TISSUE CHANGES PRODUCED BY BACTERIA. 165 



cells are guided towards the focus of infection by chemiotaxis, 

 and thus we find that different bacteria attract different cells. 

 The most rapid and abundant supplyof phagocytes is seen in 

 the case of suppurative conditions, where the rieutrophile leuco- 

 cytes of the blood are chiefly concerned. When the local lesion 

 is of some extent there is usually an increase of these cells in 

 the blood — a neutrophile leucocytosis. And further, recent 

 observations have shown that associated with this there is in the 

 bone-marrow an increased number of the mother-cells of these 

 leucocytes — the neutrophile myelocytes. The passage of the 

 neutrophile leucocytes from the marrow into the blood, with the 

 resulting leucocytosis, is also apparently due to the absorbed 

 bacterial toxins acting chemiotactically on the marrow. These 

 facts abundantly show that the means of defence is not a mere 

 local mechanism, but that increased proliferative activity in 

 distant tissues is called into play. In addition to direct phagocy- 

 tosis by these leucocytes, there is now abundant evidence that an 

 important function is the production in the body of bactericidal 

 and other antagonistic substances. In other cases the cells 

 chiefly involved are the mononuclear hyaline leucocytes, and 

 with them the endothelial cells, e.g. of serous membranes, often 

 play an important part in the defence; this is well seen in 

 typhoid fever, where the specific bacillus appears to have little 

 or no action on the neutrophile leucocytes. In other cases, 

 again, the reaction is chiefly on the part of the connective 

 cells, though their proliferation is always associated with some 

 variety of leucocytic iniiltration and usually also with the forma- 

 tion of new blood-vessels. Such a connective-tissue reaction 

 occurs especially in slow infections or in the later stages of an 

 acute infection. The tissue changes resulting from cellular 

 activity in the presence of bacterial invasion are naturally very 

 varied — of this examples will be found in subsequent chapters ; 

 but they may be said to be manifestations of the two funda- 

 mental processes of («) increased functional activity — movement, 

 phagocytosis, secretion, etc. — and (3) increased formative activity 

 — cell growth and division. The exudation from the blood- 

 vessels has been variously interpreted. There is no doubt that 

 the exudate has bactericidal properties and also acts as a diluting 

 agent, but it must still be held as uncertain whether the process 

 of exudation ought to be regarded as primarily defensive or as 



