Howard C. Warren and Leonard Carmichael, Elements of Human Psychology, Houghton Mifflin, 1930

SEEING AND ACTING

St. = stimulus; light waves from the ball. R = receptors; the eyes. S = sensory nerves, running from eyes to brain. C = center of nervous system; the brain. M = motor nerves, running from brain to arm. E = effectors or motor organs; muscles of arm and hand.