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TEXTBOOK OF BOTANY 



than in fresh air. Plenty of fresh air and sunshine are there- 

 fore very important, and especially so in the house where a 

 tuberculous person lives or has 

 lived. It is in crowded, unventi- 

 lated, and poorly lighted tene- 

 ments that a large proportion of 

 cases of tuberculosis occur, both 

 because conditions there are more 

 favorable for the bacteria, and 

 because persons living under such 

 conditions are less healthy and 

 so less able to throw off the 

 disease. 



37. Diphtheria. This is caused 

 by a bacterium that finds lodg- 

 ment in the throat (Fig. 9, B). 

 Living and multiplying here, the 

 bacterium gives off a toxin which 

 is carried by the blood to other 



parts of the body and there produces the serious symptoms 

 of the disease. The presence of the toxin in the blood of 

 the sufferer causes the blood to form an antitoxin a sub- 

 stance which counteracts the poisonous effects of the toxin. 

 If the antitoxin is formed in large enough quantity, the 

 most serious effects of the disease do not appear ; the 

 patient finally gets rid of the bacteria that are producing 

 the poison and recovers. If the antitoxin is not formed 

 rapidly enough, the disease grows worse and the patient 

 dies. Fortunately it has been found that a horse 'or mule 

 inoculated with diphtheria toxin produces a large amount of 

 antitoxin, and that this antitoxin, taken from the animal 

 and injected under the skin of the human sufferer, has the 

 same effect as the human antitoxin. 



To be effective in curing diphtheria, the antitoxin must 

 be given early in the course of the disease ; so it is most im- 



FIG. 10. Photograph show- 

 ing the growth of bacterial 

 colonies on an agar plate that 

 had been exposed for a short 

 time to the air. The bacteria 

 floating about in the air settled 

 upon the plate and there grew 

 and multiplied. 



