182 PHYSIOLOGY. 



cussed under the head of "Coagulation." They are believed to help 

 maintain the needed proportion of proteids in the blood. 



Their most evident function is the protection of the economy 

 from both harmless and pathogenic bacteria. This they accomplish 

 by two methods. The first is by generating a defensive proteid' 

 which, when imbibed by the bacteria, kills them. The second and 

 more usual method is that of drawing into their interiors the 

 various bacteria, together with the debris resulting from lesions, and 

 digesting them. From this apparent consumption of foreign par- 

 ticles they have gained for themselves the name of phagocytes, and 

 the act is known as phagocytosis. The seat of the presence of the 

 bacteria marks a miniature battlefield, with the hosts of bacteria 

 drawn up on one side in battle array against the leucocytes, the two 

 armies to become engaged in a death-struggle. If the leucocytes, 

 now termed phagocytes, are victorious, they not only kill their 

 adversaries, but even remove every vestige of the combat, aided by 

 the fixed connective-tissue cells. Those leucocytes which come out 

 of the affray unharmed and are no longer needed, find their way 

 back into the blood-stream. 



If, however, the bacteria, with their toxic secretions and excre- 

 tions, are too powerful for the phagocytes, the latter succumb, to 

 become pus-corpuscles. When the pus has been removed by drain- 

 age and the action of other leucocytes, the broken-down tissues are 

 replaced by regenerating connective tissues. 



Bacteria alone are not the provocation for attack by the phago- 

 cytes, for the presence of other foreign matters will also call out 

 an assault. It is well known that surgical ligatures of gut and silk 

 that are allowed to remain within the body-cavity and tissues are 

 gradually removed, particle by particle, by the phagocytic action of 

 the leucocytes. 



The absorption of the tails of tadpoles and other batrachians 

 is due to phagocytic action. 



Diapedesis. By reason of their locomotive tendencies the leu- 

 cocytes and red corpuscles are able to make their way through the 

 walls of the capillaries ; this emigration has been styled diapedesis. 

 There are several stages before the leucocyte finally makes its exit, 

 namely: slowing of the current with the adherence of the cell to 

 the side of the blood-vessel, and projection of processes, to be fol- 

 lowed by the gradual exit of the entire leucocyte. This process 

 occurs to some extent in health, but is greatly exaggerated by 

 inflammation, presence of bacteria, etc. Circumscribed collections 



