EXCITATION AND INHIBITION 



383 



G' 



,F 



U..IH 



glycerol ; the action is perhaps more strictly physical, since it seems to depend on 

 the removal of water. 



Mechanical methods, such as pinching, tapping, shaking, or snipping with 

 scissors, are also effective stimuli, but obviously not capable of graduation. They 

 produce more or less injury, so that they are used chiefly as a means of control 

 when it is desired to exclude the possibility of a particular result obtained from 

 a nerve being due to escape of electrical current to neighbouring parts. A simple 

 apparatus for exciting nerves by dropping mercury upon them from different 

 heights is that due to Schiifer (1901), which 'is capable, to a certain degree, 

 of graduation in strength and rate of stimulus, and does not injure the nerve 

 to an appreciable extent. 



There are reasons for regarding all artificial modes of excitation as more or 

 less unlike the natural one coming from the cell body of which the nerve fibre is 

 a prolongation. It is possible, however, to exaggerate this difference ; as we 

 shall see later, the natural excitation is accompanied by waves of electrical 

 disturbance, similar 

 to those produqed 

 by artificial stimu- 

 lation, and the 

 optimal rate of in- 

 cidence of energy, 

 Waller's "char- 

 acteristic," is prob- 

 ably very close to 

 the natural one. 



Nerves can be 

 excited, then, by 

 many and various 

 forms of stimuli 

 and, supposing that 

 the nerve is in con- 

 nection with some 

 indicator, such as 

 a muscle, different 

 strengths of stimu- 

 lation are found to 

 produce different 

 degrees of contrac 

 tion. 



"All or No- 

 thing." Now the 



most careful experiments (see especially those by Keith Lucas (1909)) have 

 shown that the degrees of contraction of a muscle, that can be produced by 

 varying the strength of the excitation of the nerve to it, are not .as numerous 

 as the degrees of strength of the exciting stimulus, but take place in a series 

 of steps, which are no more numerous than the number of motor nerve fibres 

 supplying the muscle. This fact obviously indicates that the varying degrees 

 of contraction are due to differences in the number of muscle fibres in the 

 state of contraction at one time, and that each fibre can only be excited to its 

 maximal capacity or not at all. The fact had previously been established by 

 Bowditch (1871, p. 687), for the heart muscle excited by stimuli applied directly 

 to it. The possibility of its applying also to nerve itself was discussed by Gotch 

 (1902, p. 407), who came to the conclusion that the magnitude of the electrical 

 disturbance in nerve is conditioned far more, if not entirely, by the number of 

 fibres excited, than by possible differences of intensity of the disturbance in 

 individual fibres ; but the actual proof was not given until the work of Adrian 

 (1912). If we consider for a moment the case of a muscle being excited by shocks 

 of varying intensity applied to its nerve, it will be clear that all the nerve fibres 

 are not in exactly the same favourable position for receiving the stimulus, either 



B 



Fio. 104. ELECTRODES OF KEITH LUCAS. 



For the prevention of escape of current when immersed in saline. The points of 

 stimulation are at D, where the current density suddenly increases to a high 

 value. 



A, Side view. 



C, Loose cover, kept in position by an elastic band, G ; can be tipped up by 



pressing at H. 



D, Channel for nerve 



B, View from above. 



E and P, Spirals of fine platinum wire, connected with the wires at the right- 

 hand end. 



(Keith Lucas, 1913, 2.) 



