CHEMICAL CEPTORS 103 



their power to digest waste matter in the body they 

 were termed phagocytes by Metchnikoff, who first fully 

 demonstrated their properties and who designated as 

 phagocytosis the whole process by which certain bac- 

 teria, foreign materials and the dead tissues of the 

 animal itself are destroyed. 



The similarity of the process to unicellular digestion 

 may be observed in a sponge or in the larva of the 

 echinoderm. If a foreign substance be introduced 

 into the body cavity of these organisms, cells analo- 

 gous to the leucocytes of higher organisms collect 

 around the invader and prevent its further progress by 

 fusing into a hard plasmodial mass. The protecting 

 cells then adhere to the invaders and gradually ingest 

 and absorb them. This operation continues until every 

 particle of dead matter has been absorbed, after which 

 the protecting cells move away from the seat of injury, 

 and the damage is repaired by normal cell proliferation. 

 This action is in substance that which takes place in 

 the human organism when the body is attacked by a 

 local infection which gains entrance through an exter- 

 nal wound. The live or dead organic substance serves 

 as a stimulus by which the protective activity of the 

 leucocytes is excited, first in the local area, and then, 

 if this be insufficient, by calling out the reserve forces 

 of leucocytes throughout the whole organism. 



Immunity 



For the common daily menaces of pyogenic infec- 

 tions, these phagocytic mechanisms are, in the main, 

 adequate. But there are many infections which can- 

 not be successfully met at the local point of entrance 



