SENSATION AND THE SENSIFEROUS ORGANS. 279 



the most wonderful revelations of embryology is the 

 proof of the fact that the brain itself is, at its first 

 beginning, merely an infolding of the epidermic layer 

 of the general integument. Hence it follows that the 

 rods and cones of the vertebrate eye are modified epi- 

 dermic cells, as much as the crystalline cones of the 

 insect or crustacean eye are ; and that the inversion of 

 the position of the former in relation to light arises sim- 

 ply from the roundabout way in which the vertebrate 

 retina is developed. 



Thus all the higher sense organs start from one 

 foundation, and the receptive epithelium of the eye, or 

 of the ear, is as much modified epidermis as is that of 

 the nose. The structural unity of the sense organs is 

 the morphological parallel to their identity of physio- 

 logical function, which, as we have seen, is to be im- 

 pressed by certain modes of motion; and they are fine 

 or coarse, in proportion to the delicacy or the strength 

 of the impulses by which they are to be affected. 



In ultimate analysis, then, it appears that a sensa- 

 tion is the equivalent in terms of consciousness for a 

 mode of motion of the matter of the sensorium. But, 

 if inquiry is pushed a stage farther, and the question 

 is asked, What then do we know about matter and 

 motion? there is but one reply possible. All that we 

 know about motion is that it is a name for certain 

 changes in the relations of our visual, tactile, and mus- 

 cular sensations; and all that we know about matter 



