694 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



the infection, niay become chronic, and in such a case the inflamma- 

 tion after passing through the acute stage, as indicated above, stimu- 

 lates a proliferation of the connective tissue in the part infected. 

 In such cases, around the outside of the ring of leucocytes, which 

 have been unable to ingest the bacteria, young embryonic connective- 

 tissue cells which are known as round cells are found. In case the 

 inflammation progresses, the leucdcytes are destroyed and the round 

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

 are known as epithelioid cells: On the outside of this layer of epi- 

 thelioid 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. Frequently the newly produced 

 connective tissue passes on to the adult type and in this instance 

 completely walls off the area of infection and the infecting micro- 

 organisms. In such cases the inflammation and the infection are 

 checked. Among the diseases caused by microorganisms which have a 

 tendency to produce chronic inflammation may be mentioned those of 

 tuberculosis, leprosy, syphilis, actinoniycosis and glanders. It is not 

 an uncommon observation in man 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 ninety-five per cent of all individuals Uving in 

 civihzed communities are infected with Bact. tuberculosis some time 

 during their lives. The inflammation produced by this microorganism 

 passes through the acute stage and into the chronic before being suc- 

 cessfully combated and thoroughly waUed off. Such an area is known 

 as a tubercle, and in the other diseases mentioned, similar .areas of like 

 structure 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 

 animal cells. The question arises as to whether these animals are 

 immune to the toxins on account of the presence in their bodies of 

 natural antitoxins or other substances. If antitoxin is present, it 

 can be detected by experiments made by drawing off the blood serum 

 of the animal and combining it in varying proportions with the toxin in 



