tural in that the perfected microscope
was just now claiming all fields for its own. A troop of observers soon
entered upon the study of the nerves, and the leader here, as in so
many other lines of microscopical research, was no other than Theodor
Schwann. Through his efforts, and with the invaluable aid of such other
workers as Remak, Purkinje, Henle, Muller, and the rest, all the mystery
as to the general characteristics of nerve tracts was cleared away. It
came to be known that in its essentials a nerve tract is a tenuous fibre
or thread of protoplasm stretching between two terminal points in the
organism, one of such termini being usually a cell of the brain
or spinal cord, the other a distribution-point at or near the
periphery--for example, in a muscle or in the skin. Such a fibril may
have about it a protective covering, which is known as the sheath of
Schwann; but the fibril itself is the essential nerve tract; and in
many cases, as Remak presently discovered, the sheath is dispensed with,
particularly in case of the nerves of the so-called sympathetic system.

This sympathetic system of ganglia and nerves, by-the-bye, had long been
a puzzle to the physiologists. Its ganglia, the seeming centre of
the system, usually minute in size and never very large, are found
everywhere through the organism, but in particular are gathered into a
long double chain which lies within the body cavity, outside the spinal
column, and represents the sole nervous system of the non-vertebrated
organisms. Fibrils from these ganglia were seen to join the cranial and
spinal nerve fibrils and to accompany them everywhere, but what special
function they subserved was long a mere matter of conjecture and led to
many absurd speculations. Fact was not substituted for conjecture
until about the year 1851, when the great Frenchman Claude Bernard
conclusively proved that at least one chief function of the sympathetic
fibrils is to cause contraction of the walls of the arterioles of the
system, thus regulating the blood-supply of any given part. Ten years
earlier Henle had demonstrated the existence of annular bands of muscle
fibres in the arterioles, hitherto a much-mooted question, and several
tentative explanations of the action of these fibres had been made,
particularly by the brothers Weber, by Stilling, who, as early as 1840,
had ventured to speak of "vaso-motor" nerves, and by Schiff, who was
hard upon the same track at the time of Berna