IMBEDDING IN ELDER PITH FOR CUTTING SECTIONS. 25 
half, somewhat corresponding in depth to the thickness of the 
tissue to be cut; place the latter in the groove thus formed 
on one piece of pith, and lay the other upon it. Holding 
the mass between the thumb and finger, force it by gentle 
pressure into the microtome from above, bevelling off the 
extremity of the pith, if necessary, to make it enter more 
readily ; and then with the thumb press down the whole to a 
level rather below that of the plate of the instrument. 
The mass now, if properly fitted, should remain tolerably 
firmly fixed in the tube by the force of compression employed, 
but not so tightly as to enable one to cut sections at once ; 
for if this be the case, the addition of water, will on expand- 
ing the pith, cause such pressure as might injure the specimen. 
Experience alone can suggest the exact amount of force to 
employ. Should the mass be too small, it must be either 
fitted to a still smaller tube, if there be one, or some extra 
slips of pith should be packed around. In either case the 
next step is to invert the microtome into a saucer of water ; 
or, in the case of an instrument already fixed in the upright 
position, water must be poured upon it. In about three 
minutes the pith will have swollen sufficiently to hold the 
specimen firmly in its place, to have moulded itself so as to 
fill all inequalities or irregularities in the outline of the 
tissue enclosed, and to prevent effectually any chance of its 
revolving in the microtome when the section is being cut. 
Where larger pieces of tissue are used, say of } to } inch 
in breadth, they must be packed in a manner similar to that 
described. Only four or more separate pieces of pith must be 
employed according to circumstances. Support on two sides 
only is often enough, especially in the case of chromic acid 
specimens that have undergone a full degree of hardening. 
Besides causing it to swell, the moistening of the pith takes 
away that extreme brittleness that characterises it in the dry 
state, and which, by causing it to break from the specimen 
embedded, would effectually frustrate any attempts to obtain 
a good section ; for this reason pith, even if held in the 
fingers only, should be previously moistened. As in cutting 
wax specimens. the razor should be dipped in spirit. 
To those who have never tried it this process will probably 
at first sight offer several objections ; these, however, I hope 
to show may not only be overcome, but that real advantages 
are to be found in it. The pressure exerted upon the 
specimen by the swelling pith naturally first suggests itself 
as the principal drawback. In answer I can only say that if 
the tissue be hardened in the usual manner and to the usual 
degree, nothing is to be feared on this score; I have both 
