PHYSIOLOGY OF NERVE 279 



is invisible under ordinary circumstances, the black precipitate is 

 invariably obtained in the axis cylinder at every injured spot. 

 Both neutral red and toluidine blue are readily precipitable by 

 KC1, and the granules stained by these dyes are probably caused 

 by the diffusion of this electrolyte. 



(8) The theory deduced from these observations is that the axis 

 cylinder is surrounded by an impervious wall except at Ranvier's 

 nodes, and is filled with a colloidal solution with potassium chlorides 

 adsorbed on the surface of the colloid molecules. As the salt is 

 attached in this manner it is supposed to be masked, so that it 

 does not give the potassium reaction, and does not exert osmotic 

 pressure, but can still conduct electricity. Injury (and to a less 

 extent excitation) causes these colloid molecules to aggregate and 

 become larger ( 62 ). Their surface is therefore diminished, and the 

 electrolytes are set free into simple aqueous solution. From the 

 nature of the precipitates by Macallum's methods the electrolyte 

 is inferred to be KC1, and the neutral red and toluidine blue 

 precipitates confirm this. The concentration of salt is from the 

 " concentration law " about 10 per cent. This theory is therefore 

 a purely physical one, and has a close relationship to the theory 

 propounded by Sutherland ( 63 ). Both these theories give an 

 intelligible explanation of what has always been a difficulty, 

 namely, the peculiar rate of transmission of the nervous 

 impulse ( M> **). 1 



While this Macdonald- Sutherland hypothesis would explain 

 the observations just referred to, there are still many difficul- 

 ties to be faced, some of which are of a formidable character. 

 Potassium chloride is supposed to be masked in such a manner 

 that it exerts no osmotic pressure, yet that the ions can move so 

 as to conduct electricity. It is supposed to be present as KC1, 

 yet Cl appears at once on the use of AgN0 3 , while the K is masked 

 and only appears when there is an injury a return by a back door 

 to the "alteration-theory" that had been abandoned. The rate 

 of diffusion of the + K ion is -00066, of the - Cl ion is -00069, 



1 The most recent determinations ( M ) give the rate at 30 metres per secoml in the 

 frog's nerves at ordinary temperatures, 65 metres per second at 35 C. Helmholtz 

 gave 33 metres ]>er second for human nerve (which would make this process fat 

 man at 36 occur at the same rate as in the frog at 15 C.) But as Tigerstedt 

 points out, the arm of this subject was cooled by a casing of plaster of Paris, and 

 so this value is too low. Waller gives 50 metres per second ; Alcock as the mean 

 of forty experiments on man gave 66 metres per second. 



