HYGIENE OF RESPIRATION 183 



(Fig. 156). This explains why so many germs are neces- 

 sary to transmit a disease. 



Second. The plasma of the blood and lymph, and even 

 mucus, are able in some unknown way to destroy germs of 

 disease. 



Third. The serum of the blood often develops a sub- 

 stance called an antitoxin, 

 which poisons the germs 

 and stops their action. 

 When the disease has pro- 

 gressed for a time, the anti- 

 toxin is formed in sufficient 

 quantities to kill the germs 

 and the disease is cured. 



a, germ destroyed by white blood cell; o, cell 

 317. Malarial fever arises destroyed by germs and the germs multiply- 



f rom the presence of a one- ing ' using the ceU for food 

 celled animal similar to the ameba, that lives through one 

 phase of its existence in the body of a certain kind of 

 mosquito (Fig. 157), and reaches the human system by the 

 mosquito's bite. It then multiplies and destroys the red 

 corpuscles, causing the pale skin of a person suffering 

 from malarial fever. It has not been proved that 

 malarial fever comes in no other way. It may be possible 

 that this fever is caused by the vapors that arise from 

 marshy places. It increases in any region where the trees 

 are felled, as their destruction interferes with the drainage, 

 and thus gives rise to ponds and marshy places where 

 mosquitoes breed. The wiggle-tail takes about ten days to 

 become a mosquito. If kerosene is poured upon the water 

 at shorter intervals than ten days, no mosquitoes will hatch. 

 Tin cans, barrels, and hollow stumps containing water, 

 furnish breeding places for mosquitoes (Fig. 157). 



318. The negroes in the Southern states seem to have 

 developed in the blood an antitoxin, which enables them to 

 live in " bottom lands " along rivers. In Africa they do 



