97 



§ 43- Dehydration. After the various steps pertaining to the 

 proper fixing and hardening (§21) of the tissue have been prop- 

 erly pursued it may be stored in alcohol of 82^-95% strength de- 

 pending on the tissue and its purpose. The dehydration necessary 

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

 strength. For more delicate work it is perhaps better to employ 

 stronger (absolute) alcohol. 



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

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

 days, does no harm and is preferable to ensure complete dehydra- 

 tion. For larger pieces of tissue or entire organs a correspondingly 

 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 1-3 times, letting 

 the tissue remain in it for a longer rather than a shorter period. 



§ 44. Clearing. The alcohol must next be replaced by some 

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

 as clearing . Cedar-wood oil is perhaps most generally serviceable, 

 although for special purposes other media, such as xylol, bergamot 

 oil, or chloroform may be preferred. When the tissue is dehydrat- 

 ed, it is removed to a vial of thickened cedar- wood oil. It will float 

 at first, but as the alcohol used for dehydration is displaced by the 

 oil, it sinks to the bottom, the currents of alcohol rising from it. 

 When the alcohol is entirely removed by the oil, such currents will 

 have ceased to rise from it and the tissue will be clear and translu- 

 cent, — except, of course, such as is dark in color. 



§ 45. Infiltration. After the tissue is completely cleared by 

 the cedar-wood oil, remove.it to a dish of melted infiltration paraffin 

 and place in the paraffin oven for 2 to 24 hours, depending on 

 the size of the piece. Quite large pieces may be left longer. The 

 melted paraffin replaces the cedar-wood oil, filling in the interstices 

 of the tissue. Paraffin melting at about 48° C. is used, and is pre- 

 pared by mixing equal parts of 43^^ and 54° paraffin. It is best not to 

 expose to a higher temperature than is necessary, or for a long period 

 of time. The paraffin oven will be maintained at a temperature of 

 5o°-52" C. 



§ 46. Imbedding. It is best to use fresh paraffin for imbed- 

 ding and generally with a melting point higher than that of the infil- 



