EXCITATION AND INHIBITION 395 



k 

 so that a convenient measure of is log 6. Lucas calculates the value of log (9 



a 2 



for various excitable tissues from his own experimental data, to which reference 

 will be made presently. It may be noted that, in all probability, this quantity is 

 essentially the same thing as Waller's " characteristic." In fact, any considerable 

 alteration in the shape of the curve correlating excitation with stimulus is due to 

 changes in 6, since A. and //, are not so readily affected, as may be seen by the 

 consideration of what they mean. 



A is the smallest current that will excite at all, however long it be continued. 

 If t becomes very large, 1 \iQ l becomes unity, and i is equal to A.. 



fi refers only to the distance from the membrane at which the change of 

 concentration is being considered, and will not be liable to important changes. 



C enters into A. but not into fj. or 6. 



I fear that this necessarily brief account gives but an imperfect view of this important 

 work ; the original papers of Hill and Lucas should be consulted. 



There is another point to which a little attention must be given. The fact, that on closing 

 a current through a nerve, the excitation wave starts from the cathode shows that the cations 

 are the important agents. How then is the fact of excitation at the anode, which occurs on 

 breaking the circuit, to be explained? It is pointed out by Keith Lucas- (1912, p. 519) that 

 "the one feature which is common to the cathode when the current is made, and the anode 

 when the current has just ceased to flow, is an increase of the concentration of cations above 

 the value which occurred at each of these points immediately before." At the anode, however, 

 the concentration of cations only rises to its normal level by diffusion, after having been 

 decreased: Nernst and A. V. Hill give what is essentially the same explanation on the 

 ground of the " combination" of ions with some substance in the nerve. During the passage 

 of the current, the diminished concentration of cations at the anode results in a different 

 equilibrium in the reversible "compound," or adsorption, between the ions and the assumed 

 substance. When the current ceases to flow, there is a sudden concentration of cations in 

 the system in excess of that with which it was previously in equilibrium ; a condition which 

 is the same as that at the cathode when the current is first established. Thus the excitation 

 at the anode and the failure of slowly rising currents to excite appear to depend on the same 

 conditions. It will be clear that more experimental work is required before the question 

 can be decided. 



A word is perhaps necessary as to the position of the membranes about which 

 we have been speaking. There is no evidence of the existence of transverse 

 membranes and, in fact, their assumption would raise considerable difficulties. It 

 seems most likely that it is the cell membrane covering the axis cylinder that is 

 concerned. This axis cylinder, as we shall see, is a part of a long cell, the 

 "neurone," which includes the cell body with its nucleus, etc. Bernstein (1902), 

 indeed, put forward the view that this surface membrane is the structure responsible 

 for the electrical phenomena of nerve and muscle. 



As already pointed out in Chapter III., it is not necessary to suppose that 

 this membrane is in the form of a distinct film, or separate phase, which could be 

 picked off. Being formed by condensation of constituents present in either or 

 both of the two phases, cell protoplasm and surrounding liquid, of which it is the 

 contact surface, it may be looked upon as belonging, in a certain sense, to both. 

 We may notice that Mines (1912, p. 230) comes to similar conclusions in explana- 

 tion of the results of his work on the effect of ions on the electrical charge of 

 surfaces. 



Our consideration of Nernst's theory of excitation may be best concluded with 

 the words used by Keith Lucas (1912, p. 524): "It is not a complete theory, 

 ready for acceptance, but it is an indispensable guide to the strengthening of our 

 experimental data, and so to the ultimate elaboration of a hypothesis, which shall 

 be free from those difficulties which are at present so obvious." The nature of the 

 local excitatory process is especially in need of further investigation. It seems, 

 however, from what has already been done, that the final solution will be on the 

 lines of that proposed by Nernst. 



Structure of Nerves. The fact is familiar that some nerve fibres are encased in 

 a sheath of considerable thickness. This consists chiefly of lecithin and similar 

 lipoids, containing the radicles of unsaturated fatty acids and, therefore, staining 

 with osmic acid. The function of this " medullary sheath " is problematical. It is 

 obviously not necessary as an insulator, since many nerve fibres are devoid of it, 



