GENERAL PRINCIPLES 169 



out with another by drawing it slowly along the first, sloped 

 to it at an angle of about 45. The slides used must be 

 clean, and are usually stored in absolute alcohol, which 

 is burnt off just when using. When the film is stained, 

 it can be examined at once, with a drop of cedar oil, and 

 afterwards mounted, if desired. 



Fixation is accomplished by methyl alcohol after air- 

 drying, when using the Romanowsky method. For other 

 processes it is attained by one of the following methods : 



(a). Half an hour in a hot-air chamber at 120 C. 



(b). Half an hour in a mixture of alcohol and ether 

 (equal parts). Wash, and dry. 



(c). Five minutes in formol-alcohol (19). Wash, and 

 dry (Gulland). 



(d). Two to three minutes in saturated solution of 

 corrosive sublimate. Wash well, and dry. 



For wet films, which give the histology better, the methods 

 are varied. The films, while still wet, are placed film 

 downwards in the fixative. Gulland's combination of (b) 

 and (d) is said to be an excellent one. 



INOCULATION OF ANIMALS. 



Inoculation of animals, or the animal experiment, as it 

 is called, is used for a variety of purposes, is made in a 

 variety of ways, and the number of different animals used 

 is now considerable. It is an important way of getting a 

 pure culture in difficult cases. It also determines 

 the pathogenicity of a pure culture injected. By the 

 occurrence of special symptoms following injection of 

 suspected material, it serves to establish the presence of a 

 particular micro-organism in the material. If the injection 

 of known products of certain organisms is followed by 

 certain reactions, the presence in the animal's body of 

 the organism from which the product has been derived is 

 inferred (tuberculin and maDein tests). By passing a par- 

 ticular organism through a series of susceptible animals 

 in succession, its virulence may be exalted, and the same 

 experiment through resistant animals may depress its 

 vitality ; this is Pasteur's " method of passage." Animal 

 inoculation is also used for the production of anti- 

 toxins and antibacterial bodies. 



