Need and Action of Reagents g 



required firmness and rigidity to the tissue. The material is 

 usually washed in several changes of 70 per cent, alcohol and 

 then, for preservation, is placed in several times its volume of 

 80 per cent, alcohol and set aside till needed. 



Dehydration. — In order to imbed by either of the ordinary 

 methods, all water in the tissue must be removed. Not only 

 would water be detrimental in itself, but neither of the imbed- 

 ding media, parafifin nor celloidin, is miscible with water in the 

 slightest degree. Therefore, to get the necessary infiltration by 

 either, the tissue must be thoroughly dehydrated. The strong 

 affinity which alcohol has for water enables it to remove the 

 water readily. 



The ,grades of alcohol. — The mixing of alcohol with water is 

 accompanied by violent action. The violence of the action 

 varies with the strength of the alcohol. For this reason the 

 tissue should not be transferred from water to a high grade of 

 alcohol, nor vice versa, lest the violence of the diffusion and the 

 conseqiient violent or unbalanced osmotic action shrink or rup- 

 ture the cell or break down its internal structure. ■ Dehydration, 

 therefore, is usually accomplished by passing the tissue suc- 

 cessively through grades of alcohol of gradually increasing 

 strength up to " absolute." The grades ordinarily used in dehy- 

 drating are 35 per cent., 50 per cent., 70 per cent., 85 per cent., 

 95 per cent., and absolute alcohol. These percentages need not 

 be absolutely exact, but should be approximately so. 



The water removed, the tissue is subjected to a bath of some 

 reagent which is a better solvent than alcohol is for the imbed- 

 ding medium to follow. In case of paraffin especially, where 

 this solvent is an oil, this procedure is called clearing. 



The preparation of the grades of alcohol. — With the exception 

 6f absolute alcohol, the different grades are usually made up in 

 the laboratory from, the ordinary commercial alcohol. The 

 strength of the commercial or " stock " alcohol may be determined 

 by the alcoholometer. It usually varies from 90 per cent, to 97 

 per cent. 



Digitized by Microsoft® 



