THE CELLULAR MECHANISMS OF DEFENCE 1149 



of injury with the objects of removing injured tissue, of destroying 

 micro-organisms, of protecting the body from general infection, and 

 of preparing the way for reintegration of tissue. 



Prior to the work of MetchnikofE, the changes in the blood-vessels 

 fettered the attention of physiologists, and the accumulation of 

 leucocytes was regarded as secondary to these changes. Though the 

 alteration of the capillary wall, by permitting the adhesion of the 

 leucocytes, must no doubt favour their emigration and their passage 

 from, all parts of the body into the inflamed part, we know that the 

 same accumulation of leucocytes occurs in the entire absence of 

 a vascular system. The movement of the corpuscles towards dead 

 or injured tissue must therefore have some other explanation. We 

 have abundant evidence to show that the essential factor in this 

 aggregation of leucocytes is their chemical sensibility, and that 

 the phenomenon is simply one of chemiotaxis. A capillary glass tube 

 containing a suspension of dead micrococci, or peptone, or broth 

 extracted from dead tissue, if introduced into the anterior chamber 

 of the eye or into the subcutaneous tissue, is found after a short time 

 to be full of leucocytes. We must assume that the chemical products 

 diffusing out of the ends of the capillary tube have acted like the malic 

 acid discharged by the cells forming the female organ, the arche- 

 gonium, of ferns. Just as the latter causes a movement of the anthero- 

 zoids, the male cells, towards the ovule, so the chemical substances 

 diffusing from the capillary tube have occasioned a positive chemio- 

 taxis on the part of the leucocytes. It is worthy of note that the 

 positive chemiotactic influence exerted by any given species of patho- 

 genic bacterium is roughly inversely proportional to its virulence. 

 A culture lacking in virulence may cause a very pronounced aggrega- 

 tion of leucocytes which speedily ingest and destroy the micro- 

 organism, whereas if a culture of a more virulent variety of the same 

 microbe be injected, there may be all the signs of inflammation, 

 swelling, and large effusion of fluid, but the tissues may contain very 

 few leucocytes. Under these circumstances the micro-organism 

 rapidly proliferates and spreads from the seat of the lesion, giving rise 

 finally to general infection. 



So far we have spoken merely of leucocytes or phagocytes, and 

 have not attempted to distinguish between the parts played by the 

 various types of leucocyte which are found in the blood and con- 

 nective tissues. In the higher animals there are, however, very many 

 varieties of leucocytes belonging partly to the blood, partly to the con- 

 nective tissues. The following Table, modified from Adami, enu- 

 merates the leucocytes which may be concerned with inflammation in 

 a mammal or man : 



