PATHOLOGICAL RESULTS, ETC. 337 



together with the distribution of the lesions at varying dates 

 after infection, will form the subject of the present article. 



The two great channels by which infection occurs are 

 provided in the respiratory and the alimentary tracts. In 

 many cases of the natural disease the results of a post- 

 mortem examination show that one or other mode of 

 infection has occurred. Thus in the ordinary cases of 

 chronic pulmonary tuberculosis, the stages of the disease — 

 old at the apex, more recent in the lower parts of the lungs, 

 with the absence or scarcity of tuberculous lesions elsewhere 

 -show that the mode of infection has been by means of 

 the lungs. Also, where there is primary tuberculosis of the 

 intestines or peritoneum and a second more recent tuber- 

 culosis of other parts of the body, it is evident that infection 

 has taken place by means of the intestinal tract. But there 

 are very numerous cases of tuberculosis which are by no 

 means so simple as this ; there are, for example, cases in 

 which both lungs and intestines are affected ; where there 

 is no lesion in the lungs or intestines, that is, at the site of 

 infection ; and, thirdly, where the local lesion at the site of 

 infection is out of all proportion to the tuberculosis which 

 subsequently develops. It is on these two last points that I 

 would lay special stress, namely, the absence of the local 

 lesion at the site of infection and the disproportion between 

 the tuberculosis of the body and the size of the local 

 lesion. 



The importance of these two points is brought out by the 

 results obtained in the experiments made for the Royal 

 Commission on Tuberculosis. 



In experimental tuberculosis (and indeed in natural tuber- 

 culosis) the severity of the disease — that is, its acuteness 

 and its generalisation — depends on three factors : the dose of 

 the virus, the vital activity of the bacilli in the infective 

 material used, and the resistance or refractoriness of the 

 animal used for experiment. The last point is one .which 

 is controllable by experiment, inasmuch as if an animal 

 susceptible to the disease is chosen for experiment, and the 

 results compared with those obtained in more refractorv 

 animals, the resistance to the disease can be estimated with 



