Page:Mind (New Series) Volume 12.djvu/307

PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 293 drained to its lowest ebb, i.e., their tonus is low, and there- fore that of the muscles is also low, body and limbs lie limp and flaccid. Such is the condition of the nervous system during deep sleep. When, as in the case described above, stimuli of moderate strength fall at short intervals upon the sensory ends of one or other set of afferent neurones of a nervous system in this condition, they may fail to evoke any response, whether as consciousness or as movement, until after a con- siderable number of repetitions. A familiar instance of this delayed response is the gradual waking of a sleeper by a gentle, oft-repeated knocking on his door. The earlier stimuli, which fail to produce any directly ascertainable effect, are yet not without effect, for it is by the summation of their effects that the nervous system is gradually brought into such a state that a further repetition of the stimulus may cause a reflex movement. A similar summation of effects has been shown to occur in the waking state in the case of sensory stimuli of minimal strength, as also in the isolated spinal cord. In the case in hand, it would seem that the neurin liberated in the auditory afferent neurones fails at first to discharge through any reflex lower-level path because it is absorbed and rendered latent or invisible, as we may say, in raising the tonus of those neurones, and also, no doubt, because at first part of it drains rapidly away into other systems of neurones with which they are connected on all sides, thus serving in some degree to raise the tonus of all or of a considerable mass of neurones of the lower-level systems. And this goes on until the tonus of the directly stimulated neurones is so far raised that a repetition of the stimulus determines a discharge through some purely reflex lower-level path into some group of muscles. Then at once a great step towards the waking state is made, for the contraction of the group of muscles, thus re- flexly excited, stimulates the afferent nerves of these muscles and their associated structures, tendons, joints, ligaments and so forth, and the movement, being the occasion of fric- tion and new relations of pressure between parts of the skin and the bed and bedclothes, causes indirectly stimulation of the sensory nerves of those parts of the skin. The reflexly produced movement therefore determines a rapid setting free of neurin in a considerable number of afferent neurones, the quantity being in proportion to the extent of the move- ments and the mass and number of the muscles involved. This increment of neurin will to some extent diffuse itself through the neurones of the lower -level systems and