CH. VII] ISOLATION OF ELEMENTS 173 



(| 280, 300). The first step in the series is Dehydration, that is, the water is dis- 

 placed by some liquid which is miscible both with the water and the next liquid 

 to be used. Strong alcohol (95% or stronger) is usually employed for this. Plenty 

 of it must be used to displace the last trace of water. The tissue may be soaked 

 in a dish of the alcohol, or alcohol from a pipette may be poured upon it. Dehy- 

 dration usually occurs in the thin objects to be mounted in balsam in 5 to 15 min- 

 utes. 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 refrac- 

 tive in most cases, and consequently this step is called clearing and the liquid a 

 clearer. The clearer displaces the alcohol, 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. 



ISOLATION OF HISTOLOGICAL ELEMENTS 



\ 259. 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 that the cells and fibers may be separated from one another, 

 and at the same time the cells are preserved. In fixing and hardening, on the 

 other hand, the cell cement, like the other parts of the tissue, are made firmer. 

 In preparing the isolating solutions it is better to dilute the fixing agents with 

 normal salt solution [\ 331) than merely with water. 



\ 260. Isolation by Means of Formaldehyde. — Formaldehyde in normal 

 salt solution is one of the very best dissociating agents for brain tissue and all the 

 forms of epithelium. It is prepared as follows : 2 cc. of formal, (that is, a 40% 

 solution of formaldehyde) are mixed with 1000 cc. of normal salt solution. This 

 acts quickly and preserves delicate structures like the cilia of ordinary epithelia, 



