54 INNER VATION. [CHAP. XVII. 



It appears by a simple experiment, for the principle of which we 

 are indebted to Mariotte, that the small portion of the retina cor- 

 responding to the entrance of the optic nerve, is incapable of ex- 

 citing visual sensation though it receive the image of an object. 

 Place the thumbs together at arm's length, shut the left eye and fix 

 the right eye steadily on the left thumb; then the right thumb, if 

 moved gradually outwards (so that its image on the retina of 

 course traverses inwards), ceases to be visible in a particular spot, 

 but is again seen beyond it. It will be remembered that the fibrous 

 lamina of the gray nervous layer of the retina is here evolving 

 itself from the nerve, and is not yet invested with the vesicular or 

 other lamince; a circumstance of great interest in regard to the 

 modus operandi of the constituents of the retina in vision. 



It has indeed been denied by an eminent physiologist, that the 

 retina is insensible to light at this point, on the ground that, if such 

 were the case, we should see a dark spot in our field of view when- 

 ever we use only one eye. To produce the physical sensation of 

 darkness, however, the retina seems as necessary as the nerves of 

 ordinary feeling are to the production of the physical sensation of 

 cold. Both sensations are occasioned by the absence of the respective 

 stimulus, but the specially endowed nerve is as essential to acquaint 

 us with the absence as with the presence of the stimulus. What 

 Mariotte' s experiment proves is simply that over that spot no nervous 

 matter, having the peculiar power of excitability by light, exists; 

 and as far as the faculty of seeing with that spot is concerned, it is 

 as though the piece of retina had been punched out. We no more 

 see a dark spot corresponding to it, when we look with one eye, 

 than we see everything dark behind us where there is no nervous 

 expansion visually endowed. For, in strict language, a distinction 

 must be drawn between the sensation of darkness and the absence 

 of the sensation of light. 



This incapacity of vision at the entrance of the optic nerve, seems to 

 be essential to the mode of junction of the retina with the nerve, since 

 it appears to have been the chief reason why the nerve was not made 

 to enter in the axis of the eye. If the blind spot had been situ- 

 ated in the axis, a blank space would have always existed in the 

 centre of the field of vision, since the axes of the eyes, in vision, are 

 made to correspond. But, as it is, the blind spots do not corre- 

 spond when the eyes are directed to the same object; and hence the 

 blank, which one eye would present, is filled up by the opposite 

 one. 



Though no other part of the retina is insusceptible of luminou 



)US 



