 

 procedure on the one that had been already adopted in con- 

 nection with the preparation of vaccines for other diseases, 

 Koch succeeded in obtaining a substance with which, in a 

 certain degree, at any rate, he was able to combat the 

 advance or to modify the tuberculous disease in animals. 



As this advance in the treatment of tuberculosis marks a 

 most important point in the history of the disease, and in 

 order that there may be no misconception as to Koch's exact 

 position, it may be well to give in his own words the descrip- 

 tion of the discovery, composition, and probable mode of 

 action of the remedy. 



He says, in describing the observations, by the considera- 

 tion of which he was led to take the lines of experimentation 

 that ultimately led him to success: " If a healthy guinea-pig 

 be inoculated with a pure cultivation of tubercle bacilli, the 

 inoculation wound generally becomes glued over or sealed, 

 and appears to heal up during the next few days. It is only 

 in the course of from ten to fourteen days that a hard nodule 

 is formed, which soon opens, forming an ulcerating spot which 

 persists until the death of the animal ; if an animal that is 

 already tuberculous be inoculated the course of events is 

 very different. The most suitable animals for this experi- 

 ment are those which have already been successfully inocu- 

 lated four or six weeks previously. In the case of an animal so 



