30 FUNCTIONAL INERTIA 



are, in fact, internal as well as external conditions 

 of growth, and the former are the more important 

 for they are really the determining conditions. It is 

 with the organic cell and its conditions as it is with 

 the individual and his circumstances ; the latter 

 may greatly modify character and are necessary for 

 development, but the essential fact, which deter- 

 mines the limit of the modifying power of circum- 

 stances, is the nature implanted in the individual." 

 This is a most interesting presentation of what I 

 mean by functional inertia as limit-setting. 



Rhythms. I regard all natural or spontaneous 

 rhythms of centres, e.g., the respiratory, the vaso- 

 motor,&c., as examples of phenomena in which func- 

 tional inertia is a causal factor. Not, of course, that the 

 rhythm in the intact animal and all the characteristics 

 of the discharges from the respiratory centre are 

 altogether inertial ; on the contrary, the affectability 

 of the centre is conspicuous as regards voluntary 

 impulses, heated blood, CO 9 , poisons, &c., but its 

 functional inertia is none the less obvious. For 

 instance, its uncontrollability by voluntary impulses 

 is significant : we can only, for a minute or two, 

 cease to breathe (voluntary apncea), and conversely 

 we can accelerate the pace of breathing only to a 

 certain rate beyond which it is impossible to go. 

 No doubt cerebral refractory period as above alluded 

 to is a factor limiting the rate of output of cortical 

 impulses ; but the rhythm of the respiratory centre 

 is its own, voluntary impulses are comparatively 

 powerless over it, so that a more detailed examina- 



