438 Dynamic Theory. 



fused into the substance of the lens hardening and making it impervious 

 to the light. The lens may be removed and if the vitreous humor is not 

 disturbed, the loss of the lens may be compensated by the use of spec- 

 tacles of very short focal distance. The convexity of the lens is altered 

 automatically to adjust its focus to objects at different distances. It is 

 supposed that the suspensory ligament of the lens habitually pulling it 

 by the outer edge tends to flatten it against its natural elasticity so that 

 it is focused ordinarily for long distances. When near objects are to be 

 seen, the ciliary muscle comes into play, and by its contraction the 

 choroid is pulled forward and the suspensory ligament with it, by the re- 

 laxation of which the lens is allowed to draw in its edges and increase 

 its convexity in front and so shorten its focal distance. ( See fig. 175.) 



The retina of the eye is the part in which the wave movements of the 

 ether are converted into nervous energy and transmitted through the 

 optic nerve to the interior parts of the body and especially to the brain, 

 in which alone the sensation of light arises. Of the nine layers of the 

 retina only one the outside one next the choroid coat appears to be di- 

 rectly concerned in this process of converting the waves, the other layers 

 being chiefly accessory parts for the propagation or transmission of the 

 new nervous stimuli. This outside layer constitutes about one-fourth of 

 the total thickness of the retina and is called the layer of the rods and 

 cones. The rods and cones are of the shape implied by their names, and 

 standing in a dense multitude on the fine membrane marked 8 in Fig. 

 176, point outward in a direction parallel with the radius of the eye- 

 ball toward the choroid coat. Beginning at the inner ends of the rods 

 and cones a great number of delicate nerve fibres pass inward, spread 

 out into branches and traverse the other layers to the inside limitary 

 membrane. Next to the limitary membrane is a layer of nerve fibres, 

 subdivisions of the optic nerve ( No. 2 ). Outside of this is a layer of 

 ganglionic corpuscles ( No. 3 ) with which the fibres of the optic nerve 

 freely communicate, as also do the radial fibres from the rods and 

 cones. Nos. 4, 5, 6 and 7 are layers of granular matter of differing 

 consistencies, 4 and 6 being finely subdivided, while 5 and 7 are of larger 

 granulations. The blood-vessels of the retina lie between No. 5 and 

 the limiting membrane. 



The light, in passing to the rods and cones, must therefore go through 

 the tracts holding the fibres of the optic nerve and the blood vessels. 

 As the rods and cones are the essential organs of sight, they can seethe 

 blood vessels just in front of them, as may be proved by an experiment 

 which is performed in a dark room with a dark wall and a single candle. 

 Look toward the dark wall steadily, and move the light to and fro past 

 the eyes, and so as to shine into them or one of them obliquely, and 

 after a little practice it is possible to see the red intersecting lines of the 



