Quoted by T. Quick, 'Disciplining Physiological Psychology: Cinematographs as Epistemic Devices, 1897-1922', Science in Context 30 (4), pp. 423-474.
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' McDougall conveyed his findings in terms that went directly against the mechanistic tendencies of late nineteenth century British physiological physiology (Young, 1990). He began his career by rejecting the widely-held proposition amongst British physiological psychologists that mental faculties could be identified with one or another anatomical part of the brain or nervous system. Instead, he proposed, awareness was ‘immediately determined' by 'neural processes' as a whole (McDougall 1898, 15, 365). Thus an increase in nervous excitability could be attributed to a 'diminution of the resistance offered by that delicate and complex inter-cellular substance which, as it seems to me, we have to regard as the seat of the psycho-physical processes.' This substance, according to McDougall, was a vital fluid which he termed 'neurin.' On the stimulation of a sense, its associated nerves ‘[produce] neurin far more rapidly than it can escape by leakage’ into adjoining cells. Build-ups of neurin overcame the resistance of synaptic barriers, causing chains of like reactions through the whole system (McDougall 1901b, 590, 614 and 616).'

 

Relevant passages from McDougall:

 

'It is well known that the simple reaction-time may be somewhat shortened by practice until it becomes a purely automatic or reflex act, and in a similar manner reactions involving more complex mental processes, such as discrimination and choice, may on frequent repetition become more and more automatic in character, i.e., they are performed more easily and regularly and with less and less clear conscious accompaniment, until they, too, resemble rather a reflex action, psychical activity being reduced to a minimum. And parallel with this progressive loss of psychic accompaniment goes a progressive shortening of the time lost at the synapses.

This shortening of the lost time at the synapses by repetition of the passage of the excitation across them may be best conceived as due to a diminution of the resistance offered by that delicate and complex inter-cellular substance which, as it seems to me, we have to regard as the seat of the psycho-physical processes.' (590)

 

'The idea of a certain charging of neurones has then been entertained by authors of high authority, but I have not been able to discover that anything has been said as to the nature of that something with which the neurones are supposed to be charged, although an effective working conception of its nature must be of the highest importance if there be any truth in this view. I think that for the present it may be best conceived as a fluid, and I propose that this fluid shall be called "neurin." [note: ' I am indebted for the suggestion of this word, the adjective form of νευρoν, to my friend Prof. G.C. Moore Smith.] It might of course be called the nervous fluid, or nervous energy, or "animal-spirits," or a very subtle ether, but the name I suggest is preferable because it implies nothing beyond the fact that the thing named has to do with nerves.' (614)

 

'When any adequate stimulus falls upon a sense-organ the sensory neurones affected by it are excited to the production of much larger quantities of neurin per unit of time. Let us take the case of a ray of light falling upon the retina and there setting free a substance that chemically stimulates the nerve-endings, and let us call the neurones forming a conduction-path leading from the sense-organ to any motor neurone, a, b, c, d, &c., each one, as b, being efferent to its predecessor in the chain a and afferent to its successor c. Then a, continuously stimulated by the exciting substance in the retina, produces neurin far more rapidly than it can escape by leakage into b, so that in a small fraction of a second the potential of the charge in a will rise to the level of the threshold of the synapsis a-b. A sudden discharge of neurin then takes place across the synapsis from a to b, and its sudden arrival in b acts upon it as a stimulus to the rapid production of more neurin, so that b then has a double charge of neurin and therefore rapidly discharges into c; and so the process of discharge and stimulation of afferent into efferent neurone is repeated, until the last of the chain discharges itself into the muscle and brings about contraction. To go back now to the sensory neurone a; as it discharges, the exciting substance in the retina continues to stimulate it to rapid production of neurin, and therefore, in a very short time after the first discharge, the potential of its charge rises again to discharging point, and so once more the whole chain of neurones is charged, stimulated and discharged in turn, and a second impulse flows out to the muscle, and so on again and again.' (616)