150 MALARIA 



and most widely distributed species is Plasmodium vivax, which 

 causes tertian malaria. Of somewhat more limited geographic 

 range, being confined to tropical and subtropical countries, but 

 of infinitely more importance on account of the deadly nature of 

 its attacks, is Plasmodium falciparum, the cause of the sestivo- 

 autumnal type of malaria, also called malignant tertian or subter- 

 tian fever. During the hot part of the year in the tropics 96 per 

 cent of malarial cases are of the sestivo-autumnal type. The third 

 species, Plasmodium malarice, causing quartan malaria, is relatively 

 uncommon, though more frequent in temperate than in tropical 

 countries. These three species of malarial parasites differ from 

 each other in a number of important details of structure and 

 life history and in the diseases which they produce. 



Life History of Plasmodium falciparum; Human Cycle. — 

 The life history of malarial parasites may well be exemplified 

 by that of the malignant sestivo-autumnal parasite, Plasmodium 

 falciparum, as diagrammatically shown on Fig. 43. When first 

 injected into the human blood by a mosquito the animal is 

 exceedingly minute (Fig. 43 A). It immediately enters or at- 

 taches itself to a red blood corpuscle, where it grows until it 

 occupies one-half or two-thirds of the corpuscle, meanwhile un- 

 dergoing a number of different forms. It first goes through a 

 "signet ring" stage (Fig. 43B), the ringlike appearance being 

 due to the presence of a transparent area occupying the middle 

 of the parasite, while the tiny round nucleus occupies a position 

 at one side of the parasite, simulating the setting in a ring. As 

 the parasite grows larger it becomes irregular in shape (Fig. 43C) 

 and quite active, constantly changing its form, thrusting out 

 little clublike processes or pseudopodia, now here and now 

 there. Although it has been taken for granted that malarial 

 parasites penetrate the blood corpuscles and live inside of them 

 recent investigations by Mary R. Lawson (Mrs. Johnson) indi- 

 cate that this may not be the case at all, but that the parasites 

 may attach themselves to the surface of the corpuscles, squeezing 

 up little mounds of the substance of the corpuscles and encircling 

 these mounds with their bodies, just as a bit of skin might be 

 squeezed up between the fingers. Sometimes several parasites 

 attach themselves on top of each other around a single mound. 

 A number of facts give support to Mrs. Johnson's theory: it 

 affords a logical explanation for the ring forms of the parasite; it 



