CHAPTER I 

 GENERAL TECHNIC 



Certain body fluids, such as blood and urine, may be ex- 

 amined by simply placing them on a slide under a cover-glass. A 

 few tissues, for example, thin membranes, such as the omentum and 

 the mesentery, may be examined in the fresh or living condition by 

 immersing them in some such inert medium as blood serum or normal 

 salt solution (a 0.75-per-cent. solution of socUum chlorid in distilled 

 water). For such examination the tissue is immersed in the salt 

 solution on a slide and covered with a cover-glass. IMost tissues and 

 organs, however, require much more elaborate preparation to render 

 them suitable for microscopic examination. Tissues too dense and 

 thick to be readily seen through with the microscope must be so 

 treated as to make them transparent. This is accomplished either 

 by pulling the tissue apart into hne shreds, leasing, or by cutting it 

 into thin shces, section cutting. Some tissues admit of teasing in a 

 fresh condition; others can be satisfactorily teased only after they 

 have been subjected to the action of a chemical which breaks down 

 the substance holding the tissue elements together, maceration. 

 Fresh tissue can rarely be cut into sections sufficiently thin for micro- 

 scopic examination. It must first be treated in such a manner as to 

 preserve as nearly as possible the living tissue relations, fixation. If 

 too soft for section cutting it must next be put through a process 

 known as hardening. If, however, as in the case of bone, the tissue 

 is too hard, it must be softened by dissolving out the mineral salts, 

 decalcification. If very thin sections are to be cut, it is further 

 necessary to impregnate the tissue with some fluid substance which 

 will harden in the tissue and give to the mass a firm, even consistency. 

 This is know^n as embedding. 



Furthermore, most tissue elements have so nearly the same color, 

 and possess refractive indices so similar, that their dift'erentiation 

 under the microscope is often extremely difficult. To overcome this 

 difiiculty, recourse is had to staining the tissue with dyes which have 

 an affinity for certain only of the tissue elements, or which stain 



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