42 Professor Sims Woodhead, The Relationship between 



mesenteric tubercle plus the presence of the tubercle bacilli in 

 such a large proportion of milk taken by these children are closely 

 associated. 



Let us now come to another aspect of the question. All those 

 who have studied tuberculosis are satisfied that very numerous 

 and very distinct species are affected by tuberculosis, and as Sir 

 John McFadyean points out, we know of no single organism which 

 will produce disease in more than half a dozen species and which 

 affects either the human subject or cattle which does not also 

 affect them both. It has of course been pointed out that the 

 tubercle bacilli met with in various animals exhibit very varying 

 degrees of virulence. They also differ considerably as to their 

 rate and luxuriance of their growth, and it is quite possible that 

 they may differ in other respects and perhaps differ very 

 materially. In this, however, they resemble many of the other 

 infective organisms. Washbourn and Eyre, for example, pointed 

 out that- not only could they modify the virulence of the pneumo- 

 coccus by passing it through rabbits and mice, but that by 

 cultivating it as a saprophyte on sterile media in test tubes, they 

 were able to render its growth much more luxuriant and at the 

 same time to diminish its virulence, these alterations taking place 

 much more rapidly in certain of the individual organisms than in 

 others, as after cultivating them for some time they could separate 

 from the same culture highly virulent organisms, growing slowly, 

 and slightly virulent organisms, growing rapidly and luxuriantly. 

 Pasteur's experiments on hydrophobia afford another example of 

 increase and diminution of virulence by passage through different 

 species of animals. Numerous observers have obtained similar 

 results by the passage of a streptococcus through different species 

 of animals; the streptococcus passed through the rabbit becoming 

 more virulent for that animal and less so for the mouse, whilst a 

 similar organism passed continuously through a series of mice 

 acquires increased virulence for that animal, this being accompanied 

 by a corresponding diminution in virulence of that organism for 

 the rabbit. It is unnecessary to multiply examples. Anyone 

 who will take the trouble to go through the appendix to the 

 Second Interim Report of the Royal Commission on Tuberculosis 

 will be very much struck by the marked differences in virulence of 

 the tubercle bacilli separated from different cases and inoculated 

 into animals. They will find that, speaking generally, the tubercle 

 bacillus, that is what the Commission terms "dysgonic," is dis- 

 tinctly more virulent than the rapidly and luxuriantly growing 

 (eugonic) form. All varieties both as to growth and virulence have 

 been found in the human subject, some appearing to produce 

 extensive and rapidly generated lesions, others producing minimal 

 lesions or, in some cases, none at all. It is known that alterations 



