THE RECEPTORS AND THEIR STIMULI
Introduction. --As has already been pointed out, the understanding of the nature of human response depends to a large extent upon a knowledge of the various types of stimuli which affect man; of the places where such stimuli must be applied in order to produce appropriate action; and of the various physical and physiological factors which must be taken into account in the control of both the organism and the stimulus. To make this concrete we need only to mention the fact that if the skin of certain animals is stimulated by light of high intensity an overt response will be produced. In man, light, neglecting it's heat value, must fall upon a certain part of the eye if action is to be produced. Furthermore, light necessary to produce an overt response in man must have a wave length not less than 397μμ (violet) and not greater than 760μμ (red). To bring these factors out, we must to a certain extent "dissect" our human being and find the parts which are sensitive to stimulation (bodily areas which each sense organ comprises) and the adequate stimuli which, when applied upon these sense organ areas, will produce action. It must be borne in mind that this procedure is somewhat artificial and like the one the physiologist uses when he studies the heart action, respiration, etc., to the exclusion of the other bodily functions. In later chapters, however, we will put the organisms together, as it were, and study our creation from the standpoint of its reactions as a whole. We must never lose sight of the fact that when man reacts to even the most minute sensory stimulus, the whole body coöperates in the reaction, even if he only raises a finger or says the word "red."
A General Neuro-Muscular Consideration. --We shall find in Chapter V that in every simple reflex act, such as the withdrawal of the hand from a hot object, there is involved on the structural side a receptor, or sense-organ structure, a set of neural conductors and an effector (muscle or gland). When a sense