THE TUBERCLE BACILLUS 191 



the same time numbers of leukocytes begin to appear in the neigh- 

 borhood. When living bacilli are present and sufficiently viru- 

 lent to multiply the lesion increases, the central cells degenerate 

 into a cheese-like mass, and later a cavity results. 



The most characteristic lesions caused by the tubercle bacilli 

 are the so-called miliary tubercles which, before they undergo 

 degeneration, appear as hard, gray, translucent nodules rather 

 smaller than a millet seed in size. Instead of the miliary tuber- 

 cles, however, a diffuse growth of tissue may occur similar in struc- 

 ture to the former and which also tends to undergo cheesy degen- 

 eration. 



The general symptoms of tuberculosis, fever, perspiration, and 

 emaciation are due to the absorption and distribution throughout 

 the body of the bacterial poison. 



Modes of Infection. Occasionally the organisms attack the 

 abraded skin or mucous membranes and lupus develops or a 

 nodular growth. Their main entrance to the body, however, is 

 through the respiratory tract or the digestive tract. 



The organisms leave the body chiefly in the sputum of open 

 cases of pulmonary tuberculosis and in other cases in any dis- 

 charges from tuberculous lesions opening into the skin. In pul- 

 monary cases the sputum is often swallowed, with the result that 

 tubercle bacilli may be excreted in the feces. Thus all of the dis- 

 charges from the body may be infective. 



Because pulmonary tuberculosis is of far more frequent occur- 

 rence than any other form, tuberculosis was for a long period 

 considered as an air-borne infection. The opinion was strongly 

 expressed by Koch in 1884 and was for many years practically 

 universally accepted. An interesting point in support of this 

 theory is that it requires very few organisms by inhalation to give 

 rise to the disease, whereas thousands are necessary by mouth to 

 produce infection of the alimentary canal. On the other hand, 

 the lungs are greatly protected from external infection both by 

 their location and by the moist ciliated epithelium lining the nasal 

 and pharyngeal passage. Also the fact that the lesion in the 

 lungs is usually at the apex and not in the direct line that floating 



