EXCITATION AND INHIBITION 391 



demonstrated. It would be desirable to test the effect of a simple vacuum, although the 

 experimental difficulties of exposing the nerve to the vacuum, while allowing access of oxygen 

 to the muscle, seem insuperable ; the use of the electrical change as indicator would be possible, 

 though less satisfactory. 



In any case, it will have been abundantly clear, from the various facts given 

 in previous pages of this book, that the food requirements of cell machinery in 

 general are extremely small ; food is required to afford energy for the numerous 

 physiological processes, and this is done by oxidation under the action of the cell 

 mechanisms. An infinitesimal amount of oxygen may actually be necessary in such 

 a process as that of conduction in nerve, where there is practically no energy change 

 involved. We shall meet with some further facts bearing on the question presently. 



Summation and Facilitation. In the experiments of Adrian and Lucas 

 (1912), from which the curve of Fig. 108 (p. 389) was constructed, we see evidence 

 that the refractory period is succeeded by one of slightly increased excitability, 

 in which a less strength of stimulus is required to excite a propagated disturbance. 

 This phenomenon is met with in any part of the nerve after the passage of a 

 disturbance. There is, however, another form of increased excitability, shown at 

 the point of excitation only (Adrian and Lucas, 1912, pp. 69-72). The first effect 

 of a stimulus at its place of application, is a process which, on Nernst's theory of 

 excitation (see later, page 393), we should interpret as a concentration of ions against 

 a semipermeable membrane. Now this happens even when the stimulus is too 

 weak to set up a propagated disturbance. It is shown by the fact that a second 

 stimulus, also inadequate by itself, following the first after about O0008 second, 

 sets up a propagated disturbance. It is clear that the first stimulus has left behind 

 it a change of some kind which persists for a measurable time, and is added on to 

 that produced by the second stimulus when this is put in. The propagated 

 disturbance, on the other hand, as we have seen, leaves behind it a stage of 

 diminished excitability at this interval after a previous stimulus ; so that there are 

 evidently two factors involved in the excitatory process, one t)f which is confined 

 to the point of application of the stimulus. The importance of this fact for the 

 theory of excitation will be seen presently. 



A narcotic, such as alcohol, does not prolong the time required for recovery, the refractory 

 state, even at the stage in which the disturbance is conducted with considerable decrement 

 and slowing of rate of conduction (Keith Lucas, 1913). This fact suggests that the recovery 

 process is not of the nature of a chemical, oxidation process under the control of living 

 protoplasm. In fact, it seems to exclude the view of the necessity of oxygen for recovery, 

 as an oxidative process. 



The Electrical Response. We have seen that the disturbance in nerve is 

 associated with a temporary state of negativity. The meaning of this will be 

 discussed presently. It is held by some observers that the two processes are not 

 necessarily connected, but Keith Lucas (1912, pp. 502-508) shows that none of 

 their experimental results are free from objection and that there is no reason for 

 doubting the identity of the two. On the other hand, he points out that more 

 strict proof is desirable before definitely accepting the electrical change as a basis 

 for a physico-chemical explanation of the excitatory process. In the case of 

 muscle we shall find evidence that, although the excitatory process and the 

 electrical response may be the same phenomenon, yet both these may be present 

 without a contractile response, which is, so to speak, an additional process, whose 

 conditions of appearance may be absent. 



Macdonald (1902) gives good reasons for regarding the potential difference 

 between cut end and longitudinal surface of nerve as due to the high concentration 

 of inorganic salts in the axis cylinder, in connection with the presence of 

 membranes impermeable to these. This potential difference was found to be 

 abolished by a certain concentration of ions outside the nerve, 7 to 10 per cent, of 

 potassium chloride being necessary. The salts of the nerve cannot be regarded as 

 combined chemically and split off on excitation, but must be adsorbed on surfaces 

 of colloids in the axis cylinder. In this way, as is pointed put, these salts are 

 prevented from manifesting their great osmotic pressure. The difficulty, however, 

 still exists, since ions, in order to give the necessary Helmholtz double layer, must 

 be free and not adsorbed. The electrical state of nerve and muscle is often spoken 



