NONSPECIFIC INFLAMMATION 319 



portion. The pyogenic membrane previously described is 

 formed and the bacteria are prevented from spreading from 

 the initial source of infection. The portion of the tissue 

 killed by the bacteria gradually softens and is discharged 

 through the surface, that is, the boil is said to "come to a 

 head. ' ' The purulent discharge which follows is a mixture 

 of white blood cells, serum, bacteria and disintegrated 

 tissue. When the bacteria have been cleansed from the 



t 





4 41 

 \- 



Fig. 63. — Staphylococcus aureus. 



injured area, cicatricial tissue or scar tissue grows into the 

 wound, replacing the tissue destroyed by the microorganism. 



A somewhat similar condition, frequently becoming 

 chronic, is the fistula on the withers or shoulders of a horse. 

 The organisms have gained access through the skin as a 

 result of a bruise or injury, grow between the muscles and 

 layers of fascia, producing severe inflammation. In chronic 

 fistula the area is more or less surrounded by a growth of 

 scar tissue. Poll-evil is a somewhat similar affection on the 

 top of the poll or head of the horse. 



Immunity. — As in infection with Streptococcus, immu- 

 nity, when developed, is probably due to the presence of 

 sufficient quantities of opsonin. The popular conception 

 that boils are beneficial because they drain impurities from 

 the blood is fallacious. A person is subject to boils primarily 



