SENSE ORGANS AND SENSATIONS 241 



external objects. Thus, if in driving a nail the hammer 

 misses the nail and hits a finger, we refer the pain to the 

 finger and not to the hammer ; and we similarly refer sensa- 

 tions of hunger and thirst to the body and not to external 

 objects. If, on the other hand, the skin is cooled by a 

 piece of ice, we do not say that the skin is cold, but that 

 the ice is cold; we refer the sensation to the external object 

 which causes it, not to the skin in which it actually origi- 

 nates. In the case of the sense of sight, this reference of 

 the sensation to the external object which sends light into 

 the eye is so complete that unless we stop and reflect upon 

 it we do not realize that it is the condition of the eye of 

 which we are conscious rather than the condition of the 

 external object at which we are looking. 



3. Sense organs. A few sensations, like pain, are aroused 

 by the condition of most, if not all, parts of the body; 

 there is no one organ set apart to produce them. Some, like 

 hunger, although at times more or less general in origin, 

 are commonly aroused by the condition of some one organ 1 

 which ordinarily performs other functions. Other sensations 

 arise in organs set apart for the purpose and constructed to 

 react to only one kind of stimulus (special sense organs, or 

 organs of special sensation). To this latter class belong tl.e 

 eye, the ear, the olfactory mucous membrane of the nose, 

 the touch organs in the skin, etc. We therefore speak of 

 general sensations and special senses, although no sharp line 

 of division can be drawn between the two. 



4. The brain the seat of sensation. In all cases, however, 

 the sensation, although originating elsewhere, is developed 

 in the brain and not in the sense organ. If the optic nerve 

 be cut, blindness ensues, although light falling on the retina 

 produces the same effect in the eye itself as when the nerve 

 is intact ; it even starts nervous impulses toward the brain ; 

 but, since these impulses go no farther than the cut, they 



1 In the case of hunger, the stomach. 



