CHAPTER XV 

 THE ARMY FOR HOME DEFENCE 



" Whatever uncertainty and variety may appear in the world, we remark, 

 nevertheless, a certain secret concatenation and regular order at all times carried 

 on by Providence, which causes everything to proceed in its course, and to follow 

 the law of its destiny." LA ROCHEFOUCAULD. 



THERE are certain cells and certain cell-communities whose 

 function it is to guard the organism from the invasion of its cells 

 by noxious substances and by predatory parasites. 



1. Cells. The leucocytes or white cells of the blood are amoeboid 

 in character and act as scavengers and dust cart combined. 

 They absorb, kill and digest micro-organisms which have invaded 

 the organism. They also ingest and digest effete tissues, such 

 as the tail of the tadpole when the frog stage is reached. This 

 is termed phagocytosis. Certain drugs, e.g. iodoform, are supposed 

 to increase the amoeboid movements of leucocytes and so increase 

 their phagocytic power. This takes for granted that the bacteria, 

 etc., are enclosed by the pseudopodial processes of the leucocyte. 

 In the blood vessels it is very rare for leucocytes to be seen with 

 extruded processes. They are found as spherical cells, and 

 Ledingham has shown that when a leucocyte collides with a 

 bacterium, ingestion, death, and digestion of the bacterium 

 follows as a matter of course. They act just as we saw that 

 amoeba acted, and phagocytosis is just the same as any amoeboid 

 organism feeding. There seems no doubt but that they are 

 subject to chemiotaxis like other amoebae chemical lowering 

 of surface tension applied unilaterally. Exposure to an electric 

 field causes the leucocytes to move towards the cathode. Certain 

 substances, so called opsonins, cause leucocytes to take up bacilli 

 more readily. This is due, as Ledingham has pointed out, to the 

 agglutination of the bacteria so that more are ingested at each 

 encounter. The clumping of the bacteria by an opsonin follows 

 the law of colloidal adsorption. It was once thought that the 



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