THE MICROTOMIST'S VADE-MECUM

Paraffin (No. 231) used pure, as an interstitial mass, allows
of the cutting of the thinnest sections.                          (,
Collodion (made with celloidin) does not afford sections
quite so thin as those that can be obtained by means of
paraffin; the collodion method is the least hurtful of all hub
methods to delicate tissues; it is the surest and the easiest to
carry out of all methods of interstitial imbedding.               ttion
Wax and oil is a trustworthy and very convenient simple         t til
imbedding-mass.
221. Section-Stretching.-The media with which the knife         ehicl
used for section-cutting should be moistened are indicated for
each mass. Some masses are now generally cut dry, paraffin,
for instance. By this means thinner sections are obtained, but Ieof
a difficulty generally arises owing to the tendency of sections subjet
so cut to curl-up on the blade of the knife. It is often im-       1evote
possible by any means to unroll a thin section that has curled.    tr
To obviate this, during the cutting the edge of the section         t
that begins to curl is caught and held down on the blade of        othr
the knife by means of a small camel-hair brush with a flat         hI
point, or with a small spatula made by running a piece of          f
paper on to the back of a scalpel, or by means of an ingenious     pa
little instrument called a " section-stretcher."  This consists    Mati
essentially of a little metallic roller suspended over the object  lhreft
to be cut in such a way as to rest on its free surface with a      Must)
pressure that can be delicately regulated so as to be sufficient   pOints
to keep the section flat without in any way hindering the ti o4
knife from gliding beneath it.'                                    Lkuild
Vaselin added to paraffin gives a mass that reduces the ten-      hich
dency to curl on the part of the sections. A curled section        Oi (31
may sometimes be caused to unroll by putting it into alcohol       1id
or by gently warming it. A delicate sawing movement given           k')io
to the section-knife will sometimes suffice to prevent curling. i1t
I have sometimes found it convenient, instead of holding the     dich
I See the descriptions of various forms in ' Zool. Anzeig,' vol. vi (1883),  qEIo
p. 100; and in 'Journ. Roy. Mic. Soc. (N.S.), vol. iii, pp. 450 and 916.  E

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