BACTERIA. 40A 



it lives in the intestines of those attacked by the disease, and 

 gives off a strong poison which enters the body. It is easily 

 cultivated as a saprophyte. 



A great many circumstances seem to show that a number of 

 other infectious diseases (syphilis, small-pox, scarlet-fever, measles, 

 yellow-fever, etc.) owe their origen to parasitic Bacteria, but this 

 has not been proved with certainty in all cases. 



It has been possible by means of special cultivations (ample 

 supply of oxygen, high temperature, antiseptic materials) to pro- 

 duce from the parasitic Bacteria described above (e.gr. the fowl-cholera 

 and the anthrax Bacteria) physiological varieties which are distinct 

 from those appearing in nature and possess a less degree of 

 "virulence," i.e. produce fever and less dangerous symptoms in 

 those animals which are inoculated with them. The production 

 of such physiological varieties has come to be of great practical 

 importance from the fact that they are used as vaccines, i.e. these 

 harmless species produce in the animals inoculated with them 

 immunity from the malignant infectious Bacteria from which 

 they were derived. This immunity is effected by the change of 

 the products of one or more of the Bacteria, but we do not yet 

 know anything about the way in which they act on the animal 



FIG. 34. a and b The same blood-cell 

 of a Frog : a in the act of engulfing an 

 anthrax-bacillus ; b after an interval of 

 a few minutes when the bacillus has 

 been absorbed. 



organism. The white blood corpuscles, according to the Met- 

 schnikoff, play the part of "Phagocytes" by absorbing and 

 destroying the less virulent Bacteria which have entered the 



