INTRODUCTION. 3 



observation, and even in an indifferent fluid, tissues 

 sooner or later undergo considerable alterations, so 

 that they cannot be permanently preserved. A 

 still more important deficiency in this method is the 

 lack of distinctness in structural details which it in- 

 volves. Most of the fresh animal tissues are nearly 

 transparent in thin pieces, and their structural ele- 

 ments have so nearly the same refractive power, 

 that we see through them, but do not see them ; or, 

 if we do see them, it is not with that definiteness 

 which our purposes demand. Now these difficulties 

 are usually met by the use of agents which harden 

 and preserve the tissues and at the same time ren- 

 der the details of their structure visible, by changing 

 the refractive power of one or other of their ele- 

 ments ; or, we employ certain coloring agents, 

 which, being taken up with different .degrees of 

 avidity by different parts, assist in the recognition 

 of details by differences in color ; or, such agents are 

 used as. both harden and stain at once ; or, finally, 

 which is the most common method, we employ two 

 or more of the different classes of agents, one after 

 the other. We shall consider here only some of the 

 most common and useful of these agents 



HARDENING AND PRESERVATIVE AGENTS. 



Alcohol is one of the most valuable of these. It 

 causes a considerable shrinkage of most tissues, partly 

 by the withdrawal of water from them, and, like 



