24 MAMMALIAN ANATOMY 
Dehydrating. —In order to use the material, thus hardened 
and preserved, for microscopic sections it must first have all the 
water removed from it by the successive transfer to higher grades 
of alcohol until 100% (absolute) alcohol 1 is reached. Thus from 
70% alcohol in which it is preserved (and may be kept indefinitely 
until needed), it must be transferred to 95% in which it should 
remain for several hours, and from this to 100% in which it should 
be kept for only a few hours at the longest, as this tends to harden 
and shrivel the tissues. 
Clearing. —This process consists in replacing the alcohol with 
some oil (usually xylol or turpentine) which, being on the one 
hand miscible with absolute alcohol, will replace it, while on the 
other hand it is a solvent of the paraffine in which the material 
must later be imbedded, and may thus be readily replaced in turn 
by the paraffine. The material is placed in the clearing oil and 
allowed to remain until its transparent appearance, when sub¬ 
jected to a strong transmitted light, indicates that the oil has 
quite replaced the alcohol. In the clearing oil the material may 
be kept indefinitely, though a prolonged stay in this medium is 
likely to render it brittle. 
Imbedding. —This requires the use of a paraffine oven or other • 
device for keeping melted paraffine at a uniform temperature 
slightly above the melting point of the paraffine used (54°-56° 
centigrade). 2 Place the cleared tissue in the melted paraffine 
and leave it for a sufficient length of time to insure the com¬ 
plete replacement of the clearing oil by the paraffine which will 
thus impregnate the material. The process requires about half 
an hour for each millimeter of thickness (least dimension). When 
the tissue is to be removed from the paraffine, have in readiness 
all of the articles and implements which may be needed, for the 
paraffine hardens quickly and the transfer must be made before 
the hardening begins. 
1 Acetone may be substituted in this and subsequent processes for 100% alcohol. 
2 A very simple and convenient device for student use consists of a jelly tumbler 
which is first filled two-thirds full of melted paraffine and set aside to harden, after 
which it may be used at any time by melting the top to a depth of about an inch by 
bringing directly over it an ordinary electric light bulb. So long as the material to 
be imbedded rests upon the solid surface of the lower paraffine it is in no danger from 
overheating, and the amount of melting may be regulated by raising and lowering 
the electric light bulb. 
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