PATHOGENIC MICROBES— INFECTION 127 



cellular tissue. The virus of hydrophobia spreads from the 

 region of the bite to' the nerve centres along the nerve trunks. 

 Tuberculosis appears different according as it is inoculated 

 along one path or another ; in natural disease different methods 

 of propagation are associated. The two following ideas are of 

 great importance. 



1. The role of the intestine in those diseases in which the 

 infection is not purely intestinal. — The question was long ago 

 put by Chauveau in relation to tuberculosis. Behring has 

 taken it up again, and maintains that every case of pulmonary 

 tuberculosis in the adult is the extension of an intestinal tuber- 

 culosis acquired from milk in the earliest infancy, but remain- 

 ing latent for years. Tuberculosis in general, according to 

 him, does not attack the lung until it has pierced the barrier 

 of the intestine. A similar origin has been claimed for other 

 infections which finally settle in the lung, such as the pneurno- 

 coccal inflammation. A comparative case was found in the 

 anthracosis of coal-miners (the impregnation of the lung with 

 coal-dust), and since it is easier to experiment with inert dust 

 particles than with virulent bacteria, anthracosis has become 

 the field of study and discussion in the question for and 

 against intestinal infection. 



It has been settled by experiment that living bacteria can 

 pass through the intestinal mucosa like dust particles without 

 leaving any lesion as a mark of their passage. But such 

 passage only occurs as a rule when massive, repeated doses are 

 ingested, and when there are in the intestine such injuries as 

 favour penetration. It is therefore chiefly by inhalation and 

 by penetration of the lymphatics and blood-vessels in the 

 neighbourhood of the pharynx that the tubercle bacillus 

 reaches the lungs. 



2. Seats of election and receptive cells. — The hydrophobia 

 virus fixes itself on the nerve-tissue, the parasite of malaria on 

 the red corpuscle of the blood, other protozoa on the white 

 corpuscles. The dysentery bacillus, ' inoculated under the 

 skin, proceeds to make a home for itself in the large intestine. 

 The bacillus of swine-erysipelas inoculated in the pigeon 



