INFLAMMATION AND PHAGOCYTOSIS 137 



animals, and the sensations of a plasmodium obey like ours 

 the law of Weber. 



Chemiotaxis is a sort of chemical sense. 



The phagocytes have also a sort of tactile sensibility. The 

 leucocytes in their reaction apply themselves to the exciting 

 body over the largest surface possible. 



In defensive phagocytosis, the struggle of the body against 

 the parasitic invaders, it is not necessary to suppose any 

 purposeful cause but simply a function developed by evolution 

 and selection. " Those lower animals in which the motile cells 

 directed themselves towards the enemy, engulfing and destroying 

 them, survived, whereas others in which the phagocytes did not 

 act, were condemned to perish. All the useful characters and 

 among them those which are concerned in the inflammatory 

 reaction have become fixed and transmitted without the inter- 

 vention of any preconceived purpose whatever" (MetchnikorT). 

 Thus in the invertebrates with soft skins in which bacterial 

 invasion occurs easily there has been a selective process at 

 work in the phagocytic apparatus and the defensive measures 

 have become perfected. Among the invertebrates possessing 

 a natural protection, such as a chitinous covering, infection is 

 rarer, but the means of defence have not found suitable 

 conditions for their employment and development, so that the 

 infected organism succumbs. The phagocytic arrangements 

 are much reduced in the Insects, and in these the parasitic 

 fungi have great difficulty in penetrating the cuticle, but if 

 they are successful the insect is destroyed (for example in the 

 beetle Cleonus punctiventris invaded by Isaria destructor}. The 

 nematode worms which are protected by a thick skin do not 

 even possess cells capable of movement. 



The phagocytes of man are both fixed and motile. Among 

 the fixed are the large mononuclears, the Kuppfer cells of the 

 liver, certain endothelial cells of the lung (the dust-cells) and 

 the myeloplaxes of the bone-marrow. The motile phagocytes are 

 the white corpuscles or leucocytes in general (except the small 

 lymphocytes), the polymorphs, the eosinophils, the large 

 mononuclear cells of the blood and of the lymphatic organs. 



