VII. METHODS OF DEMONSTRATING BACTERIA 

 IN TISSUES. 



FOR bacteriological purposes, sections of tissue are 

 most conveniently prepared by either the freezing 

 method or the paraffin method. 



The latter is decidedly preferable, but as it is of 

 greater importance to demonstrate the bacteria, if such 

 are present, than to preserve the tissue elements un- 

 altered, the "frozen" sections are often of value. 



Whichever method is selected, it is necessary to take 

 small pieces of the tissue for sectioning, 2 to 5 mm. 

 cubes when possible, but in any case not exceeding half 

 a centimetre in thickness. Post-mortem material should 

 be secured as soon after the death of the animal as 

 possible. 



The tissue is prepared for cutting by 



(a) Fixation; that is, by causing the death of the 

 cellular elements in such a manner that they retain 

 their characteristic shape and form. 



The fixing fluids in general use are : Absolute alcohol ; 

 corrosive sublimate, saturated aqueous solution; cor- 

 rosive sublimate, Lang's solution (vide page 82) ; 

 formaldehyde, 4 per cent, aqueous solution. (Of 

 these, Lang's corrosive sublimate solution is decidedly 

 the best all-round " fixative.") 



(6) Hardening; that is, by rendering the tissue of 

 sufficient consistency to admit of thin slices or " sec- 

 tions" being cut from it. This is effected by passing 

 the tissue successively through alcohols of gradually 

 increasing strength-: 30 per cent, alcohol, 50 per cent, 

 alcohol, 75 per cent, alcohol, 90 per cent, alcohol, 

 absolute alcohol. 



In both these processes a large excess of fluid should 

 always be used. 



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