THE PROPERTIES OF EACH PART OF THE REFLEX ARC 789 



It is often a difficult matter to determine just exactly what it is in 

 the nature of the stimulus that makes it capable of affecting one receptor 

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

 of vibration of the stimulus. Light and heat rays are both due to 

 vibration of the ether which fills space. When these vibrations are 

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

 tion of temperature, but when they are rapid and exist as rays of light, 

 they no longer affect the temperature receptors but only the highly spe- 

 cialized receptors of the retina. Similar vibrations of the air in place 

 of the ether cause sound and stimulate the auditory receptors. It is 

 quite likely that the receptors in different groups of animals are attuned 

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

 higher pitched 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 adapted. In this connection it is of in- 

 terest to note that the touch receptors of the skin respond so promptly 

 to stimulation that one hundred vibrations of a tuning fork per second 

 can be felt as separate stimuli, whereas to the ear at this frequency the 

 fork emits a continuous note. The receptors of touch are therefore more 

 prompt in their response than the receptors of the auditory nerve. 



When once the receptor has been stimulated, the impulse passes and 

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

 ticular sensation. The conditions are really not unlike those which ob- 

 tain in the case of the various physical instruments used to receive and 

 convert into the electric current stimuli of heat, light, chemical energy, 

 etc. The receiver required to bring about this transformation must be 

 especially constructed in each case, that for light being the actinometer, 

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

 chemical energy the concentration cell. Each of these physical instru- 

 ments may be considered as a specialized receptor for the purpose of 

 producing an electric current out of other forms of energy. 



In accepting the above analogy we must not fail to bear in mind 

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

 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 feeble electric current may operate to complete the circuit from 

 independent sources of electric discharge and thus set in motion a much 

 larger amount of energy. 



These general considerations of the nature of a receptor naturally 

 lead us to the law of the specific properties of nerve, which is to the 

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

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



