THE PEESENT EVOLUTION OF MAN — PHYSICAL 211 



from the blood-vessels into the tissues.^ When a solid 

 organ is injured — e. g. the skin by a cut or a bruise — 

 these cells appear in great numbers at the damaged 

 spot, infiltrate the surrounding tissues, proliferate 

 rapidly, adhere together, and undergo gradual differen- 

 tiation, till a greater or lesser mass of dense connective 

 tissue, a scar, is formed and healing is completed. 

 Should the injury be severe or prolonged, more or less 

 of them perish from various causes, — e. g. the poisons 

 secreted by bacteria, — and the clear lymph is 

 rendered turbid by their dead bodies.; that is, pus is 

 formed as in an abscess or on the surface of a sore. 

 Should the blood be injured by the presence of solid im- 

 purities non-living — e. g. artificially introduced particles 

 of vermilion or carbon — or living — e. g. bacteria or pro- 

 tozoa — the leucocytes, called phagocytes from their 

 function, here also perform reparative functions. The 

 impurities, living or non-living, are taken up by them 

 into their substance, and an attempt (successful or unsuc- 

 cessful as the case may be) is made at digestion ; that 

 is, to convert the impurities into food for the cells, or 

 into soluble substances capable of being excreted by 

 such organs as the skin and kidneys. Solid non-living 

 foreign impurities, from the nature of the case, must 

 under normal circumstances be rare in the blood. 

 Living foreign impurities occur in many of the zymotic 

 diseases — e.g. anthrax — when the bacteria producing 

 these diseases may often be seen enclosed in the cell- 

 substance of the phagocytes. 



The pathogenic micro-organisms of some zymotic 

 diseases — e. g. diphtheria and tetanus — do not enter the 

 blood stream, and so pervade the whole body, but 

 remain limited to a definite area of the inner or outer 



1 Other allied but fixed mesoderm cells perform like functions, 

 but it is needless to particularize tbem. 



