134 CONTAGION AND CONTAGIOUS DISEASES. 



leucocytes accumulate around the point of entrance, take up 

 the germs into their substance and digest them ; in fact, they 

 kill and eat their enemies. 



Again, some germs are antagonistic to others, and it is now 

 well known that the microbes of putrefaction are destructive to 

 those of anthrax ; but whilst anthracic blood and tissue are thus 

 rendered impotent, it does not follow that they are rendered 

 innocuous, for inoculations with such blood have proved that 

 death may result, not irom anthrax, but from septicaemia ; and 

 again, the ptomaines of pathogenic germs in many instances, 

 when properly prepared, attenuated, and introduced by inocula- 

 tion into the bodies of healthy animals, have the power of 

 giving protection or immunity to such animals against the specific 

 effects of the microbes themselves. A knowledge of this fact, 

 first elaborated by Pasteur, has revolutionised modern medicine, 

 and is likely to confer even greater blessings both upon man and 

 the lower animals. 



Many microbes that require a free supply of oxygen are 

 called aerobic, whilst others seem to be destroyed or fail to grow 

 when oxygen is present ; these are called anaerobic fungi. Some, 

 however, can live and grow with or without oxygen. Microbes 

 are consequently divided into three groups by Liborius : — 



I. OUigatory anaerobes, which flourish in the absence of 

 free oxygen — e.g., Bacillus hutyriciis, and of malignant 

 oedema. 

 II. Facultative anaerobes. — These grow in free oxygen, but 

 continue to live and multiply, but more feebly, in 

 the absence of it — e.g., Koch's comma bacillus. 

 III. Obligatory aerobes, which do not grow in the absence of 

 oxygen — e.g.. Bacillus anthracis and Bacillus subtilis. 



Pleomorphism. — Lister in 1873 (Quarterly Journal of Micro- 

 scopic Science) indicated that several micro-organisms in their 

 life-cycle exhibit successively the forms of cocci, bacteria, bacilli, 

 and streptothrix. This view is now generally adopted ; and 

 Lankester, also in 1873, observed a series of form-phases in the 

 case of a peach-coloured bacterium, which led him to suppose 

 that the natural species of these plants were within the proper 

 limit protean, and that the existence of true species of bacteria 

 must be characterised not by the simple form-features used by 



