40 g APPENDIX. 



3. Staining of the tissue in toto. 



4. Embedding in paraffin. 



5. Sectioning. 



6. Fixing sections to the slide. 



7. Mounting. 



8. Finishing, labelling, and storing. 



i. Fixation of the Tissue. By "fixation" is understood the 

 killing of the tissue so rapidly that its elements are retained exactly 

 as they were while living when first met by the fixing reagent ; thus, 

 for example, extended cells should remain extended after death, or 

 rapidly-effected changes, as those of karyokinesis, should be retained 

 in the stage in which first encountered, and not be allowed to com- 

 plete their cycle, and consequently disappear, as when the tissue 

 slowly dies. It is evident that absolutely fresh and, for many investi- 

 gations, still living tissues are essential for satisfactory results where 

 the condition of the cells is a matter of importance, as in the study 

 of the structure of the nucleus or of the protoplasm. 



While so evidently desirable, the fulfilment of this condition in the 

 case of human tissues is often a matter of impossibility, or, at best, 

 of extreme difficulty, the restrictions imposed upon immediate autop- 

 sies rendering it usually almost impossible to secure the more delicate 

 tissues while their cells are still alive. Fortunately, however, for the 

 majority of investigations, the exact condition of the cell is a matter 

 of less moment than its general form and its relations to the surround- 

 ing elements ; for such purposes the slow death of the cells may 

 work no serious detriment to the usefulness of the tissue, but it is to 

 be accepted as a histological maxim, that the fresher the tissue and 

 the more accurate the fixation of its elements, the more valuable and 

 satisfactory will be the preparation. 



When, then, really fresh material is to be prepared for subsequent 

 histological examination, it is to be subjected, without previous wash- 

 ing in water, first to the action of some fixation fiuid ; the choice 

 of the reagent to be employed must be determined by the purposes 

 in view and the character of the tissue. 



a. Muller's Fluid. 



Potassium bichromate 2.5 gm. 



Sodium sulphate i.ogm. 



Water 100 c.c. 



This fluid, when properly employed, is probably the most generally 

 useful fixation reagent ; for successful results, however, strict attention 

 to the manner of its employment is imperative. The quantity of 

 fluid must always be largely in excess of the volume of the tissue 



