BACTERIACE.E : THE TUBERCLE BACILLUS 307 



all stages of evolution. At any stage in its evolution the develop- 

 ment of the tubercle may become arrested and it may retrogress- 

 and heal if the infected tissue is able to overcome the bacilli. 

 If this occurs early the bacilli may be entirely destroyed and the 

 abnormal tissue may disappear completely or remain only as a 

 little hyaline or fibrous tissue. After caseation has occurred, 

 healing results in the formation of a dense fibrous nodule, usually 

 with calcareous material in the center, in which living tubercle 

 bacilli can usually be demonstrated. 



The mode of infection in human tuberculosis has been a matter 

 of some controversy and much of the evidence concerning it 

 has been derived from animal experimentation. Unquestionably 

 tubercle bacilli may pass through epithelial surfaces, especially 

 of mucous membranes, without production of any demonstrable 

 lesion. Ingested bacilli readily pass through the intestinal 

 mucosa, especially during the digestion of fat, and they may 

 first produce lesions in the mesenteric lymph glands, the liver 

 or in the lungs. In the latter instance, they doubtless pass with 

 the absorbed fat through the thoracic duct, superior vena cava 

 and right heart to the pulmonary arteries. In man, the most 

 important mode of infection is through inhaling the dust of dry 

 powdered sputum, as a result of which lesions develop in the 

 lungs. Tuberculosis may occur in any tissue of the body, reach- 

 ing it through the blood and lymph. A massive infection of 

 the blood stream often leads to generalized miliary tuberculosis 

 with minute tubercles in all the organs. 



The bacteriological diagnosis of the disease depends upon 

 finding the tubercle bacilli in discharges from the suspected 

 lesion. In sputum an acid-proof bacillus of the proper size and 

 shape is almost invariably a tubercle bacillus and a diagnosis 

 based upon such a finding by an experienced microscopist is 

 justly regarded as very accurate. Inoculation of guinea-pigs 

 will clinch the proof. The latter procedure will also sometimes 

 detect tubercle bacilli when careful microscopic search has failed. 

 In discharges from the intestine or urinary organs one may 



