330 ISOLATION OF TISSUE ELEMENTS [Cn. X 



Dehydration usually occurs in the thin objects to be mounted in 

 balsam in' 5 to 15 minutes. If a dish of alcohol is used it must not be 

 used too many times, as it loses in strength. 



The second step is clearing. That is, some liquid which is miscible 

 with the alcohol and also with the resinous medium is used. This 

 liquid is highly refractive in most cases, and consequently this step is 

 called clearing and the liquid a clearer. The clearer displaces the alco- 

 hol, and renders the object more or less translucent. In case the 

 water was not all removed, a cloudiness will appear in parts or over 

 the whole of the preparation. In this case the preparation must be 

 returned to alcohol to complete the dehydration. 



One can tell when a specimen is properly cleared by holding it over 

 some dark object. If it is cleared it can be seen only with difficulty, 

 as but little light is reflected from it. If it is held toward the window, 

 however, it will appear translucent. 



The third and final step is the displacement of the clearer by the 

 resinous mounting medium. 



The specimen is drained of clearer and allowed to stand for a short 

 time till there appears the first sign of dullness from evaporation of 

 the clearer from the surface. Then a drop of the resinous medium 

 is put on the object, and finally a cover-glass is placed over it, or a 

 drop of the mounting medium is spread on the cover and it is then put 

 on the object. For abundant examples see the next chapter. 



ISOLATION or HISTOLOGIC ELEMENTS 



514. Isolation, general. For a correct conception of the forms 

 of the cells. and fibers of the various organs of the body, one must see 

 these elements isolated and thus be able to inspect them from all sides. 

 It frequently occurs also that the isolation is not quite complete, and 

 one can see in the clearest manner the relations of the cells or fibers 

 to one another. 



The chemical agents or solutions for isolating are, in general, the 

 same as those used for hardening and fixing. But the solutions are 

 only about one-tenth as strong as for fixing, and the action is very 

 much shorter, that is, from one or two hours to as many days. In 

 the weak solution the cell cement or connective tissue is softened so 



