action an efferent stimulus follows an afferent promptly and quite mechanically. It is only where efferent stimuli do not immediately become entirely transmuted into outwardly moving impulses that mental action comes in and an animal feels. There appears to be a direct relation between sensation and motion. For instance, the shrieks and other instinctive violent motions produced by pain, "shunt off" a certain amount of nervous impression that would otherwise register itself as additional painful sensation. Similarly most women and children understand the comfort of a "good cry," and its benefit in shifting off a disagreeable mental state.
Text Book of Biology, Part 1: Vertebrata · The Wunder Library — complete classics, free to read, with narration.