ndicates the presence of
cell-bodies, and white matter the presence of nerve fibers.

100 In very early life the spinal cord entirely fills the spinal cavity,
but as the body develops the cord grows less rapidly than the spinal
column, and, as a consequence, separates at the lower end from the
inclosing bony column.

101 Fibers passing between the spinal cord and the cerebrum cross to
opposite sides--most of them at the bulb, but many within the cord--so
that the right side of the cerebrum is connected with the left side
of the body, and _vice versa_. This accounts for the observed fact
that disease or accidental injury of one side of the cerebrum causes
loss of motion or of feeling in the opposite side of the body.

102 In general, _afferent_ neurons or fibers are those that convey
impulses _toward_ the central nervous system (brain and cord), while
_efferent_ neurons or fibers are those that convey impulses _from_
the central system.

103 At different times the nervous impulse has been regarded as a
current of electricity; as a progressive chemical change, likened to
that in a burning fuse; as a mechanical vibration, such as may be
passed over a stretched rope; and as a molecular disturbance
accompanied by an electrical discharge. The velocity of the nervous
impulse, which is only about one hundred feet per second, proves
that it is not a current of electricity. It takes place with little
or no exhaustion of the cell protoplasm and consequently is not due
to chemical action. And the loose, relaxed condition of the nerves
prevents their transmission of physical vibrations, like those on a
stretched rope. The view that the impulse is a progressive molecular
disturbance, accompanied by an electrical discharge, has much
evidence in its favor, but it has only recently been proposed and is
likely to be modified upon fuller investigation.

104 The surface of the body includes the linings of the air passages,
food canal, and certain cavities, as well as the external covering
or skin.

105 Derived from the Latin _re_, back, and _flectere_, to turn or bend.

106 A frog from which the brain has been removed is suspended with its
feet downward and free to move. If a toe is pinched, the foot is
drawn away, and if dilute acid, or a strong so