152 PHYSIOLOGY FOR DENTAL STUDENTS. 



man serum two or three weeks after a previous injection, the 

 animal will go into a very profound state of shock. The blood 

 pressure will be lowered, the heart's action weakened, and breath- 

 ing interfered with. This condition is known as anaphylactic 

 shock. The reaction is a general one for proteins and is specific 

 for each protein used. The phenomenon is explained by assum- 

 ing that the first injection, while producing the bodies which we 

 referred to above as precipitins, also produces an excess of a fer- 

 ment which is able to break down the foreign protein very quick- 

 ly when the second injection takes places. The products of the 

 broken protein molecule, as they are produced in the blood, are 

 poisonous to the body and produce the phenomenon above de- 

 scribed. 



Phagocytosis. By far the greater number of pathogenic or- 

 ganisms do not excrete a poisonous toxin into the surrounding 

 medium, but they cause disease by directly attacking the tissues. 

 The diphtheria bacillus does not enter the body, but only ex- 

 cretes a soluble toxin which the body absorbs. When the disease 

 involves the infection of the tissues themselves by a micro-or- 

 ganism, other types of defense than those described above a in- 

 used. This defense depends on the fact that some of the leu- 

 cocytes of the blood and lymph have the ability to ingest and de- 

 stroy foreign bodies which are present in the blood and tissues, 

 in much the same way that the amoeba takes its food. This func- 

 tion of the leucocytes to destroy foreign bodies is known as pha- 

 gocytosis. In the changes which accompany the metamorphosis 

 of certain forms of larva, the leucocytes are the agents which re- 

 move those parts of the body which are no longer of service to 

 the animal. Likewise the leucocytes of the blood can be shown 

 to ingest pathogenic bacteria and to destroy them. There are a 

 number of varieties of white cells in the blood, and up to this time 

 the part played by each is not definitely known. In active in- 

 flammatory processes the polymorphonuclear leucocytes are by 

 far the most numerous. On the other hand, in cases of chronic 

 infection, as in tuberculosis, the number of lymphocytes is in- 

 creased. Some of the forms of white cells do not take an active 

 part in the ingestion of bacteria, and therefore cannot directly 



