154 



PLANTS WITHOUT CHLOROPHYLL 



-400 



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Tuberculosis is caused by the growth of bacteria, called the 

 tubercle bacilli, within the lungs or other tissues of the human body. 

 Here they form little tubers full of germs, which close up the deli 

 cate air passages in the lungs, while in other tissues they give rise 

 to hip-joint disease, scrofula, lupus, and other diseases, depending 

 on the part of the body they attack. Tuberculosis may be con 

 tracted by taking the bacteria into the throat or lungs or possi- 



bly by eating meat or 



drinking milk from 

 tubercular cattle. Es 

 pecially is it communi 

 cated from a consump 

 tive to a well person by 

 kissing, by drinking or 

 eating from the same 

 cup or plate, using the 

 same towels, or in com 

 ing in direct contact 

 with the person having 

 the germs in his body. 

 Although there are al 

 ways some of the germs 

 in the air of an ordinary 

 city street, and though 

 we may take some of 

 these germs into our 

 bodies at any time, yet 

 the bacteria seem able 

 to gain a foothold only 

 under certain conditions. It is only when the tissues are in a 

 worn-out condition, when we are " run down," as we say, that 

 the parasite may obtain a foothold in the lungs. Even if the 

 disease gets a foothold, it is quite possible to cure it if it is 

 taken in time. The germ of tuberculosis is killed by exposure 

 to bright sunlight and fresh air. Thus the course of the disease 

 may be arrested, and a permanent cure brought about, by 

 a life in the open air, the patient sleeping out of doors, taking 



too 



1850 I860 18TO 1880 1890 19001906 



This curve shows a decreasing death rate from 

 tuberculosis. . Explain. 



