IMMUNITY AND SUSCEPTIBILITY. 549 



which have been produced upon them. In certain instances they 

 will ingest the bacteria and destroy them. In such cases, the bacteria 

 having been removed, the inflammation rapidly subsides and the infection 

 is, therefore, checked. Such are the characteristics of an acute inflam- 

 mation. Inflammation, however, not infrequently, depending especially 

 upon the microorganism producing the infection, may become chronic, 

 and in. such a case the inflammation, after passing through the acute 

 stage as indicated above, stimulates a proliferation of the connective 

 tissue in the part. In such case, around the outside of the leucocytes, 

 which have been unable to ingest the bacteria, are found young embryonic 

 connective-tissue cells which are known as round cells. In case the 

 inflammation progresses, the leucocytes are destroyed and the round 

 cells next the infected area assume more of an elliptical shape, and are 

 known as epithelioid cells. On the outside of this layer of epithelioid 

 cells will be found newly produced round cells, and on the outside of 

 the round cells an area of recently migrated leucocytes, those passing 

 out in the beginning having been destroyed by the toxic action of the 

 infecting microorganisms. Not uncommonly the newly produced con- 

 nective tissue passes on to the adult type and in this instance completely 

 walls off the area of infection and the infecting microorganisms. When 

 this occurs the inflammation and the infection are cured. Among the 

 diseases caused by microorganisms which have a tendency to produce 

 chronic inflammation may be mentioned tuberculosis, leprosy, syphilis, 

 actinomycosis and glanders. It is not an uncommon observation in 

 the human to note in the lungs and in other parts of the body healed 

 areas of tubercular infection; areas that have been completely walled 

 off by the development of adult fibrous tissue. It is probable that about 

 95 per cent of all individuals living in civilized communities are infected 

 with Bact. tuberculosis some time during their life. The inflammation 

 produced by this microorganism passes through the acute stage and into 

 the chronic before being successfully combated and thoroughly walled 

 off. Such an area is known as a tubercle, and in the other diseases 

 mentioned similar areas are produced. It depends entirely upon 

 the virulence of the infecting microorganisms and the resistance of 

 the connective tissue of the individual infected as to whether healing 

 will result. 



Natural Antitoxins. It is an observed fact that certain animals 

 resist the action of toxins produced by bacterial and other plant and 



