98 MICROSCOPIC METHODS. 



eosin before mixing. Thereafter the bacteria, etc., may be 

 stained with methylene-blue or other stain, as described 

 below. This method has the advantage over the previous 

 that, as a small amount of corrosive sublimate is used, less 

 washing is necessary to remove it from the preparation, and 

 deposits are less liable to occur. 



3. Examination of Bacteria in Tissues. For the examin- 

 ation of bacteria in the tissues, the latter must be fixed and 

 hardened, in preparation for being cut with a microtome. 

 Fixation consists in so treating a tissue that it shall per- 

 manently maintain, as far as possible, the condition it was 

 in when removed from the body. Hardening consists in 

 giving such a fixed tissue sufficient consistence to enable a 

 thin section of it to be cut. A tissue, after being hardened, 

 may be cut in a freezing microtome, but far finer results can 

 be obtained by embedding the tissue in solid paraffin and 

 cutting with some of the more delicate microtomes of 

 which, for pathological purposes, the small Cambridge rocker 

 is by far the best. For bacteriological purposes embedding 

 in celloidin is not advisable, as the celloidin takes on the 

 aniline dyes which are used for staining bacteria, and is apt 

 thus to spoil the preparation, and besides thinner sections 

 can be obtained by the paraffin method. 



The Fixation and Hardening of Tissues. Absolute 

 alcohol may be used for the double purpose of fixing and 

 hardening. If the piece of tissue is not more than ^ inch 

 in thickness it is sufficient to keep it in this reagent for one 

 or two days. If the pieces are thicker a longer exposure is 

 necessary, and in such cases it is better to change the 

 alcohol at the end of the first twenty-four hours, as the first 

 alcohol is diluted by water which comes out of the fluids 

 of the tissue. The tissue must be tough .without being 

 hard, and the necessary consistence, as estimated by feeling 

 with the fingers, can only be judged of after some experience. 

 If the tissues are not to be cut at once, they may be pre- 

 served in 50 per cent spirit. 



Corrosive sublimate is an excellent fixative agent. It is 

 best used as a saturated solution in .75 per cent sodium 



