288 PHYSIOLOGY FOR DENTAL STUDENTS. 



The fibers all converge to the optic disc, which is a little to the 

 inside of the posterior pole of the eyeball. At this point the 

 fibers of the nerve fiber layer bend backwards at right anglos 

 and run into the optic nerve, thus crowding out the other layers 

 and causing the existence of a blind spot, which can be readily 

 demonstrated by closing one eye, say the left, and with the other 

 regarding the letter B in the next line. Although the S is 



B S 



also distinctly visible in most positions, yet if the book be 

 moved towards and away from the eye, the S will become in- 

 visible at a certain distance corresponding to that at which the 

 rays from it are impinging upon the blind spot. As we alter 

 the distance of the book from the eye, the line of vision, or 

 visual axis, being fixed on the B, the image of the S travels 

 from side to side across the inner or nasal half of the retina. 

 and at a certain position strikes the optic disc. Ordinarily we 

 are unaware of the blind spot, partly because we have two eyes 

 and, the blind spot being towards the nasal side of each side, 

 the image of an object does not fall on it in both eyes at the 

 same time; and partly because we have learned to disregard it. 

 The area or extent of the blind spot may become so increased, as 

 by excessive smoking, that it becomes noticeable. 



At another portion of the retina called the fovea centralis, all 

 the layers become thinned out except that of the rods and cones, 

 especially the cones. This, as we should expect, is by far the 

 most sensitive portion of the retina, and is indeed the portion on 

 which we cause the image to be focused when we desire to see 

 an object clearly. The remainder of the retina is only suffi- 

 ciently sensitive to give us a general impression of what we are 

 looking at. Thus when we view a landscape, we can see only a 

 small portion clearly at one time, although we have a general 

 impression of the whole. The portion which we see clearly is 

 that which is focused on the fovea, and we keep moving our 

 eyes in all directions so that every part of the landscape may in 

 turn be properly seen. We see with the fovea what the rest of 

 the retina informs us there is to be seen. 



