B. The stimulus that directly arouses a mental process is often "central". C. Brain activities of all sorts influence the muscles by way of the motor area and the lower motor centers. D. Brain action in skilled movement. E. Brain action in speech. F. Brain action in sensation. G. Brain action in recognizing seen or heard objects. H. Relations of reactions of different levels. 2. Define and illustrate these classes of stimuli: A. Peripheral: (1) External. (2) Internal. B. Central. 3. Show by a diagram how one cortical center arouses another. Compare the diagram in Fig. 9, p. 37. 4. Facilitation of the patellar reflex or "knee jerk". Let your subject sit with one leg hanging freely from the knee down. With the edge of your hand strike the patellar tendon just below the knee cap. (a) Compare the reflex movement so obtained with a voluntary imitation by the subject. Which is the quicker and briefer? (b) Apply a fairly strong auditory stimulus (a sudden noise) a fraction of a second before the tap on the tendon, and see whether the reflex response is reinforced, (c) Ask the subject to clench his fists or grit his teeth, and tap the tendon as he does so. Reinforcement? (d) Where is the reflex center for the patellar reflex, and whence comes the reinforcing influence? 5. Construct a diagram showing the different centers and connections involved in making the skilled movement of writing; and consider what loss of function would result from destruction of each of the centers. REFERENCES Herrick's _Introduction to Neurology_, 1918, Chapter XX, on the "Functions of the Cerebrum". Stile's _Nervous System and Its Conservation_, Chapters X, XI and XII. {68} CHAPTER IV TENDENCIES TO REACTION HOW MOTIVES INFLUENCE BEHAVIOR, AND HOW THEY FIT INTO A PSYCHOLOGY WHICH SEEKS TO ANALYZE BEHAVIOR INTO REACTIONS. One advantage of basing our psychology on _reactions_ is that it keeps us "close to the ground", and prevents our discussions from sailing off into the clouds of picturesque but fanciful interpretation. Psychology is very apt to degenerate into a game of blowing bubbles, unless we pin ourselves down to hard-headed ways of thinking. The notion of a reaction is of great value here, just because it is so hard-headed and concrete. Whenever we have any human action before us for explanation, we have to ask what the stimulus is that arouses the individual to activity, and how he responds. Stimulus-response psychology is solid, and practical as well; for if it can establish the laws of reaction, so as to predict what response will be made to a given stimulus, and what stimulus can be depended on to arouse a desired response, it furnishes the "knowledge that is power". Perhaps no more suitable motto could be inscribed over the door of a psychological laboratory than these two words, "Stimulus-Response."