282 INFECTION AND RESISTANCE 



vessel by sending out pseudopodia which slowly penetrate it. Adami 

 states that if, at this stage, the tissues be excised, fixed in osmic 

 acid, and stained, leukocytes may be seen crowding the inner sur- 

 face of the vessel in all stages of transition from its anterior to 

 the lymph spaces on the outside. 



In the staphylococcus infection, after from 12 to 48 hours, there 

 will be seen the results of an active and destructive struggle between 

 the invading bacteria and the defending cells. In the center of the 

 area of invasion tissue has been destroyed and disintegrated. Amid 

 the necrotic detritus, closely packed, lie leukocytes and cocci and 

 active phagocytosis has taken place. In some cases the intracellular 

 bacteria appear swollen and disintegrating, in others the leukocyte 

 itself, overcome by the larger number of bacteria it has taken in, 

 becomes vacuolated, indefinite in outline, and apparently is being 

 itself destroyed. The presence of blood serum, which is aiding in 

 the destruction of bacteria both by its bactericidal powers and its 

 reenforcement of the phagocytic process, renders this mass fluid or 

 semi-fluid, and the whole mixture constitutes what is known as pus. 

 Around the periphery cocci and leukocytes become more scattered 

 and sparse, and bacteria, together with leukocytes, loaded with cocci, 

 may be seen lying within large mononuclear cells (macrophages). 

 Whether the process goes on to further extension or is eventually 

 walled off into a distinct abscess by the formation of granulation 

 tissue and new connective tissue depends upon the balance of forces 

 between attacking agent and defensive factors. 



If we inject a similar emulsion of cocci into the pleural or peri- 

 toneal cavity of an animal a process similar in principle may be 

 observed. 



Normally the peritoneum contains a small amount of this serous 

 fluid and a moderate number of white blood cells, chiefly lympho- 

 cytes. When any substance, broth or salt solution, an aleuronat or 

 a bacterial emulsion, is injected into the peritoneal cavity, there 

 follows a brief period during which there is a diminution of the 

 free cellular elements in the peritoneal fluid. At this time there is 

 a clumping of cells in the folds of the omentum and mesentery, a 

 transient stage of flight away from the point of injury. This, how- 

 ever, is soon over. Within one to two hours an active immigration 

 of leukocytes into the serous cavity occurs and if, during the next 

 12 to 24 hours small quantities of fluid are, from time to time, with- 

 drawn with a capillary pipette, a rapid and constant increase of 

 leukocytic elements, chiefly of the microphage or polfnuclear type, 

 is observed. If the injected substance has been a sterile, harmless 

 fluid, a gradual return to normal within 48 hours then ensues. If, 

 however, we have injected bacteria, a struggle similar to the one 

 described above takes place within the peritoneum, and active 

 phagocytosis of the micro-organisms takes place. 



