14 
PHYSIOLOGY: FORBES AND RAPPLEYE 
to a motor nerve can be raised as high as 300 per second and a corre- 
sponding action current rhythm will still appear in the innervated mus- 
cle. If the muscle can respond separately to as many as 300 nerve 
impulses per second, he argues that it will not respond as slowly 
as fifty times per second unless it is stimulated only fifty times per 
second. 
An explanation of the paradox is found in an analysis of the series 
of events involved in the excitation of a muscle through its nerve, in 
the light of certain functional properties shown by Lucas and Adrian 
to be essentially common to the two tissues. In either tissue a re- 
sponse is followed by a brief 'refractory period,' at first absolute when 
the tissue cannot be excited, then relative when the tissue can only be 
excited by a stronger stimulus than usual, and when the response is of 
subnormal magnitude. During the relative refractory period the ex- 
citabiHty and the magnitude of response both return gradually to nor- 
mal. At all times, including the refractory period, the magnitude of 
response in nerve is independent of the strength of stimulus and de- 
pendent only on the state of the tissue. The refractory period in nerve, 
though similar to that in muscle is much shorter. 
If we assume that the stimulating effect of the nerve impulse on 
muscle is correlated with its magnitude as shown by other criteria, 
then we find no confhct between the abiHty of muscle to respond sepa- 
rately to 300 impulses per second and its response at a slower frequency 
to nerve impulses of a higher frequency. For as each nerve impulse 
is made to come earlier in the nerve's refractory period which was due 
to the preceding impulse, its magnitude is reduced till, coming at so 
early a stage in the refractory period of the muscle, it fails to excite it. 
One or more nerve impulses will be ineffective following each that is 
effective. The higher the nerve frequency the weaker will be each 
impulse and the more must the muscle recover to be again excited. Thus 
after the critical frequency is reached, the higher the nerve frequency 
the slower the muscle rhythm. In this respect we depart from Buchan- 
an's view to the extent of supposing an indirect dependence of muscle 
rhythm upon nerve rhythm. 
In view of this analysis, since the nerve rhythm evidently does not 
correspond with the muscle rhythm, it must have a higher frequency 
than the critical value (300 or more per second) below which the mus- 
cle can follow the nerve rhythm; otherwise the muscle rhythm would 
correspond with it. 
Inferences based on these considerations together with observations 
of Lucas, Adrian and Garten, lead us to suppose that the normal fre- 
