the spinal cord is irritated in precisely the same way, only half of
these effects is produced. All the muscles which are supplied with
fibers from that trunk contract, but no pain is experienced. Conversely,
if the posterior bundle of nerve-fibers is irritated, none of the
muscles to which the filaments of the nerve are distributed contract,
but pain is felt throughout the entire region to which these filaments
are extended. It is evident, from these facts, that the fibers composing
the posterior bundles of nerve-roots only transmit sensory impulses, and
the filaments composing the anterior nerve-roots only transmit motor
impulses; accordingly, they are termed respectively the _sensory_ and
the _motor_ nerve-roots. This is illustrated by the fact that when the
posterior root of a spinal nerve is divided, all sensation in the parts
to which the filaments of that nerve are distributed is lost, but the
power of voluntary movement of the muscles remains. On the other hand,
if the anterior roots are severed, the power of voluntary motion of the
muscles is lost, but sensation remains.

It appears from these experiments, that, when a nerve is irritated, a
change in the arrangement of its molecules takes place, which is
transmitted along the nerve-fibers. But, if the nerve-trunks are
divided, or compressed tightly at any point between the portion
irritated, and the muscle or nerve-centre, the effect ceases
immediately, in a manner similar to that in which a message is stopped
by the cutting of a telegraph wire. When the nerves distributed to a
limb are subjected to a pressure sufficient to destroy the molecular
continuity of their filaments, it "goes to sleep," as we term it. The
power of transmitting sensory and motor impulses is lost, and only
returns gradually, as the molecular continuity is restored.

From what has been said, it is plain that a sensory nerve is one which