96 VARIATIONS IN PATHOGENICITY [ch, vii 



each case is attributable to anatomical differences in the 

 parts affected. 



The determining factor may be in one case the route of 

 infection, in another the lowered vitality of the particular 

 organ attacked. In a third case neither explanation appears 

 adequate and we are forced to conclude that the organism 

 itself has some influence in determining the site of the 

 disease. 



Such an hypothesis is required, for example, to explain 

 the fact that different strains of the tubercle bacillus, morpho- 

 logically and culturally indistinguishable from one another, 

 may produce in one patient phthisis, in another a tuberculous 

 osteitis or arthritis, and in a third lupus. Here the analogy 

 with lead poisoning or pneumococcal infection fails, for in 

 both the latter conditions, although one system or organ may 

 be for a time affected almost alone, the disease shows a 

 tendency to extend to other regions of the body and to 

 produce characteristic symptoms as each fresh region becomes 

 involved. In the case of tuberculous infection there appear 

 to be certain limitations. A patient with phthisis may develop 

 meningitis or peritonitis or general tuberculosis, but it is rare 

 for a phthisical patient to develop lupus or a tuberculous 

 joint, or for one suffering from a tuberculous ostitis to develop 

 either lupus or phthisis — facts which are significant when one 

 considers how widespread are these diseases. 



Nield and Dunkley (1909) quote an instance of a phthisical 

 patient who moistened a scratch on her arm with her own 

 saliva and developed lupus at that spot. Examples have been 

 given by various observers (quoted Stelwagon, 1910) of lupus 

 occurring on the hands of women employed in washing the 

 clothes of patients with phthisis and of the same disease 

 following such operations as ear piercing, tatooing and cir- 

 cumcision when performed by operators themselves suffering 

 from phthisis, but such instances are sufficiently uncommon 

 to be regarded as exceptional. 



The contrast already referred to between the gonococcus 

 and the meningococcus, in respect to the organs they par- 

 ticularly — one might almost say exclusively — attack and the 



