140 COMPARATIVE PHYSIOLOGY 



To satisfy the above equation the factor /x^* on the left 

 must vanish for ^=00 , so that A is the smallest current which 

 will excite at all, i.e. A =60 in this experiment. Substituting 

 J, t^ and A for any two pairs, yu may be eliminated by dividing 

 one equation by the other : then becomes 0*375 ^^^ ^» ^Y 

 substitution, 0*909. The recalculation of i for corresponding 

 values of t by means of the formula gives the following : — 



? (observed) 175 115 91 76 68 64 61 60 



I (calculated) 178 115 91*2 75'9 68-9 65-1 62*4 60 



For instances of specific phenomena on which further 

 light is shed by Hill's analysis, the paper by Keith Lucas may 

 be consulted. It is of considerable interest to note that the 

 equation holds equally for direct excitation of muscle, so that 

 we may infer that the event which initiates the changes 

 described in an earlier chapter is of the same type in both 

 tissues. This is a fact which it is most important to bear in 

 mind when discussing the origin of the electrical variation 

 in muscle ; and in any attempt to unravel the anomaUes which 

 beset the study of the relation of electrolytes to muscular 

 activity {cf. pp. 21, 22). 



Minimal Duration of Stimulus for Excitation in Muscle (Lapicque). 



Gastrocnemius of Rana temporaria . . 

 Foot of snail, Helix pomatia . . 

 Adductor of claw of crab, Carcinus msenas 

 Mantle of sea-slug, Aplysia punctata 

 Ventricle of tortoise, Testudo graeca 



(b) Conduction. — ^When a nerve is stimulated the dis- 

 turbance set up at the seat of stimulation is propagated along 

 each neurone at a measurable rate. The rate of conduction 

 can be determined directly by observing the difference in the 

 latent period of muscular contraction, when a nerve-muscle 

 preparation is stimulated at points along the nerve separated 

 by a measured distance apart : the difference in the latent 

 period of contraction then represents the interval taken for 

 the nervous impulse to traverse this distance. This now 

 familiar class experiment was first performed by Helmholtz 

 (1852), before whose time it has been supposed that the nervous 

 impulse travelled at a rate comparable with the velocity of 



