56 INFECTION, PHAGOCYTOSIS, OPSONINS 



these factors had not been developed in the evolution of the higher 

 animals the latter would at all times be exposed to the invasion of 

 pathogenic bacteria, and toxin production in an infected animal 

 would go on until death resulted. 



In the first place, pathogenic bacteria cannot, as a rule, gain en- 

 trance into the tissues of the body through a perfect skin or mucous 

 membrane. Animals constantly inhale and ingest pathogenic bac- 

 teria. However, as long as the mucosa of the respiratory and gastro- 

 intestinal tracts and the skin are covered by unbroken layers of 

 healthy epithelium, pathogenic bacteria have generally no chance of 

 penetrating into the tissues. When breaks do occur in the epithe- 

 lium the bacteria, of course, enter the tissues, and to counteract 

 this invasion the body of the higher animals possesses a number of 

 protective agencies which are always at work against the multiplica- 

 tion and the toxic effect of the invading microorganisms. These pro- 

 tective agencies are here considered systematically and in detail. 



PHAGOCYTOSIS. 



Metchnikoff was the originator and is today the strongest and 

 most ardent exponent of the theory of phagocytosis, which owes its 

 firm position today to the untiring efforts of its originator and his 

 pupils. 



Description. The word phagocyte (derived from two Greek roots, 

 means an eating or feeding cell) and the word phagocytosis indicate 

 the act by which small fragments of bacteria are incorporated into 

 a phagocyte and therein subjected to digestion and assimilation. 

 Metchnikoff first observed phagocytosis in low animal organisms of 

 the class tunicata, called actinia. These possess a common body 

 cavity, not yet differentiated into a thoracic and abdominal cavity, 

 known as a celom. If food or small particles of any kind are intro- 

 duced into the celom they float around unchanged until they come in 

 contact with the epithelial cells lining the celom cavity. As soon as 

 this occurs the epithelia send out protoplasmic processes, pseudo- 

 podia, which surround the small particles, until finally they become 

 completely incorporated in the cell protoplasm. If suitable they are 

 then digested; if not, they are expelled. This is the process of phago- 

 cytosis as first observed by Metchnikoff. 



The question then arose, Does anything like this process exist in 

 higher animals? It was quite easy to show this to be the case. If 

 the blood of a goose is defibrinated by whipping it with a bundle 

 of pieces of wood or wire there is obtained a mixture of blood 

 corpuscles and serum, minus the fibrin removed by whipping. This 

 defibrinated blood can be mixed with physiologic salt solution, 

 centrifuged, and the clear supernatant fluid pipetted off. If this is 

 done several times there is obtained what is called in experiments of 



