MALARIA AND ITS TRANSMISSION 19 



evenly spreading the droplet by means of a needle. 

 The film is fixed and stained with a good malarial 

 stain. If parasites are present in the blood, they should 

 be visible after careful microscopical examination as 

 pigmented intracorpuscular bodies, or as crescents 

 in sestivo-autumnal fever of ten or more days' duration. 

 Microscopic examination under an oil-immersion lens 

 is desirable, though crescents can easily be seen under 

 lower powers. 



The ease with which parasites can be discovered in 

 a blood smear depends on several important factors, 

 — first, on the length of time that the patient has had 

 malaria, i.e. the parasites may be so few that they will 

 be hard to find. Ross states that the parasites "will 

 not generally be numerous enough to cause illness 

 unless there is at least one parasite to 100,000 hsema- 

 tids ; that is, 50 parasites in 1 c.mm. of blood ; or 

 150,000,000 in a man 64 kilograms in weight. . . . 

 Such calculations demonstrate the absurdity of sup- 

 posing that there are no plasmodia present in a person 

 because we fail in finding one after a few minutes' 

 search. As a matter of fact, even if as many as 150,- 

 000,000 plasmodia are present in an ayerage man, the 

 chances are that ten to fifteen minutes' search will be 

 required for each plasmodium found ; while if we are 

 careless or unfortunate, we may have to look much 

 longer." 



In the second place, if the patient has recently taken 

 quinine, the chances for the discovery of parasites are 

 virtually reduced to nil. 



