Section 104. the Simplest Example of the Action of the Nervous

system is reflex action. For instance, when the foot of a frog, or the hand of a soundly sleeping person, is tickled very gently, the limb is moved away from the irritation, without any mental action, and entirely without will being exercised. And when we go from light into darkness, the pupil of the eye enlarges, without any direct consciousness of the change of its shape on our part. Similarly, the presence or food in the pharynx initiates a series of movements-- swallowing, the digestive movements, and so on-- which in health are entirely beyond our mental scope.