400 HISTOLOGICAL TECHNIQUE. 



When a simple scraping of the natural or freshly cut surface does 

 not yield useful preparations, showing isolated tissue-elements, some 

 process of maceration may be employed. Bits of the tissue are 

 soaked for a time in some solution that serves to soften the cement- 

 substances lying between the elements of the tissues, so that they 

 may be easily separated with needles (teasing). Such specimens 

 are usually best examined when mounted on a slide in some of the 

 macerating-fluid. Many of the macerating-solutions not only favor 

 the separation of the constituents of tissues, but also preserve them, 

 so that a fair idea of their natural size and shape may be obtained 

 from such preparations. It is evident, however, that with this 

 method very little can be learned of their arrangement in the tis- 

 sues before they were separated, and a knowledge of that arrange- 

 ment is often of greater importance in the determination of the 

 character of the tissue than a knowledge of the exact shape and 

 size of the tissue-elements. 



The third method, that of preparing sections of the tissues, is the 

 one most commonly employed, because it yields the most useful 

 results. The structural elements composing the tissues are seen in 

 their natural relative positions, and can be distinguished from each 

 other and identified by the use of dyes and other reagents that 

 affect them in some characteristic manner. But in order to ob- 

 tain useful sections the tissues must almost always undergo some 

 preliminary treatment with reagents, to give them a proper consist- 

 ency for cutting and to hold the tissue-elements together so that 

 the sections shall not fall apart after they have been cut. This may 

 be accomplished by freezing the tissue before cutting it ; but more 

 satisfactory results are obtained by causing a coagulation of the 

 albuminous substances and subsequently extracting some or all of 

 the water contained in the tissues. These changes in the tissues 

 give them a firmness which favors the preparation of very thin sec- 

 tions ; but sometimes even they are inadequate, and then the tissues 

 are usually impregnated with some substance, like paraffin or col- 

 lodion, which fills the interstices of the tissues and can then be 

 hardened, when it serves to hold the tissue-elements together and 

 retain them in their natural positions. The paraffin or hardened 

 collodion is cut with the tissues and keeps the sections from disinte- 

 grating. Before mounting the section, the substance used for im- 

 pregnation may be removed from the section, or it may be retained 



