THE AVENUE OF INFECTION AND TISSUE SUSCEPTIBILITY 99 



that these tissues are particularly weak in defensive power, so that the 

 bacteria naturally lodge where conditions are most favorable for their 

 growth. 



Selective Tissue Affinity. While the primary focus of infection is 

 determined largely by the route of invasion, the selective affinity of micro- 

 organisms or their toxins for certain tissues and the inherent tissue sus- 

 ceptibility to the toxins are best in evidence in the location of secondary 

 foci or localization of the infection in general bacterernias. Thus the seat 

 of the principal local lesions in pneumonia is the lungs, and in typhoid 

 fever the lymphoid tissues, especially that of the spleen, and Fever's 

 patches in the intestine. It is true that mechanical factors may aid in 

 this selection, as, e. g., the occlusion by emboli of microorganisms caught 

 in the capillaries of organs; but, in general, we must conclude that either 

 (1) Microorganisms tend to be destroyed in every tissue or organ 

 except those that are poor in defensive forces and are susceptible, or (2) 

 that microorganisms . or their products circulate passively through a 

 tissue and do not lodge because they possess no affinity for these cells. 

 In many infections both processes are probably operative, and at least we 

 are led to the very important conclusion, laid down by Adami, that "in 

 infections the body is never involved as a whole. Coincidentally with 

 the growth of the specific germs in individual organs there tends to be a 

 reaction to, and destruction of, the same in other parts." 



Nickols and Hough 1 and Reasoner 2 have isolated strains of Treponema 

 pallidum from the nervous tissues that appeared to have selective affin- 

 ities for the cornea, choroid, and retina of the eyes of rabbits; Noguchi 3 

 has noticed that various types produced lesions in rabbits of certain 

 distinct characters and with considerable constancy. Further and simi- 

 lar studies may show that various strains of Treponema pallidum may 

 possess selective tissue affinities and thereby explain the early develop- 

 ment of lesions in the central nervous, cardiovascular, and cutaneous 

 systems of different persons. According to the investigations of Rose- 

 now 4 various bacteria, and particularly streptococci, exhibit extreme 

 degrees of tissue affinity and produce various constant and distinct 

 lesions in rabbits after inoculation by various routes. 



The numeric relationship of bacteria to infection is very important, 

 and the number alone may determine whether or not it shall occur. 



1 Jour. Amer. Med. Assoc., 1913, 50, 108. 



2 Jour. Amer. Med. Assoc., 1916, 56, 1917. 



3 Jour. Lab. and Clin. Med., 1917, 2, 472. 



4 Jour. Infect. Dis., 1915, 16, 240; ibid., 1915, 16, 367; ibid., 1915, 17, 219; ibid., 

 1915, 17, 403; Jour. Amer. Med. Assoc., 1914, 63, 1835; ibid., 1915, 64, 1968. 



