neuron
with which it makes an end-to-end connection, so that a series of impulses
is produced along a given nerve path (Fig. 129).

The special _function of the axon_ is to transmit the impulse. By its
length, structure, and property of conductivity it is especially adapted
to this purpose. The axis cylinder, however, is the only part of the axon
concerned in the transmission. The primitive sheath and the medullary
layer protect the axis cylinder, and, according to some authorities, serve
to insulate it. The medullary sheath may also aid in the nourishment of
the axis cylinder.

*Nerve Stimuli.*--While the properties of irritability and conductivity
supply a necessary cause for the production and transmission of nervous
impulses, these alone are not sufficient to account for their origin. An
additional cause is necessary--a force not found in the nerve protoplasm,
but one which, by its action on the protoplasm, makes it produce the
impulse. In this respect, the neuron does not differ essentially from the
cell of a muscle. Just as the muscle cell requires a stimulus to make it
contract, so does the neuron require a stimulus to start the impulse.
Hence, in accounting for the activities of the body, it is not sufficient
to say they are caused by nervous impulses. We must also investigate the
_nerve stimuli_--the means through which the nervous impulses are started.
Most of these are found outside of the body and are known as external
stimuli.

*Action of External Stimuli.*--In the arrangement of the nervous system the
most favorable conditions are provided for the reception of external
stimuli. Not only do vast numbers of neurons terminate at the surface of
the body,(104) but they connect there with delicate structures, called
_sense organs_. The purpose of the sense organs is to _sensitize_ (make
sensitive) the terminations of the neurons. This they do by supplying
special structures through which the stimuli can act to the best advantage
upon the nerve endings. Moreover, there are different kinds of sense
organs, and these cause the neurons to be sensitive to different kinds of
stimuli. Acting through the sense organs adapted for receiving them,
light, sound, heat, cold, and odors all act as stimuli for starting
impulses. Indeed, the arrangement is so complete that the nervous system
is subjected to the action of external stimuli in some form practically
all the time. The work of the sense organs is further considered