392 PRIXCIPLES OF GENERAL PHYSIOLOGY 



of as that of a concentration battery. As we shall see later (page 393 and Chapter 

 XXII.) it is only in a modified sense that this statement can be made. 



Under certain conditions it is possible to observe an electrical change in the opposite 

 direction, after cessation of the stimulus, both after tetanising and after single stimuli 

 (Garten, 1903, p. 59). This phenomenon, which was first noticed by Ewald Hering, was 

 correlated by him with the restitution or assimilation process, by which the excited nerve 

 returns to its original state. This view is in agreement with Bering's well-known theory 

 of assimilation, which will be discussed under the head of "inhibition." But it cannot 

 be said that we have, as yet, a satisfactory explanation of this positive electrical response. 

 It may, perhaps, have some connection with the stage of increased excitability of Adrian 

 and Lucas. According to Ve'szi (1912), however, the magnitude of an electrical response is 

 decreased in the stage of positivity after a prolonged tetanic excitation, but it would \*e 

 more to the point if the observations had been made on the actual propagated disturbance 

 itself. Cremer (1909) suggests that nerve in the resting state may be in a condition of 

 partial excitation or negativity, which disappears, of course, immediately after a disturbance, 

 and would give rise to the appearance of a stage of less negativity, that is, of positivity, until 

 the normal tonic stattt is re established. If the resting state is a balance of two opposite 

 processes, as is likely, there is some justification for Cremer's view, although no other 

 evidence has been brought forward in favour of the existence of such a tonic condition of 

 partial excitation. 



Certain support is given to the idea of the electro-positive response as representative of 

 a restitution process by the expei'iments of Sochor (1911), who found that, in a current of 

 nitrogen, this positive after-action is abolished much more rapidly than the negative excitatory 

 change is. The result might be interpreted as showing the necessit} 7 of oxygen for restitution, 

 but the fact that carbon dioxide was found to abolish the effect much more quickly than nitrogen 

 does suggests rather narcotic action. As Garten remarks, granting the necessity of oxygen 

 for the restoration process does not prove that it is an assimilation in the sense of Hering. 



Rate of Conduction. The fact that the nerve impulse takes time to traverse 

 a nerve was first definitely shown by Helmholtz (1850) and had an important 

 effect on views taken with regard to mental phenomena, since here was a nervous 

 process capable of numerical expression. 



The value obtained by Helmholtz for the frog was 29 m. per second. In 

 man, the latest value, obtained by Piper (1912, p. 52), is 123 m. per second. 

 This was obtained by the use of the string galvanometer and may be taken as a 

 very accurate one. 



All investigators agree that the rate is independent of the strength of the 

 stimulus. Narcotics, such as alcohol, slow the rate of conduction (Keith Lucas, 

 1913). 



The temperature coefficient as determined by the most accurate method, 

 that of Keith Lucas (1908), is T79 for 10. I have already pointed out (page 42) 

 that it is not permissible to draw conclusions as to whether a process is chemical 

 or physical from this value alone ; one may say this much, that a simple 

 chemical reaction with a temperature coefficient lower than 2, at ordinary 

 temperatures, is extremely rare, if not unknown. 



Changes in Permeability. When a nerve is cut across and electrodes placed 

 on the cut end and on the longitudinal surface, as in Macdonald's experiments 

 referred to above, there is found to be a difference of electrical potential between 

 these points, such that the cut end is negative to the normal surface. As we shall 

 see in Chapter XXII., the only satisfactory way of explaining such electrical 

 states is by the assumption of a membrane which is permeable to one of the 

 ions into which an electrolyte inside the axis cylinder is dissociated, but not 

 permeable to the oppositely charged fellow ion. We have, indeed, described 

 such a case in that of Congo-red, separated from water by a parchment-paper 

 membrane. A Helmholtz double layer is formed at the membrane or, as it is 

 sometimes expressed, the membrane is "polarised," having anions on one side. 

 cations on the other side. Suppose the membrane to become suddenly permeable 

 to both ions, what will happen ? Since the constraint preventing the two layers 

 of ions from freely mixing is removed, the ordered arrangement of ions ceases to 

 exist and, with it, the potential difference between the two sides of the membrane 

 and the possibility of polarisation. Now this is precisely what happens when a 

 nerve is put into a state of excitation. If the cut end is negative at rest and 

 the other electrode on the longitudinal surface becomes negative when excited, 

 as experiment shows, the potential difference is either greatly reduced or abolished, 



