490 THE HUMAN BODY. 



Common Sensation and Organs of Special Sense. A 



sensory nerve is one which, when stimulated, arouses, or may 

 arouse, a sensation in its possessor. The stimulant is in all 

 cases some form of motion, molar (e.g., mechanical pressure) 

 or molecular (as ethereal vibrations or chemical changes). 

 Since all our nerves lie within our Bodies as circumscribed 

 by the skin, and are excited within them, one might a priori 

 be inclined to suppose that the cause of all sensations would 

 appear to be within our Bodies themselves; that the thing 

 felt would be a modified portion of the feeler. This is the 

 case with regard to many sensations; a headache, toothache, 

 or earache gives us no idea of #ny external object; it merely 

 suggests to each of us a particular state of a sensitive portion 

 of myself. As regards many sensations, however, this is not 

 so; they suggest to us external causes, to properties of which, 

 and not to states of our Bodies, we ascribe them; and so they 

 lead us to the conception of an external universe. A knife 

 laid on the skin produces changes in it which lead us to 

 think not of a state of our skin, but of states of some object 

 outside the skin; we believe we feel a cold heavy hard thing 

 in contact with it. Nevertheless we have no sensory nerves 

 going into the knife and informing us directly of its condi- 

 tion; what we really feel are the modifications of our Body 

 produced by it, although we irresistibly think of them as 

 properties of the knife of some object that is no part of the 

 Body, and not of them as states of the latter. Let now the 

 knife cut through the skin; we feel no more knife, but ex- 

 perience pain, which we think of as a condition of ourselves. 

 We do not say the knife is painful, but that our finger is, and 

 yet we have, so far as sensation goes, as much reason to call 

 the knife painful as cold. Applied one way it produced 

 local changes arousing a sensation of cold, and in another 

 local changes causing a sensation of pain. Nevertheless in 

 the one case we speak of the cold as being in the knife, and 

 in the other of the pain as being in the finger. 



Sensitive parts, such as the surface of the skin, through 

 which we get, or believe we get, information about outer 

 things, are of far more intellectual value to us than sensitive 

 parts, such as the subcutaneous tissue into which the knife 

 may cut, which give us only sensations referred to conditions 

 of our Bodies. The former are called Sense-organs proper, 



