24. H. GOLDING BIRD. 
so where it is a question not only of imbedding, but of using 
a microtome as well. If held simply in the fingers without 
instrumental aid, pith will yield sections in most cases equal 
to those obtained by the more complicated wax method, and 
at a cost of far less time and trouble. In the histological 
laboratory of Prof. Ranvier, at the Collége de France, Paris, 
pith is the only imbedding medium employed for all varieties 
of tissue, and transverse sections of spinal cord obtained 
through its means I recollect as peculiarly good. 
The principle on which it depends is simply the swelling 
of the parenchymatous tissue of the dried pith in the pre- 
sence of moisture, so that if placed dry in a rigid tube, as 
that of the microtome, with the tissue to be cut let into its 
centre, the addition of a little water will in the course of a 
few minutes so cause the pith-cells to expand, that the 
specimen becomes immoyably fixed. Sections are then very 
readily cut. 
Any microtome may be used for this purpose; but, as a 
rule, the English instruments have so large a bore that a 
great expenditure of pith is involved: hence I prefer the 
smaller instrument known as Ranvier’s microtome, the one 
alone used in the Collége de France. It carries tubes of 
three different sizes, and is intended to be held in the left 
hand of the operator, and not to be fastened to the table ; 
and though, perhaps, a little more practice and manipulative 
skill are required, it has the advantage of so far yielding to 
the razor that the brass top need never be cut, while with a 
slight rotatory action on its own axis given to the instrument 
at the moment the section is being cut, so as to meet the 
razor halfway, as it were, the specimen may be literally 
“‘ whipped off”? without any of that pressure exerted upon it 
where a fixed microtome and a razor held in both hands are 
employed. 
The pith used is that of the common elder, and so is found 
in cylindrical pieces of varying diameter. As, however, it 
does not matter in how many pieces it is employed, provided 
the specimen be well held, the thickness of the pith is not of 
much moment; still, to avoid trouble, it will be found that 
the thicker are the better. 
Suppose it be required to cut a section of a tissue of about 
one eighth of an inch in breadth, select first a piece of pith 
about the size of the tube of the microtome, and of the same 
length as the specimen ; split it longitudinally, and with the 
finger-nail make a small furrow on the cut surfaces of each 
1 This instrument is made by Verick, of Paris. The three tubes have 
the following diameters: 19 mm., 16 mm., 12 mm. 
