Page:Mind (New Series) Volume 12.djvu/315

PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 301 liberation of neurin in these neurones, a liberation which perhaps never altogether ceases in any part, but which is- much accelerated in any groups of which the peripheral ends are in any way stimulated ; and the pressure is high when large groups are subject to stimulation, low when peripheral stimuli are few and feeble. From this reservoir neurin escapes continually into efferent tracts by channels which vary from moment to moment of waking life, chan- nels which form therefore an ever-changing tridimensional neuergic pattern. 1 And these channels are so organised and interconnected to form systems and subsystems that, when during the activity of any partial system, a rise of pressure of neurin in the reservoir is brought about in any way, this increase of pressure leads, not to an opening of other inde- pendent channels, but rather to a spreading of the excitement through other parts of the system already partially active. This mode of spreading of the excitation-process is seen in its simplest form in the spinal cord after ablation of the brain. A gentle stimulus will cause a reflex contraction of a small group of muscles, and increase of the strength of the stimulus applied to the same nerve endings w T ill then cause other groups of muscles to contract, the movements de- termined being, not irregular and haphazard, but co-ordinated and, in some degree at least, purposeful, i.e., adapted to the carrying out of some definite act such as walking or swimming, and this is true " even when the stimulus, instead of being restricted to a narrow sensory area or path, affects simultaneously large surfaces or wide channels". 2 The transition from sleep to waking brought about by the re- petition of an auditory stimulus, as we have traced it above,, is but a more complicated example of this spreading of the excitation-process through organised systems of neurones, the stimulus being in this case largely re-enforced and complicated by the internal stimuli arising in the viscera and skeletal muscles, and in this case the spreading involves conduction- paths, not only of level i. to which it is confined in the isolated spinal cord, but ultimately of all levels up to the highest. This view of the mode of functioning of the brain is fully in harmony with that view of the functions of the spinal cord to which physiologists have been led by the rapid growth 1 1 adopt the phrase ' neuergic pattern ' suggested by Mr. H. R. Marshall in his article in No. 44 of this journal. 2 Schafer's Textbook of Physiology, Prof. Sherrington's article ' The Spinal Cord,' pp. 843, 844.