444 ORGANS OF THE SENSES. 



sensation. Light is only the usual normal and physiological 

 excitant of this sensation ; the retina, being situated in the 

 depth of the eyeball, and protected by the cavity of the 

 orbit, is almost entirely removed from the influence of all 

 other agents than the rays of light; these are able to reach it 

 unobstructed by passing through the transparent media of 

 the eye. We have already seen that in cases where the 

 refringent apparatus of the media of the eye is in working 

 order, the images of external objects are painted (upside- 

 down) upon the* retina; an impression is then made upon the 

 membrane, and the excitation transmitted to the cerebral 

 centres (corpora quadrigemina and cerebral lobes), by 

 means of a peculiar mechanism, which we shall endeavor to 

 describe. 



The retina is not, however, in every part equally sensitive 

 to light ; there is a point which is quite insensitive to it, viz., 

 where the optic nerve (papilla) begins, and is called, on that 

 account, the punctum ccecum. This may be easily proved by 

 the following experiment : if two small objects, one of which 

 is white and the other red, be placed on the same plane, at a 

 certain distance from each other, and we look at either of 

 them with one eye only, we shall see the other also ; but if 

 the latter be moved so as to make its image pass over the 

 whole retina, a moment will come when this image will be 

 formed exactly on the optic papilla; at that moment the 

 object will be quite invisible, being depicted on the punctum 

 ccecum. An experiment made by Mariotte consists in mark- 

 ing two black points upon the paper, at a distance of five 

 centimetres from each other, and standing at a distance of 

 fifteen centimetres from the paper, the left eye being closed, 

 while the point on the left side (A) is observed with the 

 right eye ; in this position the point on the right side (B) 

 will not be seen, but it will become visible in nny other part, 

 whether nearer or farther off. We find, by calculation, that, 



in the position indicated, the image of the point on the right 

 side falls upon the punctum ccecum, and, consequently, is 

 invisible. 



The sensibility of the retina in other parts differs greatly ; 

 it reaches its highest point in the yellow spot (which corre- 

 sponds exactly with the posterior pole of the eye) and de- 



