Page:Mind (New Series) Volume 12.djvu/306

292 W. MCDOUGALL : 3. The excitement of complex and well-developed mental systems connected with the necessity of expelling an intruder and defending my property. 4. An emotional excitement due to the affective tone of the ideas aroused and having its basis in the instinct of defence of self and belongings. This series of states of increasing awakeness and of rising attention must now be described in terms of the physiological scheme that I have proposed in the first part of this essay. While I lie with closed eyes in a warm soft bed in a dark and quiet room, the sum of physical stimuli affecting the sensory endings of the afferent nerves is very small. The amount of ' neurin ' generated in the afferent neurones per unit of time is therefore so small that it is insufficient to make good the loss by leakage into the muscular system that is continually going on in all parts of the nervous system. During the day fatigue-products have been accumulating in the blood, and these act upon all the synapses, the junctions of neurones, of the nervous system, to increase their resistance, to raise their thresholds, just as do ether and chloroform when present in the blood, and, like these drugs, they act with most effect upon the least organised synapses, those of the highest levels. 1 Hence there is low pressure of neurin in all the afferent neurones, and a high degree of resistance in all the paths by which it tends to escape into the efferent neurones and the muscles. The pressure of neurin is therefore in- sufficient to keep open any path or paths through the higher levels, or indeed any paths but those most thoroughly organised chains of neurones that constitute the reflex paths through which the activity of the fundamentally vital viscera, the heart and lungs, is regulated. All neurones, with the exception of these last mentioned, are therefore as nearly at rest as living cells can be, their metabolism, or rather their katabolism, is minimal, and therefore the amount of neurin set free in them is minimal, and, excepting these few funda- mental reflex arcs, there are no paths discharging into efferent neurones ; the skeletal musculature is therefore motionless and there is no consciousness. Further, the amount of neurin set free in unit of time is less than can escape by leakage, hence the charges of neurin in the individual neurones is 1 Of these two conditions, which I take to be the two principal deter- minants of normal sleep, the latter is probably the more important during the early hours of the night and the former during the later hours, as fatigue passes off.