90 FUNCTIONAL INERTIA 



well known to experimental psychologists. It is, 

 in short, a post-stimulant rhythm : the two opposed 

 metabolic phases alternating by their inertias. 

 The physical analogy is the oscillations of the 

 rocking-stone after you have given it its initial 

 push, the swingings of the swing-door after a person 

 has passed through. 



This phasic alternation of states due to a single 

 stimulus is clearly comparable with the slow, 

 rhythmic tremors in muscle which result from the 

 application of a single stimulus as described in 

 chap. ii. (tremor from a " single " chemical or 

 mechanical stimulation). There is a similar phasic 

 alternation of states in retino-cerebral protoplasm 

 at a certain rhythm, the so-called "periodic dark- 

 ening in retinal rivalry." I * showed that the in- 

 tervals of comparative light between the dark periods 

 were of an average duration of about ten seconds. 



It is a matter of everyday experience that it is 

 not only in the sense of vision that we have after- 

 sensations. We : ' feel >: contacts, pressures, pains 

 long after the irritants responsible for the sensations 

 have been removed. We o ten cannot tell whether 

 the offending thorn is or is not in the finger-tip, 

 and the pain of the mote in the eye continues long 

 after the gritty particle has been removed. Mus- 

 cular and articular sensations are very prone to 

 have after-effects in consciousness : all these are 

 examples of katabolic inertia in sense-organs. 



* D. F. Harris, " On Periodic Darkening in Retinal Rivalry." 

 Proc. Phys, Soc., Nov. 8, 1902. 



