THE RECEPTOR SYSTEM 219 



trunk be stimulated in the middle of its course, we refer the re- 

 sulting sensation to its outer endings. A blow on the inside of 

 the elbow-joint, injuring the ulnar nerve, produces not only a 

 local pain, but a sense of tingling ascribed to the fingers to which 

 the ends of the fibers go. Persons with amputated limbs have 

 feelings in their fingers and toes long after they have been lost, 

 if the nerve-trunks in the stump be irritated. This persistent- 

 reference is commonly ascribed to the results of experience. The 

 events of life have taught us that in the great majority of in- 

 stances the sensory impulses which excite a given tactile sensa- 

 tion, for example, have acted upon the tip of a finger. The sen- 

 sation goes when 'the finger is removed, and returns when it is 

 replaced; and the eye confirms the contact of the external object 

 with the finger-tip when we get the tactile sensation in question. 

 We thus come firmly to associate a particular region of the skin 

 with a given sensation, and whenever afterwards the nerve-fibers 

 coming from the finger are stimulated, no matter where in their 

 course, we ascribe the origin of the sensation to something acting 

 on the finger-tip. 



Perceptions. In every sensation we have to distinguish care- 

 fully between the pure sensation and' certain judgments founded 

 upon it; we have to distinguish between what we really feel and 

 what we think we feel; and very often firmly believe we do feel 

 when we do not. 



The most important of these judgments is that which leads us 

 to ascribe certain sensations, those aroused through organs of 

 special sense, to external objects that outer reference of our 

 sensations which leads us to form ideas concerning the existence, 

 form, position, and properties of external things. Such represen- 

 tations as these, founded on our senses, are called perceptions. 

 Since these always imply some mental activity in addition to a 

 mere feeling, their full discussion belongs to the domain of Psy- 

 chology. Physiology, however, is concerned with them so far as 

 it can determine the conditions of stimulation under which a 

 given mental representation concerning a sensation is made. 

 It is quite certain that we can feel nothing but states of ourselves, 

 but, as already pointed out, we have no hesitation in saying we 

 feel a hard or a cold, a rough or smooth body. When we look at 

 a distant object we usually make no demur to saying that we 



