1028 PHYSIOLOGY 



especially in those caused by the tubercle bacillus. They do not ingest 

 bacteria. The histogenous wandering cells appear in the inflammatory 

 area at a later period than the polymorphonuclear and eosinophile cells. 

 They are actively phagocytic and are motile. As a rule their phagocytic 

 properties are exerted, not on bacteria, but on other cells and cell- 

 debris. After an acute inflammation their chief office is to clear away the 

 remains of the polymorphonuclear leucocytes and dead tissues so as to 

 prepare the way for subsequent regeneration. It is possible that these 

 cells may take a part in the formation of new connective tissue. They are 

 indistinguishable from the immature form of connective tissue-cells. It is 

 therefore difficult to be certain whether the wandering and the fixed con- 

 nective-tissue corpuscles are of identical or of different origin. Metchnikoff 

 speaks of these cells as macrophages, to distinguish them from the poly- 

 morphonuclear type, which he terms microphages. 



We thus see that several types of the wandering cells of mesoblastic 

 origin which take part in inflammation do not exert active phagocytic 

 properties and cannot therefore destroy bacteria or other invading organisms 

 by the process of ingestion and digestion. Yet we have evidence that the. 

 part played by such cells in the defence of the organism is no less important 

 than that of the actively phagocytic cells. In the alimentation of the more 

 primitive invertebrata the cells lining the digestive cavity take up the 

 particles of food directly, and the processes of digestion are carried out in 

 vacuoles within the cells themselves. In the higher animals this process of 

 intra- cellular digestion has almost disappeared, and the cells lining the 

 alimentary tract have become differentiated into those which secrete 

 digestive ferments and those which absorb the products of the action of the 

 ferments on the food-stuffs. Digestion has thus become extra-cellular. It 

 *seems that a similar modification has taken place to some extent in the 

 means adopted by the organism for its defence from infection, and that the 

 leucocytes destroy bacteria not only by the process of intracellular digestion 

 but also by the excretion of substances into the surrounding body fluids 

 which have a deleterious influence on bacteria. Thus normal blood- serum 

 is found to have a strong destructive influence on most species of bacteria, 

 whether pathogenic or not. Since this property is not shared to anything 

 like the same extent by the blood-plasma, it may be ascribed to the breaking 

 down of leucocytes in the process of clotting and the consequent liberation 

 of bactericidal substances. Extracts made from any collection of leuco- 

 cytes have a similar bactericidal effect, and it has been shown by Wright 

 that the ingestion of bacteria by normal leucocytes goes on much more 

 rapidly in the presence of blood-serum or if the bacteria have been previously 

 subjected to the action of blood-serum. This adjuvant action of blood- 

 serum on phagocytes is destroyed if the serum be heated to 55 C., so that it 

 must be due to the presence of some chemical substance in the serum which 

 is unstable and destroyed by heat at a temperature far below the coagula- 

 tion point of the serum proteins. Moreover, there are many species of 

 pathogenic bacteria which cannot infect the animal as a whole. These 



