ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 441 



fiber, be this sudden "change electrical, thermal, mechanical 

 or chemical. Of course the stimuli so far enumerated are 

 all artificial stimuli which in the normal physiology of the 

 body are never called into play. The natural nerve stimuli 

 are of two kinds: 



First, stimuli which arise in the nerve centers in a way 

 so far entirely unexplained. These are stimuli which are 

 produced in the nerve cells themselves and by these sent 

 out along the fibers. Such stimuli are those produced by 

 the cells of the cortex of the brain, which produce the con- 

 traction of the fingers or toes. They may be spoken of 

 as central stimuli. These are always called into play in the 

 production of motor impulses. 



The second kind is peripheral stimuli which are best ex- 

 hibited in the organs of special sensation. Thus in the eye, 

 light in some way excites the optic terminations in the ret- 

 ina. In the ear vibrations in the cochlea excite the audit- 

 ory hairs on the basilar membrane, while on the skin gen- 

 erally, foreign bodies in their effect upon the tactile cor- 

 puscles usually occasion a stimulus interpreted in the mind 

 as sensations of touch. 



These peripheral stimuli are, however, not limited to the 

 special sense-organs. Such stimuli may originate in any 

 part of the body. In the stomach the presence of food may 

 produce a stimulus which is carried to the brain, and there 

 in a reflex way translated into a motor impulse to move the 

 muscles of the stomach. In fact, it will be pointed out that 

 the special sense-organs are nothing more than contrivances 

 by means of which stimuli which are too feeble to excite 

 nerves in general are so manipulated in a specially con- 

 structed sense-organ as to make possible such an excita- 

 tion. 



Do Nervous Impulses Differ Among Themselves? 



Some nerves carry motor impulses, others tactile im- 

 pulses, still others visual impulses, and the question natur- 

 ally arises do these impulses differ inter se? 



The older physiologists entertained the view that the 

 different results produced by different impulses were due to 



