2^4 ON THE SENSES. 



is effected. It is highly probable that the quality of the simple sensation ia 

 precisely the same in both cases. We learn only from circumstances, especially 

 from the accompanying muscular feeling, whether pressure or pulling, compres- 

 sion or stretching of the skin, has occasioned the sensation, and associate with 

 the idea corresponding to the sensation that of the direction of the force moving 

 the portions of skin — an idea which, as well as many others, we falsely regard 

 as essentially a part of the sensation. Every one thinks he sees the direction 

 in which a ray of light comes to the eye, and thus the direction in which a 

 visible object lies ; that he hears in what direction a sound reaches his ear, 

 because the idea of the direction, to which in these respects the muscular feel- 

 ing chiefly contributes, connects itself immediately and unconsciously with the 

 sensation of light or of sound. The bare sensation can, in itself, just as little 

 express the direction of the exciting force as the electric current, which moves 

 the index of the telegraph, can furnish an indication of the direction from which 

 it proceeded. A simple example will illustrate the formation of such an idea of 

 direction in the case of sensations of touch. If some one plucks us by the 

 hair, without our seeing him, we immediately conceive the direction in which 

 he has drawn us, but not from the sensation, not even mediately from this, but 

 from the feeling which arises in those muscles of the neck that offer resistance 

 to the turning of the head in consequence of the traction ; and it is again neces- 

 sary that we should have previously learned from experience to interpret all 

 these muscular feelings, so as to know with what movement each of them is 

 connected. That it is really the muscular feeling from which we divine the 

 direction of the traction, is evident from the fact that we no longer know that 

 direction, when our head is so firmly held by a third person as to prevent its 

 following the communicated impulse. 



For compression of the skin to produce a sensation of pressure, it is neces- 

 sary that the former should not fall below a certain degree of intensity nor rise 

 above another certain degree. 



Pressure, if too strong, creates, instead of such sensation, pain ; if too light 

 it does not excite the nerves ; we feel not, for instance, the pressure which a 

 small bit of paper exerts by its weight on the skin, especially not on the parts 

 of the latter where the epidermis is of greater thickness and hence embai'rasses 

 the propagation of the pressure to the ends of the nerves in the under skin. 

 Within these limits of intensity but one and the self-same sensation, as to qual- 

 ity, is produced by whatsoever pressure ; no matter by what force or by what 

 object the latter is occasioned, whether the organ of touch move towards the 

 object or this towards the organ, and the pressure be thus created by the resist- 

 ance of either to the movement o^ the other, there is in effect only one kind 

 of sensation as regards pressure. This assertion may sound strangely to one 

 not conversant with such discussions, AvliO has been accustomed to assume that 

 it is from the different qualities of the sensation arising from the touch of an 

 object that he forms a judgment of the properties of such object, the material 

 of which it consists, &c. Yet is it one and the same kind of sensation upon 

 which is founded the perception of roughness and smoothness, hardness and 

 softness, dryness and moisture, &c.; the self-same sensation of pressure arises 

 whether the object touched be of wood, metal, gum or clay. What enables us 

 to know those properties and materials may be made clear by a single example. 

 If with closed eyes you receive a ball in your hand, a moment's examination by 

 touching enables you to pronounce a complete judgment respecting it You 

 feel that the object is, in form, round ; you can perhaps indicate its size, and 

 can certainly decide whether it be rough or smooth, heavy or light, hard or 

 soft, elastic or otherwise ; you can even rightly conjecture whether it consist of 

 wood or metal. From what impressions do you form this comprehensive judg- 

 ment, which affords a complete image of the performances of the sense of touch 



