518 



COMPARATIVE PHYSIOLOGY. 



that under constant stimulation fatigue would soon result and 

 perceptions be imperfect. Hence it happens that all the senses 



fail when exercised, 

 Q even for but a short 



period, without change 

 of stimulus leading to 

 alteration of condition 

 in the central cells. 

 The change need not 

 be one of entire rest, 

 but merely a new form 

 of exercise. Hence the 

 freshness experienced 

 by a change of view on 

 passing through beau- 

 tiful scenery. 



Exhaustion may 

 not be confined wholly 

 to the central nerve- 

 cells, but there can be 

 little doubt that they 

 are the most affected. 

 Since also there must be a certain momentum, so to speak,to 

 molecular activity, it is not surprising that we find that the 

 sensation outlasts the stimulus for a brief period ; and this ap- 

 plies to all the senses, and necessarily determines the rapidity 

 with which the successive stimuli may follow each other with- 

 out causing a blending of the sensations. 



Thus, then, in every sense we must recognize (1) an end- 

 organ in which the chain of processes begins ; (3) a conducting 

 nerve through which (3) the central nerve-cells are affected; 

 and we may speak, therefore, of (1) sensory impulses and (2) 

 sensations, when these give rise to affections of the central 

 nervous cells resulting in (1) perceptions and (2) judgments, 

 when we take into account the psychic processes ; and, from the 

 nature of cell-life generally, we must recognize p, certain inten- 

 sity of the stimulus necessary to arouse a sensation and a limit 

 within which alone we have power to discriminate (range of 

 stimulation and perception) ; and also a limit to the rapidity 

 with which stimuli may succeed each other to any advantage, 

 so as to give rise to new sensations ; and a limit to the endur- 

 ance of the apparatus in good working condition corresponding 



FiQ. 374.— Nerves with ganglion cells (ff) beneath a 

 tactile bristle ITb), from skin of an arthropod 

 ( Corethra) larva. 



