SENSE ORGANS. 



95 



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odor. The fibers of the second pair of cranial nerves are 

 specialized in such wise as to respond to impressions of 

 light, and nothing else, and any stimulation of this nerve, 

 whether by prick- 

 ing or by electric- 

 ity or otherwise, 

 gives rise to a 

 flash of light. The 

 eighth pair is so 

 specialized as to 

 respond to vibra- 

 tions of the air, 

 and any stimula- 

 tion of this nerve 

 gives rise to a sen- 

 sation of sound. 

 Similarlytheninth 

 pair is so special- 

 ized as to produce 

 the peculiar sen- 

 sation which we 

 call taste, and 

 nothing else, and 

 hence stimulation 

 of this nerve in 

 any way will pro- 

 duce this peculiar 

 sensation. This is 

 only one example 

 of the universal law of differentiation, but so admirable 

 a one that we stop a moment to dwell upon it. 



Commencing with the lowest animals — protozoa — or 

 with the earliest embryonic condition of the higher, we 

 have first a single cell, and then an unmodified cell ag- 

 gregate. As we pass upward three fundamental systems 



Uiznio citified. 

 Cellular Structure. 



Fig. 69. 



