MALARIAL FEVER. 



of the disease by infected mosquitoes. Abundant additional observa- 

 tions, with confirmatory results, were supplied by Koch, Daniels, Chris- 

 tophers, Stephens, and others. Wherever malaria has been studied the 

 result has been the same. Lastly, we may mention the striking experi- 

 ment carried out by Manson by means of mosquitoes fed on the blood 

 of patients in Italy suffering from mild tertian fever. The insects, after 

 being thus fed, were taken to London and allowed to bite the human 

 subject, Hanson's son, Dr. P. Thurburn Manson, offering himself for 

 the purpose. The result was that infection occurred; the parasites 

 appeared in the blood, and were associated with an attack of tertian 

 fever. Ross's discovery has not only been a means of elucidating the 

 mode of infection, but, as will be shown below, has also supplied the 

 means of successfully combating the disease. 



From the zoological point of view the mosquito is regarded as the 

 definitive host of the parasite, the human subject as the intermediate 

 host. But in describing the life history, it will be convenient to con- 

 sider, first, the cycle in the human body, and, secondly, that in the 

 mosquito. 



The Cycle in the Human Subject. With regard to this cycle, it 

 may be stated that the parasite is conveyed by the bite of the mosquito 

 in the form of a small filamentous blast or exotospore, which gives rise 

 to the small amoeboid organism or ameebula seen in the human blood. 

 There is then a regularly repeated asexual cycle of the parasite in the 

 blood, which cycle determines the type of the fever, e.g. the cycle of 

 the tertian parasite is completed in forty-eight hours, although a double 

 infection with this parasite may produce a quotidian type. During this 

 cycle there is a growth of the amcebulae within the red corpuscles up to 

 their complete development and sporulation. The onset of the febrile 

 attack corresponds with the stage of sporulation and the -setting free of 

 the spores, or youngest forms, i.e. with the production of a fresh brood 

 of parasites. These spores soon become attached to, and penetrate 

 into the interior of, the red corpuscles, becoming intra-corpuscular 

 amoebulae ; the cycle is thus completed. The parasites are most 

 numerous in the blood during the development of the pyrexia, and, 

 farther, they are also much more abundant in the internal organs than 

 in the peripheral blood ; in the malignant type, for example, the process 

 of sporulation is practically confined to the former. 



In addition to these forms, which are part of the ordinary cycle, 

 there are derived from the amoebulae other forms, which are called 

 gametes, or sexual cells. These gametes remain unaltered during suc- 

 cessive attacks of pyrexia, and undergo no further change until the 

 blood is removed from the human body. In the simple tertian and 

 quartan fevers (vide infra] the gametes closely resemble in structure 



