THE METHODS OF MOUNTING MICROBES 95 



the infectious matter is mixed with the animal's 

 food; (c) the infectious matter is introduced into 

 the animal body by cutaneous or subcutaneous in- 

 oculation or injection; (d) by the fourth method 

 mentioned above (i.e. special operations), the infec- 

 tious matter may be injected into the duodenum, or 

 introduced into 'the peritoneal cavity by the per- 

 formance of abdominal section/ These and other 

 operations are used as means of introducing micro- 

 bian matter into the living animal. But it cannot 

 be too firmly impressed upon the mind that all 

 operations should be performed with antiseptic pre- 

 cautions ; and the instruments, as well as the hands 

 of the operator, should be thoroughly disinfected. 



Before closing the present chapter we give a few 

 remarks on what is known as the unit of micro- 

 scopical measurement. It has been the general prac- 

 tice among bacteriologists to give the dimensions of 

 microbes in terms of a thousandth part of a milli- 

 metre, which is called a micro-millimetre, and is 

 known by the symbol fi. 1 This unit is of great 

 importance, for ' it is always easier to conceive the 

 size of any object, and especially to realise the com- 

 parative sizes of two objects, when their dimensions 

 are given in terms of a unit smaller than either; 

 for instance, it is difficult exactly to comprehend 

 the length represented by -^ of an inch, and few 

 people can readily compare such dimensions as T V 

 and 75^ of an inch. This difficulty vanishes when 

 the dimensions are expressed as multiples of a small, 

 properly chosen unit, and not as fractions of a large 



1 1 /u=0'001 mm.= T j r ^ nr in., or 0*0000393 in. 



