Page:Mind (New Series) Volume 12.djvu/309

 PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 295 continue at each recurrence of the stimulus to discharge itself through purely reflex paths of level i., but instead penetrates to and excites a path of level ii., the sensory reflex level, so determining the undiscriminated sensation of noise ? Now we know from physiological experiments on animals deprived of the brain that the reflex paths of the cord, paths of level i., are of limited capacity, i.e., they are incapable of transmitting an excitation of more than a very moderate intensity. 1 For on stimulating any given set of afferent nerves it is impossible to obtain reflex contractions of more than a moderate force and, when the sensory stimulus is increased in strength beyond the degree that suffices to bring about such contractions of moderate force, there is no further increase in the force of the contractions of the group of muscles first excited, but other muscles are set in motion. It would seem that the excitation-process overflows its most direct reflex path, the path of forward conduction of lowest resistance, and so spreads laterally to other paths of the same level with which it is connected by synapses of a normal resistance higher than that of the synapses connecting to- gether the neurones of the direct path. In the intact nervous system the spread or overflow of the excitation-process would seem to take place upwards into paths of higher level rather than laterally into other paths of the same level, and in the case in hand, the spreading of the excitation-process from paths of level i. to the paths of level ii., in which it determines the sensation of sound, may be regarded as due to overflow of the excess of neurin that cannot escape to the muscles by the lowest level path of limited conduction-capacity. Here, as we shall find in other instances, a hydro-dynamic analogy will serve to make clear the state of affairs that seems to obtain. Imagine a vessel having a single large inflow pipe and a series of smaller discharge pipes all opening about its base and leading from it to other vessels, and imagine these discharge pipes to be controlled by spring valves of different degrees of resistance. This vessel standing almost empty may represent the central nervous system during deep sleep; water dribbles into it so slowly that a small leakage about the valves prevents the accumulation of sufficient pressure to open the valve of lowest resistance. Then comes a series of gushes of water through the inflow pipe. At first the result is merely a rise of the level of the water in the vessel and a correspondingly 1 Sherrington, op. cit., p. 831.