logist, physicist, and psychologist is,
clearly, the central nervous system--the spinal cord and the brain.
The importance of these structures as the foci of nervous and mental
activities has been recognized more and more with each new accretion
of knowledge, and the efforts to fathom the secrets of their intimate
structure has been unceasing. For the earlier students, only the
crude methods of gross dissections and microscopical inspection were
available. These could reveal something, but of course the inner secrets
were for the keener insight of the microscopist alone. And even for him
the task of investigation was far from facile, for the central nervous
tissues are the most delicate and fragile, and on many accounts the most
difficult of manipulation of any in the body.

Special methods, therefore, were needed for this essay, and brain
histology has progressed by fitful impulses, each forward jet marking
the introduction of some ingenious improvement of mechanical technique,
which placed a new weapon in the hands of the investigators.

The very beginning was made in 1824 by Rolando, who first thought of
cutting chemically hardened pieces of brain tissues into thin sections
for microscopical examination--the basal structure upon which almost all
the later advances have been conducted. Muller presently discovered that
bichromate of potassium in solution makes the best of fluids for the
preliminary preservation and hardening of the tissues. Stilling, in
1842, perfected the method by introducing the custom of cutting a series
of consecutive sections of the same tissue, in order to trace nerve
tracts and establish spacial relations. Then from time to time
mechanical ingenuity added fresh details of improvement. It was found
that pieces of hardened tissue of extreme delicacy can be made
better subject to manipulation by being impregnated with collodion or
celloidine and embedded in paraffine. Latterly it has become usual
to cut sections also from fresh tissues, unchanged by chemicals, by
freezing them suddenly with vaporized ether or, better, carbonic acid.
By these methods, and with the aid of perfected microtomes, the worker
of recent periods avails himself of sections of brain tissues of a
tenuousness which the early investigators could not approach.

But more important even than the cutting of thin sections is the
process of making the different parts of the section visible, one tissue
differentiated from another. T