SENSATIONS WUERE NERVES ARE INVISIBLE. 353 



vision is simply the perception of light and darkness. The 

 changes of temperature produced by the 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 our human sense, 

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

 combination of tactile sensations with sensations of liaht. 

 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 manifestation of sensibility, warranted 

 in such an assumption ? Kolliker has shown that there is 

 no portion of the skin, however minute, which is not sensi- 

 tive. But does this prove that every point must be supplied 

 with a nerve? Admitting that sensibility resides onhj in 

 nerve-tissue (which, for my part, I doubt, and in the next 

 Chapter will furnish my reasons), I think another explanation 

 will do away with such an assumption. It is imnecessary that 

 a nerve-fibre should be directly pressed upon at the imme- 

 diate point of contact of the needle and the skin. The sensa- 

 tion will equally result if the pressure be communicated at 

 some distance /Vo??« the point of contact. Strictly speaking, 

 this is always the case when the cuticle is not pierced. The 

 needle presses on the cuticle, and the pressure is communicated 



2 G 



