[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.
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