SENSATIONS WHERE NERVES ARE INVISIBLE. 371 



the eyes of our Nudibranchs, we can have little doubt 

 that their vision is simply the perception of light and 

 darkness. The changes of temperature produced by 

 t*he absorption of the rays in their pigment, cannot be 

 elevated into the perception of an image, because the 

 optical conditions for the formation of an image are 

 absent : an indefinite sensation, resulting from change 

 of temperature, is all that they can perceive. Nay, 

 even were their eyes constructed so as to form optical 

 images, there is little doubt that vision, in om- human 

 sense, would still fail them, owing to the absence of 

 the necessary combination of tactile sensations with 

 sensations of light. We see very much by the aid of 

 our fingers. 



Apropos of tactile sensations, are those anatomists 

 who assume the existence of invisible nerves in parts 

 of the skin which, although revealing no nerve to the 

 eye, seem to reveal it to the mind by the manifesta- 

 tion of sensibility, warranted in such an assumption ? 

 KoUiker has shown that there is no portion of the 

 skin, however minute, which is not sensitive. But 

 does this prove that every point must be supplied with 

 a nerve ? Admitting that sensibility resides only in 

 nerve-tissue, I think another explanation will do away 

 with such an assumption. It is unnecessary that a 

 nerve-fibre should be directly pressed upon at the 

 immediate point of contact of the needle and the skin. 

 The sensation will equally result if the pressure be 

 communicated at some distance from the point of 

 contact. Strictly speaking, this is always the case 



