336 SCIENCE PROGRESS. 



mentally in susceptible animals. Any one who has engaged 

 in the regular performance of post-mortem examinations of 

 either men or cattle who have died of natural tuberculosis 

 must be aware of the difficulties surrounding an explanation 

 of the mode in which the disease has. originated. In 

 anthrax occurring in the human being the mode of infection 

 is, as a rule, evident in the local lesion produced at the seat 

 of inoculation. In diphtheria and tetanus the site of 

 infection is also evident; in the former by the false membrane 

 which is formed by the growth of the bacillus ; in the latter, 

 in the majority of cases, by the wound which precedes the 

 development of the disease. These three diseases are acute. 

 They differ from one another in the fact that, whereas in 

 anthrax the bacillus invades the tissues and oroans of the 

 body, in diphtheria and tetanus it is localised to the seat of 

 infection, and the symptoms and effects of the disease result 

 chiefly in the absorption of the chemical products of the 

 bacillus from the seat of infection. In tuberculosis, on the 

 other hand, we have a chronic disease which may become 

 localised, or may spread to different tissues and organs of 

 the body, the lesions produced in the different parts being 

 characterised by the growth of the bacillus at the particular 

 spot. Although tuberculosis, therefore, may become a 

 generalised disease, the infective agent is not present in the 

 blood or fluids bathing the tissues, except as an exceptional 

 occurrence. 



The mode of infection in tuberculosis becomes from 

 these considerations a very important study. In many 

 instances it cannot, as has been already stated, be satisfac- 

 torily made out from a post-mortem examination of human 

 beings dying from the disease, inasmuch as frequently at 

 death the channel of infection is obscured by the presence of 

 very extensive tuberculosis. Indeed it may be said that a 

 large collection of cases of post-mortem examinations of 

 tuberculosis in man, recorded with care from the point of 

 view of the channel of infection, is entirely wanting. Con- 

 siderable light is thrown on natural tuberculosis by the 

 results obtained from experiment. 



The channels by which infection occurs in tuberculosis, 



