CHAP. XIX.] ORGANS OF SPECIAL SENSE. 231 



nerve-fibres is formed by the expansion of the optic nerve after 

 it has passed through the sclerotic and choroid coats at the back 

 of the eye. At its entrance it forms a slight prominence, and 

 then radiates latterly in every direction in the form of a closely 

 set layer of fine fibrils. The second layer, or layer of nerve- 

 cells, consists for the most part of bipolar l cells, processes from 

 which extend into the layer above and below. 



The layer of rods and cones is the most remarkable of all the 

 retinal layers; it is composed of elongated nucleated cells, 

 which are supposed by some authorities to be highly specialized 

 epithelium cells : they are directly concerned in producing the 

 sensation of light. Rays of light produce no effect upon the 

 optic nerve without the intervention of the rods and cones. 

 This is proved by the fact that at the entrance of the optic 

 nerve there are no rods and cones, and this spot is quite blind, 

 rays of light falling upon it producing no sensation. There is 

 one point of the retina which is of great importance, and that is 

 the macula lutea, or yellow spot. It is situated about j 1 ^- of an 

 inch to the outer side of the entrance of the optic nerve. In its 

 centre is a tiny pit (fovea centralis) which is the centre of 

 direct vision ; that is, it is the part of the retina which is always 

 turned towards the object looked at. From this point the sen- 

 sitiveness of the retina grows less and less in all directions. In 

 the fovea centralis the rods are highly developed, but most of 

 the other layers of the retina are wanting. 



Light may be described as consisting of vibrations in the ether 

 which pervades space. These ethereal vibrations enter the eye 

 through the cornea, pass on through the pupil and refracting 

 media, and strike on the retina. Here they excite the rods and 

 cones (cell-bodies specially adapted for receiving stimulation 

 from rays of light), and these pass on their stimulation to the 

 fibres of the optic nerve, which in turn transmit the stimulus to 

 the visual centre in the brain, and the sensation and perception 

 of light is produced. 



Behind the layer of rods and cones, and lying on the choroid, 

 is a beautiful mosaic layer of pigment cells containing a purple 

 substance which stains the rods, and is constantly bleached by 

 the action of light and regenerated in the dark. 



The refracting media of the eye. The interior of the eyeball 



1 A bipolar cell is a cell having two processes. 



