196 PARASITISM 



or indirect destruction of organ cells by which normal functions 

 are carried on. The result is disorder in the physiological 

 balance of the human organism, leading to morbid symptoms, 

 and, if uncontrolled, to death. 



Moreover, just as these parasites have become adapted to a 

 new mode of life in the host organism, so the host organism has 

 become physiologically adapted to resist them. These adapta- 

 tions are (i) physical, through the activity of white blood cells or 

 leucocytes (phagocytes), and (2) chemical, through the forma- 

 tion of chemical bodies which counteract the poisons created by 

 the parasites. 



(i) Phagocytosis. If we inject a bit of capsicum into the skin 

 of a salamander or other amphibian, the result is a collection of 

 blood, or inflammation, in the vicinity. If the experiment is 

 made on the web of the foot, and the foot fixed under the micro- 

 scope, the course of the blood in the veins and in the capillaries 

 can be easily watched. From time to time white or colorless 

 cells come along, hesitate in the blood flow, stop and then 

 begin to work through the walls of the capillary. They pass 

 through this wall and into the surrounding fluids, the process 

 of migration being known as diapedesis. Thus by amoeboid 

 motion they move toward the seat of irritation, and if yeast 

 cells or powdered carmine be injected in the skin, the white cells 

 can be observed to engulf them exactly as an amoeba takes in 

 food. These white cells are the phagocytes of the blood mi- 

 crophages and macrophages and their function is to surround 

 and engulf any foreign bodies or irritating substances in the 

 organism. This function is phagocytosis. 



In a similar manner, the phagocytes may attack and engulf 

 bacteria or other harmful foreign objects. Once engulfed, they 

 are digested by intracellular digestion in the same way that 

 amoeba engulfs and digests living food. The phagocytes, there- 

 fore, contain some digestive substance fatal to bacteria, pro- 

 vided the bacteria are engulfed by them, and just as alcoholic 

 fermentation is possible through the action of zymase without 

 the living yeast cell, so extracts of phagocytes would be capable 

 of destroying bacteria. This apparently happens under con- 



