192 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 parasities 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 

 formation 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 microphages 

 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 substance fatal to bacteria provided the 

 bacteria are susceptible to them and just as alcoholic fermenta- 

 tion is possible through the action of zymase without the living 

 yeast cell, so extracts of leucocytes would be capable of destroy- 

 ing bacteria. This apparently happens under conditions of 



