550 ANIMAL PHYSIOLOGY. 
rise to certain molecular changes which enable the mind or 
the ego to have a perception proper; which, when taken in con- 
nection with numerous past 
experiences of this and 
other senses, furnishes the 
material for a sensory judg- 
ment, 
The chief events are, 
after all, internal, and hence 
it is found that the higher 
in the scale the animal 
ranks, the more developed 
its nervous centers, espe: 
cially its brain, and the 
more it is able to capitalize 
its sensory impulses; also 
the greater the degree of 
possible improvement by 
Fic. 401.—Nerves with ganglion cells () beneatha experience, a difference well 
tactile bristle (7b), from skin of an arthropod . . 
(Corethra) larva. seen in blind men whose 
ability to succeed in life 
without vision is largely in proportion to their innate and 
acquired mental powers. Inasmuch as all cells require rest, 
one would expect that under constant stimulation fatigue would 
soon result and perceptions be imperfect. Hence it happens 
that all the senses fail when exercised, even for but a short pe- 
riod, without change of stimulus leading to alteration of con- 
dition in the central cells. The change need not be one of en- 
tire rest, but merely a new form of exercise. Hence'the fresh- 
ness 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 af- 
fected. 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 applies to all the senses, and necessarily determines the 
rapidity with which the successive stimuli may follow each 
other without 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; (2) a conducting 
nerve through which (3) the central nerve-cells are affected; 
and we may speak, therefore, of (1) sensory impulses and (2) 
