Physiology and Hygiene for Secondary Schools

by Francis M. Walters

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[pg 311] 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[pg 312] actions and are under intelligent direction, they are brought about in much the same manner.

Voluntary Action Pathways differ in but one essential respect from those of reflex action. They pass through the cerebrum, the organ of the mind (Fig. 140). This is necessary in order that the mind may control the action. From all portions of the body surface, afferent pathways may be traced to the cerebrum; and from the cerebrum efferent pathways extend to all the voluntary organs. A complex system of intermediate neurons, found mostly in the brain, join the afferent with the efferent pathways. The voluntary pathways are not distinct from, but include, reflex pathways, a fact which explains why the same external stimulus may excite both reflex and voluntary action (Fig. 141).

Fig. 140—Diagram of a voluntary action pathway.

Choice in Voluntary Action.—In reflex action a given stimulus, acting in a certain way; produces each time the same result. This is not the case with voluntary action, the difference being due to the mind. In these actions the external stimulus first excites the mind, and the resulting mental processes—perhaps as memory of previous experiences—supply a variety of facts, any of which may act as stimuli to action. Before the action takes place, however, [pg 313]some one fact must be singled out from among the mental processes excited. This fact becomes the exciting stimulus and leads to action. It follows, therefore, that the action which finally occurs is not necessarily the result of an immediate external stimulus, but of a selected stimulus—one which is the result of choice.