MICROSCOPIC EXAMINATION 125 



obtained when the tissues are fixed and imbedded in paraffin 

 and cut in thin sections. 



From the properly selected areas small portions, not 

 more than 5 to 10 mm. in thickness, are cut and placed in 

 absolute alcohol for 8 to 10 hours if less than one-eighth inch 

 thick, and longer if thicker. For the larger pieces, it is better 

 to change the alcohol after twenty-four hours. The pieces of 

 tissue should be kept from falling to the bottom, as the higher 

 layers of alcohol remain nearer absolute. If along with the 

 bacteria one wishes to study the finer structure of the tissue 

 he should employ formalin or corrosive sublimate. The tissue 

 is put in formalin 4 to 10 per cent solution for from three to 

 twenty-four hours, and then in alcohol. Corrosive sublimate 

 as a saturated solution in 0.75 per cent sodium chloride 

 solution is also an excellent fixative agent. Dissolve the 

 sublimate in the salt solution by heat and then allow it to 

 cool, when the separation of crystals will show that saturation 

 is complete. Many prefer Zenker's * solution to the saturated 

 corrosive sublimate as a fixative. The tissues should then be 

 placed in pieces of gauze or copper wire baskets, 1 and left in 

 running water for from twelve to twenty-four hours, accord- 

 ing to the size of the pieces, to wash out the excess of sub- 

 limate. They are then placed for twenty-four hours in each 

 of the following strengths of alcohol, 67, 82, and 95 per cent, 

 and absolute alcohol for a few hours. The latter is not a 

 necessity. They are then ready to be prepared for cutting 

 according to the usual histological methods. The paraffin sec- 

 tions of tissue having been prepared and cut, they are ready 



* Zenker's Fluid. Bichromate of potassium, 2.5 grams; 



Sulphate of sodium, 1.0 gram; 



Corrosive sublimate, 5.0 grams; 



Glacial acetic acid, 5.0 cc. 



Water, 100.0 cc. 



Dissolve the corrosive sublimate and the bichromate of potassium in 

 the water with the aid of heat. 



1 See Gage, "The Microscope," 1908, p. 286, for washing appa- 

 ratus for tissues. 



