438 LIFE : OUTLINES OF GENERAL BIOLOGY 



acti\ity. But it is obviously difficult to distinguish the members of 

 the normal bodyguard of phagocytes from cells that have arisen 

 through multipHcation in persistently embryonic tissue. 



(4) Phagocytes frequently help in those drastic changes of bodily 

 architecture which are called metamorphoses. Thus they are active, 

 like microscopic sappers and miners, when the tadpole is changing 

 into a small frog. From the dwindling tail, which is in part dissolved 

 and in part disintegrated, they carry useful material forwards 

 where it may be used in reconstruction. 



Metchnikoff distinguished two kinds of phagocytes — (a) the 

 microphagcs, which devour small bodies like bacteria, and (b) 

 the macrophages, which devour dead or dying cells, such as 

 exhausted red blood corpuscles, or transport granules, as we have 

 mentioned in connection with rapid whitening of hairs or feathers. 

 He gives a \avid account of the invasion of the brain of a very 

 long-lived parrot by macrophages which proceeded to do away with 

 exhausted nerve-cells. His general thesis as regards inflammatory 

 processes in the body may be stated in his own words : "Inflammation 

 generally must be regarded as a phagocytic reaction on the part of 

 the organism against irritants — a reaction carried on by the mobile 

 phagocytes." In the strict sense inflammation is not a diseased 

 state; it is the clash and struggle between the organism's body- 

 guard and intruding microbes or irritants. 



THE RETICULO-ENDOTHELIAL SYSTEM.— We have seen 

 then that Metchnikoff disclosed more especially the rdle of the 

 migrant phagocytic (microbe-devouring) cells which serve as one 

 of the chief internal defences of the body. They form a mobile 

 bodyguard not merely brought where needed by the blood stream, 

 but able like amoebae to move independently by pseudopods. But 

 besides this mobilised force there are Metchnikoff's more or less 

 stationary "macrophages", which are definitely located — like 

 garrisons— in various tissues of the body. After long neglect they 

 have come into prominence again and been much studied of recent 

 years, e.g. by Aschoff and his pupils. They are generally referred to 

 as the reticulo- endothelial system. 



They are notably to be found in the spleen, the liver (there called 

 Kupffor's "stellate cells"), and the bone marrow. They also occur 

 as "histiocytes" scattered through the connective tissue, which 

 forms, as it were, the girder system of the body, especially when it 

 is soHdiftcd into cartilage or gristle, or reinforced by the deposit 

 of lime-salts to form bone. These histiocytes appear to be persis- 

 tently embryonic cells, for two types of them give rise to two kinds 

 of phagocytes that occur in the blood of higher Vertebrates. They 

 are also allied to the red blood corpuscles. 



Xow all the cells of the reticulo-endothelial system possess the 



