INTRODUCTION. 



GENERAL TECHNIQUE. 



ANIMAL tissues must conform to certain physical 

 conditions before they can be subjected to a satis- 

 factory microscopical examination. Portions of 

 them subjected to study must be sufficiently thin 

 to allow the light to pass readily through them, and 

 transparent enough to permit the determination of 

 the form, character, and relations of their structural 

 elements. At the same time the refractive power 

 of the different elements should not be too nearly 

 alike, since upon differences in this respect, the 

 form and characters which microscopical objects 

 present to the eye are largely dependent ; or, in 

 case they are so, the different elements must be 

 rendered visible by staining them with coloring 

 agents. 



Certain tissues naturally undergo rapid changes 

 of structure after death ; these are to be prevented 

 by the use of preservative agents. Some are too 

 soft to permit the preparation of thin sections, and 

 must be hardened ; others are too hard, and must be 

 softened. In some specimens one, in others another, 



