THE MICROTOMIST' S VADE-MECUM

water of the tissues must be removed. This is necessary for
two reasons; firstly in the interest of preservation, the          the whol
presence of water being the condition of all others that most      901b
favours post-mortem    decomposition; and secondly, because        ffD i.
all water must be removed in order to allow the tissues            at (e
to be impregnated with the paraffin necessary for section-         paid
cutting, or with the balsam    in which they are to be finally     toieft
preserved.   (The cases in which aqueous imbedding and              nIRO
preserving media are employed are exceptional, and will be          R is
treated of in the proper places.)    This dehydration is per-      tisse
formed as follows: the objects are brought gradually into           goRt
alcohol, which  is effected by transferring them    into it by      3durr
means of a small spoon or a pipette, so that they carry with       batles
them a little atmosphere of aqueous fluid into which the aIkholk
surrounding alcohol gradually diffuses. They are then passed        0I1; (
through successive alcohols of gradually increased strength,        hady
for instance, 50 per cent, two hours, 70 per cent. six to          914l'o
twenty-four hours, 80 per cent. several hours, 95 per cent.        th@ p
two or three hours, absolute alcohol, time enough for complete      its t6
saturation.  (Very small objects, so small that section-cutting     abhbels
is not necessary, may be dehydrated much quicker than this. adhbh
Infusoria may be prepared in a few minutes.)        The water       he e
having been thus completely removed, the object is brought          tqueti
into good chloroform, where it remains till saturated, which         Thel
generally happens in a few minutes in the case of fairly small     introbu
objects. The chloroform is now to be gradually saturated tostai
with paraffin. This is done by placing it, with the object,         tsin
on  a water-bath, heating it to the melting point of the            to lon
paraffin employed, and dropping into it from      time to time      esentei
small pieces of paraffin. When it is seen that no more              bPSrIiE
bubbles are given off from the object the addition of paraffin      uhshi
may cease, as that is a sign that the paraffin solution has         khiah
entirely taken the place of the chloroform in the object. This      grdSn
displacement having been gradual, the risk of shrinkage of abbe
the tissues is reduced to a minimum. The heating is then             drntie

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