MICROSCOPICAL TECHNIQUE. 401 



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permanently, since it is usually easily recognized in the specimen 

 and does not interfere with its study under the microscope. 



The study of tissues by means of sections has the disadvantage 

 that the elements of the tissues are cut, and the sections contain the 

 resulting portions as well as complete elements. The incomplete 

 portions lie near and at the surfaces of the sections, where they are 

 in clearest view, while the uncut elements are situated in the body 

 of the section, more or less obscured by the overlying portions that 

 have been cut by the knife. Moreover, the tissue-elements may lie 

 obliquely to the plane of the section, so that only a portion of them 

 can be seen at a time, the rest being brought into clear view only 

 when the foc^l plane is raised or lowered. These circumstances and 

 the fact that the tissue-elements are frequently closely crowded 

 together make the interpretation of sections a matter of some dif- 

 ficulty in many cases. These difficulties are in a measure overcome 

 by examining sections of different thicknesses, but a more satis- 

 factory way of studying the structure of a tissue is to examine por- 

 tions after maceration as well as in section. 



The processes of coagulation and dehydration, which have already 

 been mentioned as usual preliminaries to the cutting of sections, 

 deserve a few words in explanation of their purposes. 



The coagulation of the albuminous substances in the tissues has 

 for its chief aim the preservation of the minute structure of the 

 tissue-elements, so that a lapse of time or the subsequent manipula- 

 tions of the tissues shall not cause an alteration in the details which 

 it is desired to study. If this precaution be omitted, the tissues 

 undergo post-mortem changes which seriously alter the appear- 

 ance of the elements of which they are composed. Coagulation 

 brought about for this purpose is called " fixation " of the tissues. 

 It may be induced in a variety of ways : the tissues may be sub- 

 jected to heat for a few moments, thus rendering the albumins they 

 contain both solid and insoluble ; but the more usual procedure is 

 to immerse the tissues in a solution of some substance that causes 

 rapid death with coagulation. These solutions are called fixing- 

 solutions, and not infrequently the substances they contain not only 

 cause death and coagulation, but also form a union with some of 

 the structural materials of the tissues which may facilitate their 

 subsequent recognition. 



The number of formulae that have been devised for the prepara- 

 tion of fixing-solutions is very great, and some of the solutions are 



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