16 MICROSCOPY. 



CHAPTER II. 



MICROSCOPIC TECHNIQUE. 

 I. THE HISTORY OF THE SLIDE. 



The history of a slide from the crude tissue to the finished mount 

 includes the following processes : Fixing, hardening, infiltrating, em- 

 bedding, sectioning, fixation, staining, dehydrating, clearing, 

 mounting, labeling. 



1. Fixing. This process consists in so killing the cells as to pre- 

 serve them in their natural form and structure. The reagents com- 

 monly used for this purpose are alcohol, corrosive sublimate, chromic 

 acid, Perenyi's fluid, etc. To fix tissues in absolute alcohol, they 

 should be allowed to remain from one to six hours, according to the 

 character of the specimen. Objects are left in Perenyi's fluid from 

 three to twelve hours and then transferred to seventy per cent al- 

 cohol. Specimens hardened in corrosive sublimate solution should 

 be removed in one to three hours, according to size. 

 To fix tissue in chromic acid requires from a few days 

 to a few weeks. After fixing in the last reagent, the tissue should 

 be thoroughly washed with water and then run through increasing 

 strengths of alcohol in the dark. Picro-sulphuric Acid, Erlicki's 

 Fluid, Muller's Fluid, and Flemming's Solution are also commonly 

 used for this purpose. 



2. Hardening. This consists in dehydrating the tissues so as to 

 make them rigid and suited for sectioning with the microtome. For 

 this purpose alcohol is commonly used. The tissue should be run 

 through increasing strengths seventy per cent, eighty per cent, 

 ninety per cent, ninety-five per cent, and absolute alcohol. Objects 

 should not be allowed to remain long in absolute alcohol (from one 

 to five hours), as this renders the tissue brittle and causes it to crum- 

 ble under the knife of the microtome. The objects may remain in 

 the other strengths of alcohol about twenty-four hours for each. 



3. Infiltrating. This process consists in removing the hardening 

 agent and filling up the pores of the tissue with an embedding me- 



