170 IMMUNITY. 



phagocytes containing bacteria are removed from the body the phago- 

 cytes die and some of the contained bacteria demonstrate their re- 

 tention of life by growth and multiplication. It sometimes happens 

 within the body that the bacterium proves too powerful an opponent, 

 and the phagocyte is destroyed. This condition can be brought 

 about by the employment of a highly virulent culture or by placing 

 the animal after inoculation under unfavorable conditions. For in- 

 stance Pasteur and Joubert demonstrated that while the chicken is 

 naturally immune to anthrax, susceptibility may be secured by keep- 

 ing the inoculated animal at a low temperature, under the unfavor- 

 able influence of which normal phagocytic action is not possible. 



It is probable that plasmolysis is also a factor in natural immu- 

 nity. It is a well-known fact that the removal of bacteria from one 

 medium into another containing slightly more or less of certain min- 

 eral constituents may be followed by the death of all or the greater 

 part of the microorganisms. Bacteria consist of cell-waUs with con- 

 tents. Diffusion through the cell-wall is constantly taking place to 

 a greater or less extent. By altering the mineral constituents of the 

 medium, an osmosis, which may prove fatal to the bacterium, results. 

 The removal of the bacterial cell from an isotonic medium to one 

 which is either hypotonic or hypertonic leads to greater or less injury 

 to the cell. 



The subject of natural immunity to toxins is one of quite as much 

 interest as that which we have just been discussing. Instances of 

 insusceptibility to powerfiil poisons have long been known and have 

 formed the subject-matter of much theoretical discussion. For in- 

 stance, hogs can eat with impunity relatively large amounts of arsenic 

 and antimony, and certain insects not only live, but seem to thrive 

 in an atmosphere saturated with the vapor of formaldehyde. Metsch- 

 nikoff states that there are certain invertebrates which are not af- 

 fected by large doses of some of the most important bacterial toxins. 

 The larva of a large beetle (Nashornkafer) has been found to be 

 wholly immune to the cholera toxin. If one of these and a small 

 green frog of equal weight be treated with 0.5 c.c. of soluble cholera 

 toxin, the larval beetle remains apparently unaffected, while the frog 

 speedily dies. On the other hand, if both animals be inoculated with 

 the cholera bacillus, the beetle succumbs to cholera sepsis, while the 

 frog escapes infection. Metschnikoff explains these observed facts 

 on the ground that the leucocytes of the frog consume the vibrios, 

 but are powerless against the toxin, while the leucocytes of the lar- 

 val beetle are unable to cope with the bacteria, but it must be ad- 

 mitted that he does not explain how it is that the beetle succumbs to 

 the cholera toxin elaborated in its own body, while it is insusceptible 

 to that introduced from without. 



It is well known that certain vertebrates are very resistant to the 

 most potent venoms of snakes. This is notably true of the mon- 



