THE RELATION OF BACTERIA TO DISEASE 5o 



matory reactions may occur. Thus we may have a 

 ''dry pleurisy." In this form of inflammation the passage 

 of the defending leukocytes is accompanied by the exuda- 

 tion of large amounts of fibrin, as though nature deliber- 

 ately intended to glue the two opposing pleural surfaces 

 together. Or we may have a "pleurisy with effusion," 

 in which the number of defending leukocytes which pass 

 out of the blood-vessels is relatively small, while the quan- 

 tity of blood-serum which passes out is enormous. One 

 cannot help thinking that this is intended to dilute the 

 irritant which has caused the inflammation. Finally, we 

 meet with cases in which both the number of leukocytes 

 and the amount of serum are large, forming really a thick 

 pus. This form of pleurisy constitutes an "empyema." 



In some instances, as in the case of the staphylo- 

 coccus boil described above, the bacterium itself invades 

 the tissues and constitutes the irritant which gives rise 

 to the inflammation. In other cases the inflammation 

 is produced mainly by poisons given off by the bac- 

 teria. The latter is well seen in diphtheria, where the 

 bacilli remain on the tonsil and the poison (diphtheria 

 toxin) causes inflammatory changes in various parts of the 

 body. 



In case the inflammation is slight, both the serum and 

 the leukocytes which have passed out of the blood-vessels 

 may re-enter the circulation. When the cells are more 

 numerous they may first undergo a kind of digestion 

 and then be absorbed by certain scavenger cells of the 

 body. 



When pus is formed it usually works its way to the sur- 

 face and is discharged either through external openings or 

 into cavities of the body. 



