PROPERTIES OF THE RETINA. 337 



nerve fibers themselves are not acted upon by light waves is proved 

 by the existence of the blind spot. The termination of the optic 

 nerve within the eyeball, the optic disc, lies about 15 degrees to the 

 nasal side of the fovea and has a diameter of about 1.5 mms. From 

 this point the nerve-fibers spread out over the rest of the optic cup 

 to form the internal layer of the retina. But the optic disc itself has 

 no retinal structure, and light that falls upon it is not perceived. 

 The presence of this blind spot in our visual field is easily demon- 

 strated by the experiment illustrated and described in Fig. 141. 

 In the visual field for each eye, therefore, there is a gap representing 

 the projection of the area of the optic disc to the exterior, the size 

 of the gap increasing with the distance from the eye. We do not 

 notice this deficiency, inasmuch as it exists in our indirect field of 

 vision (see below), in which our perception of form is poorly de- 

 veloped; so that any disturbance in outline that might result in the 

 retinal image of external objects is unperceived. Morever, the 

 portion of the external world that falls on the blind spot of one eye 

 may fall on the retinal field of the other, and be perceived in binoc- 

 ular vision. It is to be borne in mind, also, that the projection of 

 the blind spot does not appear in the visual field as a dark area; it 

 is simply an absent area, so that no gap exists in our consciousness of 

 the spatial relations of the visual field; the margins, so to speak, of 

 the hole come into contact so far as our consciousness is concerned. 

 The Action Current Caused by Stimulation of the Retina. 

 The effect of light waves falling upon the retina is to set up a series 

 of nerve impulses in the optic nerve fibers. It is interesting to 

 find that these impulses aroused in a sensory nerve by a normal 

 stimulus are attended by electrical changes similar to those observed 

 in motor fibers when stimulated normally or artificially. The fact 

 strengthens the view that the electrical change is an invariable ac- 

 companiment of the nerve impulse, if not the nerve impulse itself. 

 If the eye is excised and connected with a galvanometer or capillary 

 electrometer by two non-polarizable electrodes, one placed upon the 

 cut end of the optic nerve and the other on the cornea, the usual 

 demarcation current is obtained due to the injury to the optic nerve. 

 If the preparation is kept in the dark and arrangements are made 

 to throw a light through the pupil upon the retina the galvanometer 

 indicates an electrical change or current whenever the light is 

 admitted.* The direction of the current in the eyeball is from the 

 fundus to the cornea, and as regards the pre-existing demarcation 

 current it is in the same direction and forms, therefore, a so-called 

 positive variation. When the electrodes are placed on the longi- 

 tudinal and the cut surface of the optic nerve, then, according to 

 Kiihne, the electrical response to light is a negative variation similar 



* Dewar and McKendrick, " Transactions, Royal Society, Edinburgh," 27, 

 1873; Gotch, " Journal of Physiology," 29, 388, 1903, and 31, 1, 1904. 

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