THE CELLULAR MECHANISMS OF DEFENCE 1075 



mingled with staphylococci, partly within, partly outside the cells. The 

 margin of the abscess is formed by connective tissue infiltrated with living 

 leucocytes. A certain number of cocci are to be seen free in the tissue out- 

 side this layer, but in the course of a day or two these free cocci disappear, 

 and there is thus a continuous layer of phagocytes surrounding the abscess 

 cavity and preventing any further invasion of the body as a whole from the 

 seat of infection. The abscess subsequently discharges on to the exterior 

 by a process of necrosis of the super jacent skin, and regeneration of tissue 

 takes place in the same manner as in the more trivial injury. Inflamma- 

 tion in warm-blooded animals thus gives rise to dilatation of 'vessels and 

 increased vascularity of the part, to alteration of the vessel wall and 

 therefore to increased effusion of fluid. There are warmth and redness of 

 the part from the vascular dilatation, swelling from the effusion of lymph, 

 and very often, as a result of the injury or the swelling and the conse- 

 quent involvement of sensory nerves, pain. The four cardinal symptoms 

 of inflammation, namely, rubor, color, turgor, and dolor, which have 

 been described for generations as typical of this condition, leave out of 

 account altogether the phenomenon which Waller's and Cohnheim's obser- 

 vations, in the light of the comparative studies of Metchnikoff, have shown 

 us to be the essential feature of the process. This is phagocytosis, the 

 accumulation of wandering mesoderm cells round the seat 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 Metchnikoff, 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 aggrega- 

 tion 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 

 archegonium, 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 chemiotaxis on the part 

 of the leucocytes. It is worthy of note that the positive chemiotactic 

 influence exerted by any given species of pathogenic bacterium is roughly 

 inversely proportional to its virulence. A culture lacking in virulence may 



