22 



THE PARAFFIN METHOD. 



50. As seen by the above scheme, the aim is to fill all the 

 interstices of the tissue with paraffin of the right degree of hardness 

 to have it cut well. Paraffin is not soluble in water or alcohol, but 

 is soluble in a number of fluids which in turn are miscible with alco- 

 hol. Hence the following steps are necessary: (1) the tissue must 

 be first water-rid, thoroughly dehydrated with strong alcohol; (2) 

 freed from the alcohol, cleared by a fluid that mixes with melted 

 paraffin which (3) takes the place of the clearer in the tissue, infil- 

 trates it, filling the spaces; (4) finally, the tissue is imbedded in 

 paraffin of the right degree of hardness, the mass cooled, and it is 

 ready (5) to cut, or section. 



51. Dehydration. After the various steps pertaining to the 

 fixing and hardening ( 7, 9) of the tissue have been properly 

 pursued it may be stored in alcohol of 82% to 95% strength depend- 

 ing on the tissue and its purpose. The dehydration necessary 

 in imbedding may be accomplished by immersion in alcohol of 95% 

 strength. For most work it is perhaps better to employ stronger 

 (absolute) alcohol. If xylene or toluene are used for clearing absolute 

 alcohol must be used. 



Immerse small pieces 2 to 3 mm. in diameter for at least 6 to 8 

 hours in 95% alcohol changed once or twice. A longer time, even 

 days, usually does no harm and is preferable to ensure complete 

 dehydration. For larger pieces of tissue or entire organs a corres- 

 pondingly longer period of dehydration should be employed, a several 

 days' stay, with the alcohol changed daily, being often advisable. In 

 any case, dehydrate thoroughly, changing the alcohol 1 to 3 times, the 

 last change usually being to absolute alcohol. Let the tissue dehy- 

 drate for a longer rather than a shorter period of time. 



52. Clearing. The alcohol must next be replaced by some 

 solvent of paraffin which is miscible with alcohol, a step spoken of 

 as clearing. Toluene* is one of the most serviceable clearers, although 

 for special purposes other media such as xylene, cedarwood oil, 

 bergamot oil or chloroform may be preferred. Toluene (and xylene) 

 mix well only with absolute alcohol, hence the dehydration needs to 

 be thorough, and the clearing and infiltrating is best accomplished by 

 a number of steps. After the absolute alcohol, the tissue is placed (1) 



*Benzene, toluene, xylene, etc.; the terms adopted by American chemists. 

 In Europe the same substances are designated xylol, benzol, etc. 



