anglia are concerned in these actions (Fig. 139).

[Fig. 139]

Fig. 139--Diagram illustrating reflex action in its relation to the food
canal. The nerve path in this case includes sympathetic neurons.

*Reflex Action in the Circulation of the Blood.*--On sudden exposure to
cold, the small arteries going to the skin quickly diminish in size, check
the flow of blood to the surface, and prevent too great a loss of heat. In
this case, impulses starting at the surface of the body are transmitted to
the bulb and then through the efferent neurons to the muscles in the walls
of the arteries. In a somewhat similar manner, heat leads to a relaxation
of the arterial walls and an increase in the blood supply to the skin.
Other changes in the blood supply to different parts of the body are also
of the nature of reflex actions. As in the work of digestion, neurons
having their cell-bodies in the sympathetic ganglia aid in the control of
the circulation.

*Purposes of Reflex Action.*--The examples of reflex action so far
considered illustrate its two main purposes--(1) protection, and (2) a
means of controlling important processes.

The pupil has but to study carefully the reflex actions of his own body
for a period, say of two or three weeks, in order to be convinced of their
protective value. He will observe that portions of his body have, on
exposure to danger, been moved to places of safety, while in some
instances, like falling, his entire body has been adjusted to new
conditions. He will also find that reflex action is quicker, and for that
reason offers in some cases better protection, than movements directed by
the mind. In digestion and circulation are found the best examples of the
control of important processes through reflex action.

*Voluntary Action.*--It is observed that reflex action, in the sense that
it has so far been considered, is not the usual mode of action of the
external organs, but is, instead, a kind of emergency action, due to
unusual conditions and excitation by strong stimuli. Voluntary actions, on
the other hand, represent the ordinary, or normal, action of these organs.
They comprise the movements of the body of which we are conscious and
which are _controlled by the mind_. But while they are of a higher order
than reflex actions and are under _intelligent_ direction, they are
brought about in much the same manner.

*Voluntary Action Pathways* differ in bu