528 Introduction to the Study of Science 



way to the salivary glands of the mosquito or to other parts of 

 the body. 



The complete development of the parasite in the mosquito 

 requires from ten to fourteen days. Only after the development 

 is complete does the mosquito become infective. Then when it 

 bites an individual, it pours into the wound a quantity of saliva 

 containing a number of the sporozoites, which at once begin 

 their life cycle unless they are destroyed by the protective 

 phagocytes of the blood plasm. 



A mosquito does not expel all of the sporozoites from the 

 salivary glands at once, but may infect several individuals 

 over a period of many days or weeks. All the parasites do not 

 escape through the salivary glands, some working their way into 

 the ovaries and entering the eggs developing there. The young 

 mosquitoes hatched from such eggs may introduce the malarial 

 parasites into human circulation just as effectively as if they had 

 been directly infected. This fact is important in understanding 

 an outbreak of malarial fever that cannot be explained otherwise. 



If the adult parasites are not taken from the human system by 

 a mosquito, they perish, except in a few cases where the female 

 forms survive in man for years. Then with no apparent means 

 of being fertilized, they begin to multiply by division. Thus 

 it happens that one who has recovered from malaria may have 

 after several years a recurrence of the disease, and without a 

 new inoculation by a mosquito. 



257. Practical tests of the theory. The popular mind is 

 usually slow to accept the results of science, and even medical 

 practitioners may reject the conclusions of scientific investigators 

 in their own field. So it was with the work of Ross, and with 

 that of others, as Pasteur, Koch, and Finlay. To secure the 

 popular recognition of truths practical tests and demonstrations 

 must be made. Even to-day, in spite of the work of science, 

 the superstitious idea persists, as it did after the results of Ross's 

 work had been announced, that malaria is caused by marsh or 

 swamp air (mat aria meaning bad air). 



