Chap. XIX.] ORGANS OF SPECIAL SENSE. 237 



to the outer side of the exit 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 sensitiveness of the retina grows less and less in all direc- 

 tions. In the fovea centralis the rods and cones are exceedingly 

 numerous, while the other retinal elements have been pushed 

 aside, as it were, forming an elevated margin around the pit. 



Light may be described as consisting of vibrations in the 

 ether which pervades space. These ethereal vibrations enter 

 the eye through the cornea, pass in through the pupil and 

 refracting media, and strike on the retina. They penetrate the 

 transparent retina until they fall upon the rod and cone cells. 

 In these there occur certain substances which are acted upon by 

 the light (much as the film of a photographic plate is acted upon 

 by the light). The chemical changes which these substances 

 undergo stimulate the adjacent dendrones, and the impulse 

 passes througli the cell-bodies and the axones to the second 

 series of neurones, and then through their dendrones and cell- 

 bodies to their axones. These last converge towards the blind 

 spot, where they unite to form the optic nerve, which, passing 

 from the eye to the brain, conducts the sensory impulses derived 

 from the chemical changes occurring in the rods and cones to 

 the visual centre, and the perception of light is produced. 



As in the case of the end organs of touch, each had its local 

 sign (see page 223), so each rod and cone has its own particular 

 local sign, but this local sign is not associated in our minds with 

 the part of the retina stimulated, as we associate touch with a 

 certain portion of the skin stimulated. In the case of the retina, 

 the local sign is associated with the source of the light which 

 acts as the stimulus. Thus when the upper part of the retina is 

 stimulated, we know that the source of light, the object which 

 we see, is below the line of direct vision ; and when the lower, 

 the right, or the left-hand portion of the retina is stimulated, 

 we know that the object lies above the line of direct vision, or 

 to the left or right of it, as the case may be. 



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

 is divided into two chambers by the crystalline lens and iris. 

 The "anterior chamber," the portion in front of the iris, is filled 

 with a colourless, transparent watery fluid, the aqueous humour. 



