INTRODUCTION. 7 



morphological elements of the object under examination. Donders* 

 has, however, correctly shown that it is very often necessary 

 to submit the tissue we may be examining to the prolonged action 

 of the chemical reagent an action not merely of hours but of 

 days. We regard both modes of proceeding as absolutely neces- 

 sary for an accurate examination. In association with Messer- 

 schmidt,t I long ago directed attention to the fact, that even in 

 objects which are easily penetrated as, for instance, pus the 

 solution of the chemical reagent only very gradually makes its 

 way into the mass of the object, and acts very unequally on the 

 parts that are differently situated; and consequently that the 

 microscopical appearance may often give occasion to very different 

 interpretations. Hence Donders especially recommends that the 

 tissues should not be submitted to microscopical investigation until 

 they have been exposed for a longer or shorter period to the action 

 of the chemical agent in a somewhat disentangled or carefully 

 prepared state. In this way the final result of the action may be 

 much better observed than by any other means, and the altered 

 parts are seen with perfect clearness. In the meanwhile, although 

 the result may be sufficiently obvious, it may often be far from 

 easy to decide in what manner the change has been brought about, 

 and what parts especially undergo solution, contraction, or gela- 

 tinisation. If we merely observe such preparations without tracing 

 the action of the chemical reagent upon them under the micro- 

 scope, we soon see how readily we may fall into error. Hence in 

 every case it is expedient when examining a preparation, at the 

 same time to observe the direct action of the same reagent on it. 

 In the latter proceeding there are various means by which we may 

 be assisted in accurately observing the direct action of the chemical 

 reagent ; thus, for instance, Henle recommends that a hair should 

 be introduced between the slide on which the object is placed and 

 its cover, in order to regulate the flow of the test- solution, and to 

 retard its action on the preparation, which is often extremely 

 rapid ; linen or cotton threads may often be more conveniently 

 used, provided they are not affected by the reagent. One's own 

 experience is, however, a better guide than any written directions 

 in successfully carrying out experiments of this kind. 



Another point of much importance in micro-chemical researches, 

 which has been often neglected even in recent times, and which 

 has been strongly insisted upon by Donders and myself, is atten- 



* Hollandische Beitrage. 1846, S. 39. 

 t Arch. f. physiol. Heilk. Bd. 1, S. 225. 



