7li THE NERVOl S s VST KM AND ITS ( '( ).\SKK\ A T1O.N 



crushing the grasses on which it is set and at the same time 

 pre>-inii- down an area much larger than that which it act- 

 ually covers. It is precisely in this way that a pencil 

 point pressed into the skin stimulates powerfully the nerve- 

 twigs just beneath the selected spot, but extends its effect 

 to others within a certain radius. 



The nerve-fibers of the skin take their rise in part from 

 fine ramifications among its deeper cells. Others lead 

 away from special terminal organs of one kind and an- 

 other. These microscopic end-organs (tactile corpuscles, 

 etc.) have been minutely studied and will be found de- 

 scribed in detail in works on histology. It is needless 

 here to discuss the relation of the several types of nerve 

 ending to the several forms of stimulation to which each 

 may respond. In the light of what has been said already 

 ( 'hapter III) we shall assume that only one result can 

 follow the stimulation of a given fiber, whatever the 

 means employed. If we are right in taking this position, 

 we must conclude that the variety of sensations which 

 can be elicited through the skin is made possible by a 

 corresponding number of afferent pathways. 



We derive from the skin at least four kinds of sensa- 

 tion pressure, warmth, cold, and pain. As we have 

 accepted, provisionally at least, the "Mi'dlei-ian prin- 

 ciple," we must believe that a fiber which is once found 

 to give the sensation of pressure when it is stimulated can 

 never give any other sensation. For the impulses, how- 

 ever they may be started on their way, are of an un- 

 varying nature and take always the same direction. 

 On their arrival within the confines of the central nervous 

 system tliev must produce over and over the same set of 

 effects. Tin- only qualifying statement which should be 

 made is that strong stimuli of any kind may, very prob- 

 ably, initiate impulse- which shall penetrate farther than 

 those originated by less intense applications. So. accord- 

 ing to one theory, /mitt may be produced by a sufficiently 

 strong excitation of any afferent nerve, its occurrence being 

 the sign of an extension of the brain pmce beyond the 



