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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'In 1891, John Smyth Macdonald had been appointed a Holt Fellow at Liverpool under the then-holder of the professorial chair, Francis Gotch. By 1899, he had attained the rank of Senior Lecturer, and had moved away from Gotch's emphasis on relating galvanometric changes to changes in body temperature (O'Connor 1991, 335-336). Instead, Macdonald began to identify electrical variability with interactions between nerve cells and their environments. Casting the production of current following the lesion of a nerve in terms of an osmotic 'concentration cell', he suggested that the rapidity of transmission of electricity through a nerve was not due to its internal polarization alone, but also to its compartmentalization in relation to its immediate environment (Macdonald 1900). As he reported to the Royal Society (via Sherrington), recent histological staining techniques revealed the presence of ion-carrying potassium granules in the fluids surrounding the so-called 'nodes of Ranvier' (the myelin sheath-free points that could be detected along the lengths of nerves). Whether large or small amounts of these granules could be observed at any one time correlated with the detectability of electrical charge. Macdonald suggested that nervous current was therefore produced not within a single, undifferentiated cell, but by interaction between a series of 'relays placed at every point of the nerve to ensure the continuous propagation of the excited state' (Macdonald 1905, 323-324). Significantly, these relays presented an explanation for the both the variability and the unidirectionality of nerve conductivity (Macdonald 1905, 340). Like his Cambridge contemporary William Bate Hardy, Macdonald also doubted the existence of an autonomously-acting protoplasmic network extending outwards from nerve cells. He proposed in its place an explanation of the variability of nerve cell conductivity which appealed to the changing proportions of ion-carrying compounds in the fluids within and surrounding cells. Inhibition of individual nerve cells was thus due to 'a reversible change during which electrolytes are set free into a state of simple solution, and are then recovered from this state back into their original condition' (Macdonald 1905, 330-333). The vibrations detected by protoplasm theorists were not the products of organic fibrils connecting inner life with its external conditions, but changes in the ionic concentrations of fluids behaving according to mathematical laws of osmosis.'

 

Relevant passages from Macdonald:

 

'I made, a few years ago, a careful and detailed inquiry into the phenomenon known as the injury-current. Taking the supposed greater conductivity of the internal solution as a main guide to the choice of appropriate methods of experiment, I examined the possibility that the injury-current was due to the diffusion from this more concentrated solution first permitted by the circumstances of injury. Electrodes placed upon the transverse-cut end and the longitudinal surface respectively were treated theoretically as if the first was in contact with the internal solution, the second in contact with the external solution, of a hypothetical single nerve-fibre represented by the nerve-trunk. Modifying the value of the external solution, I found that a very precise modification of the value of the electromotive force thus measured followed each variation in the external solution. The relation thus found between the value of the external solution and that of the electrical phenomenon I called the "Concentration Law." [note: 'J.S. Macdonald, Proc. Roy. Soc.,' vol. 67, pp. 325-328.'] Its nature was such as to confirm the opinion that in this arrangement I was dealing with a liquid concentration cell, but if so it was evidently a special case in which the conditions present were curiously simple. Judging from the numerical statement of the law alone, this simplicity indicated a relation between the solutions present such that the internal solution must be supposed to be ten times more concentrated than the external solution. Such a difference of concentration as this seemed on other grounds improbable, and the necessity for such a conclusion entails a critical examination of the kind of concentration cell supposed to be thus formed and examined in the nerve. The hypothetical contents of this cell are (1) the internal solution, (2) a membrane of undetermined but presumably limited permeability, (3) the external solution. Knowing nothing as to the probable effect of the interposition of this membrane, it seemed best to seek for the case in which the value of the cell was reduced to zero. In this case it seemed fair to infer that the peculiarities of the membrane, supposing its permeability to be the same in both directions, were eliminated, and that the cell contents were now: (1) a solution, (2) a membrane, (3) another solution similar to the first. Having sought experimentally for such instances, [note: '....] and for instances in which the value of the cell was reversed, the general conclusion arrived at remained as before. The hypothesis being correct, the value of the internal solution was ten times greater the decinormal solution (lymph) found bathing the outer surface of the nerve.' (323-324)

 

'Let us then suppose that in all these events we have a modified representation of the process of excitation and its consequences upon neighbouring segments of the fibre; a reversible change during which electrolytes are set free into a state of simple solution, and are then recovered from this state back into their original condition. Here truly there is the appearance and the withdrawal of a source of energy, a relay placed at every point of the nerve to ensure the continuous propagation of the excited state. Inorganic salts are set free to move; they move ever so little; the next segment of the fibre is charged as a consequence (let us say negatively); the colloidal state of the fibre is thus changed from its condition of equilibrium; as a result a setting free of electrolytes at this new point and the propagation of the process; in the meantime the communication of the negative charge to the onward segment has left the original segment positively charged, the state of colloidal equilibrium is thereby reproduced, and the last involved segment is brought to a condition of rest. The idea of such a progressive fall from and return to a condition of colloidal equilibrium has already been advanced in explanation of. this phenomenon, the novelty is an introduction of a new source of energy with the justification of actual observation.' (330)

 

'in pursuit of the present histological investigation I have used dyes suitable for a revelation of neuro-fibrils. I have, however, never seen these neuro-fibrils, except under such circumstances as demanded their criticism as possible artefacts, as for example after the use of fixatives, or in short stretches of nerve-fibres obviously suffering from the results of excitation. Yet the only peculiarity marking the use of these dyes in this investigation has been the fact, that they have been for the most part presented to the fibres in solutions of such salts as were least likely - as previous observations had shown-to destroy the "living state" of the fibres.

In place of these neuro-fibrils, however, the granules spoken of have made an absolutely constant appearance. Such granules I have observed forming at the cut ends of the fibres, but also in portions of the fibres remote from positions of injury. Such granules may, even in portions of intact fibres, be observed to increase and again to diminish in size. They may be seen at any one time to be of different sizes and in different number in neighbouring portions of the axis-cylinder. There is, therefore, no temptation to consider them as permanent units of structure, and yet, after the admission of "fixatives," these very granules are joined together in lines to form neuro-fibrils. The granules not being permanent units of structure, it is,therefore logical to conclude that the fibrils thus formed by their agglutination also are not permanent units of structure. The fibrils then may be said to be observed under conditions suggestive of artificial formation. This fact also has to be taken with the complementary fact, that artefacts of just this kind might be expected to appear within the nerve-fibre.' (332-333)

 

'Considering the process in terms of the state of electrical potential, the forward movement of the negative charge is ensured, or rather its backward transmission is prevented, by the pursuing positive charge. Considered in terms of osmotic pressure the facts seem somewhat simpler, since it may or must be considered that the rise and fall of osmotic pressure respectively lag a definite time behind the appearance of the causes producing them. It is also conceivable that the rise, the release, is a more sudden phenomenon than the fall, the recall. The facts then arrange themselves in this manner. The rise of pressure in B occurs at a time when there is still a region of increased pressure behind it at A, but a region of normal low pressure in front of it at C. The tendency is therefore always forwards. Arranged in these terms the wave of the nervous impulse can be described as a double oscillation in the value of osmotic pressure, the front a rise, the trough a fall.' (340)