THE PROPERTIES 01 l LOH PABT OF THE Hi HI \ ABO 



Ii is often a difficull matter to determine .just exactly what it is in 

 ihe nature of the stimulus thai makes it capable of affecting on< 

 and not another; for example, it is often merely a question of tl 

 of vibration of the stimulus. Lighl and heat rays both duo to 



vibration of the ether which fills Bpace When these vibrations are 

 slow, they stimulate receptors thai have been specialized for apprecia- 

 tion of temperature, bu1 when they are rapid and exist as rays of li^ht. 

 they no longer aftVrt the temperature r iptors bul only the highly spe- 

 cialized receptors of the retina. Similar vibrations of tin- air in pli 

 of tho ether cause sound and stimulate tho auditory receptors. Tt is 

 quite likely that tho receptors in different groups of animals are attui 

 to react tn different rates of vibration. For example, a cat can hoar 

 higher pitehod notes than man, and it is possible that the retinas of 

 some animals respond to rays vibrating with a different frequency from 

 those to which the retina of man is adaptod. Tn this connection it is of in- 

 terest to note that tho touoh roooptors of tho skin rospnnd so promptly 

 to stimulation that ono hundred vibrations of a tuning fork per seeond 

 can bo felt as separate stimuli, whereas to tho oar at this frequency th^ 

 fork omits a eontinuous note. Tho roooptors of touoh are therefore moro 

 prompt in their response than tho roooptors of tho auditory nerve. 



When onee tho roooptor has boon stimulated, tho impulse passes and 

 is transmitted to the nerve centers, where it is translated into a par- 

 ticular sensation. Tho conditions are really not unlike those whioh ob- 

 tain in tho ease of tho various physioal instruments used to reeeive and 



convert into tl lectric current stimuli <>f heat, light, chemical ene 



eto. Tho roooiver required to bring about this transformation must ho 

 especially constructed in eaoh ease, that for light boinir the aotinomoter. 

 that for motion the dynamo, that for heat the thermopile, and that for 

 chemical energy the oonoontration coll. Eaoh of these physioal instru- 

 ments may bo considered as a specialized receptor for tho purpose of 

 producing an electric current out of other forms of energy 



Tn accepting the above analogy we must not fail to boar in mind 

 that very feeble stimuli are often able to set in operation nerve impti 

 that are as potent as those produced by much stronger stimuli. Here 

 again, we have a physical analogue in the case of relay currents, in 

 which a fooblo electric current may operate to complete the circuit from 

 ind( 'it soi. •' electric discharge and thus set in motion a much 



larger amount of oner 



These general considerations ,,f the natal ptor naturally 



lead u< ti. the law of thr >- properties of > which is to the 



effect that, however excited, each nerve of special sense gives riso to 

 its own peculiar sensation. Tims, in whatever way the chorda tympani 



