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1. The rapidly exchanging 24Na fraction was found to account for approximately a third of the sodium contained in desheathed terminal abdominal ganglia which had been previously made radioactive by the injection of labelled ions into the haemolymph. 2. This rapidly exchanging sodium was associated with the extracellular spaces which, with the aid of 14C-inulin, were shown to contain 18.2% of the ganglion water. Using this figure it was possible to calculate that the extracellular sodium concentration exceeded that in the haemolymph by a factor of 1.8 and was 2.5 times greater than that in the cellular fraction of the ganglion. 3. Experiments using 22Na, 42K, 45Ca, 36Cl and 3HOH showed that the concentrations of the ions in the rapidly exchanging extracellular fractions of isolated abdominal nerve cords were different from those of the external medium. The three cations were maintained at considerably higher concentrations in the extracellular spaces, chloride ions being present at a much lower concentration than in the external solution. 4. It was concluded that the ions were distributed between the extracellular spaces and the external solution according to a Donnan equilibrium. The evidence would seem to indicate that a significant proportion of the free anion groups in the extracellular spaces may be those of protein molecules in solution. An appreciable portion of the cations may also, however, be associated with the free anion groups of structural elements such as collagen and mucopolysaccharides. 5. It is suggested that the relatively rapid depolarization observed in desheathed preparations, as compared with intact nerves, in conditions of high external potassium concentration may result from the effects of the disruption of the Donnan equilibrium as well as to any properties of the nerve sheath as a diffusion barrier. 6. It was shown that because of their positive charges the diffusion of the cations in the extracellular spaces occurred more slowly than would be expected on the basis of their free-diffusion constants. The efflux of 14C-inulin also occurred more slowly than would be expected from its free diffusion, an effect probably caused by the passage of the polysaccharide molecules through relatively small spaces in the extracellular system.

The Distribution and Exchange of Some Ions and Molecules in the Central Nervous System of Periplaneta Americana L.