154 PHYSIOLOGY FOR DENTAL STUDENTS. 



patient who is suffering from the infective agent. A high op- 

 sonic index therefore indicates a relative immunity or high resist- 

 ance to the disease in question. The bactericidal power of the 

 leucocytes for many bacteria can be greatly increased by the in- 

 jection of dead bacteria into the body. This fact is made use of 

 in the manufacture of bacterial vaccines, which consist of sus- 

 pensions of dead bacteria in saline. 



