io FUNCTIONAL INERTIA 



This point is somewhat important because the 

 term inertia being etymologically related to " inert " 

 may in time come to be used in a functional sense 

 as denoting only those cases of inactivity after 

 a stimulus physiological insusceptibility in fact. 

 It is obviously only this form of inertia which is 

 strictly the physiological counterpart of affectability. 

 This would be an unfortunate limitation of the 

 usefulness of the term on the one hand, and of the 

 symmetrical correspondences with the phenomena 

 of affectability on the other. 



Take the case of the heart : through its possession 

 of affectability it can be either accelerated or in- 

 hibited, that is, inactivity can be the result of 

 stimulus acting on an affectable organ ; but, 

 conversely, through its possessing functional inertia, 

 the heart will for a time disregard a stimulus 

 arriving during its refractory period, or, being 

 stimulated by induction shocks of, say, 50 a second, 

 will perform its rhythmic movements at a vastly 

 slower rhythm than that, one having no causal 

 relation to that of the stimuli whatever. In the 

 same way on being heated, the rapid action cannot 

 be forced beyond a certain rate (1.5 per second in 

 the frog, 6 per second maximum in the rabbit). This 

 idio-muscular activity, this activity independent 

 of stimulation, is inertial. Thus though the heart 

 possess affectability, there are the most rigid limits 

 set to its expression, limits which prevent its being 

 tetanised, being hurried beyond a certain pace, 

 limits also to the energy of its systole. Here then 



