GENERAL SKETCH 65 



tococcus is the most numerous of the micro-organisms and the probable cause of the 

 disease, for he was able to isolate the bacillus and to produce dysentery in cats by 

 injecting them with the culture obtained from it. Thus, as the matter stands, 

 Petridis's results, the most positive that have yet appeared, together with growing 

 evidence from the bacteriological side, make it exceedingly probable, although not 

 definitely established as yet, that bacilli and not Am&ba coli are the cause of this 

 disease. 



While the majority of intercellular parasites are harmless, it is 

 quite different with the intracellular forms. These, by making their 

 way into the interior of the cell and growing at the expense of the 

 cell-contents, .gradually cause degeneration of the tissues which may 

 end in death of the host. These parasites belong almost exclusively 

 to the class Sporozoa of which the Coccidiida and Haemosporidiida are 

 found in vertebrates, while the Gregarinida are confined to the inverte- 

 brates, where they are widely distributed. 



The Coccidiida are found in nearly all of the tissues of the lower 

 vertebrates although rarely in man, unless indeed, as many observ- 

 ers believe, they are the cause of various tumors and cancers. That 

 there is some reason for this belief is shown by the fact that in the 

 lower vertebrates, especially in fishes, the presence of Sporozoa leads 

 to ulcers and tumors and to the ultimate death of the fish. The sub- 

 ject, however, as far as man is concerned, is in a very unsatisfactory 

 state, and opinions differ widely as to the nature of certain elements 

 found in cancerous growths. By some observers these are regarded 

 as parasites, by others as disintegrated or pathological cells. Up to 

 the present time no satisfactory evidence has appeared to prove the 

 former view, and until such evidence is forthcoming the entire matter 

 must rest in abeyance. 



From the pathogenic point of view, the most important protozoon 

 is the malaria germ (Plasmodium malaria), a form belonging to the 

 Haemosporidiida. These organisms, in the young stages, move about 

 by amoeboid motion in the blood-vessels of men and birds. They 

 penetrate the red blood corpuscles, which slowly hypertrophy, until in 

 one type of the disease, at least, they attain a size three to four times 

 that of the normal corpuscle, the parasite in the meantime growing at 

 the expense of the haemoglobin and finally reproducing by spore- 

 formation. In this form alone there appears to be a poisonous sub- 

 stance analogous to bacterial ptomaines, which is produced by the 

 organism and periodically discharged (at spore-formation) into the 

 blood, thus causing the pyrexial attacks so characteristic of malaria. 

 The recent successful results obtained by Ross, Manson, Koch, Grassi, 

 and others, in locating the seat of the malaria germ when outside the 

 human body, leads to the hope that some successful means of guard- 

 ing against this disease may soon follow. 1 



1 Vide infra, pp. 160-165. 



