102 PRINCIPLES OF VETERINARY SURGERY 



and hence they deserve to be classed as strictly parasitic 

 bacteria. This class includes all of the micro-organisms that 

 remain closely associated to the living body of animals and 

 only produce disease when a favorable opportunity presents 

 itself. For example, the staphylococcus pyogenes aureus 

 will live upon the skin without harm until a favorable wound 

 is sustained, under which circumstance it will promptly and 

 quite certainly provoke suppuration and even more grave 

 conditions. The pneumococcus may be harmless in a 

 healthy bronchiole, but is capable of causing fatal pneu- 

 monia if the environment is made suitable by exposure or 

 debility. 



MEASUREMENT OF MICRO-ORGANISMS.— Bac- 

 teria are too minute to be conveniently measured by the or- 

 dinary commercial scale of dimensions. A special unit has 

 been adopted for this purpose. The unit applied to the mea- 

 surement of bacteria is the micro-millimeter or micron, 

 which is the equivalent of one twenty-five thousandth of an 

 inch, or one-thousandth of a millimeter. Its symbol is /a. 



In mental calculations of the size of bacteria during 

 microscopic examinations the red blood cell is largely used 

 as the unit of measurement by practical microscopists. 



Sifr)me bacteria are but a fraction of a micro-millimeter 

 in length, while others reach a length of forty or more. The 

 measurement is obtained by the eyepiece micrometer, the 

 manipulations of which must vary according to the construc- 

 tion of the instrument. The operator should follow the 

 dealer's instructions which accompany such instruments. 



REPRODUCTION.— Bacteria propagate by fission 

 (binary division). That is to say, each bacterium divides 

 into two separate beings, by which process an appalling num- 

 ber may be reproduced in a relatively short time. If the 

 process of fission continues unrestricted the number of bac- 

 teria resulting, even from a single germ, could scarcely be 



