9 6 



AN INTRODUCTION TO ZOOLOGY 



connection with the dendrons of the last layer of cells, the trans- 

 mitting cells. The bodies of these trans- 

 mitting elements, which are relatively 

 large, form the layer of nerve cells and 

 their axons, the innermost layer, that of 

 the nerve fibres. These pass out, via 

 the blind spot and are continued on 

 outside the eye as the optic nerve into 

 the brain. On the internal surface of 

 the nerve fibres is another thin inner 

 limiting membrane. Between the inner 

 and outer limiting membrane stretch 

 certain non-nervous supporting cells, 

 known as Muller's fibres, that form a 

 sort of scaffolding and whose nuclei fall 

 in the inner nuclear layer. 



The rods and cones are the actual 

 receptive elements in the eye, and it is 

 interesting to note that they are on the 

 outside of the retina, so that the light 

 has to pass through all the other layers 

 before reaching them, after doing which 

 it is stopped by the pigment cells. From 

 the evidence afforded by diseases of the 

 eye, it seems probable that the cones 

 are more particularly concerned with the 

 perception of colour and the rods with 

 variations of light intensity. The whole 

 eye forms a kind of photographic camera, 

 and the amount of light let in is regulated 

 by the iris, which therefore plays the 

 part of the diaphragm. By means of 

 the lens an image of what the animal 

 wishes to see is focussed upon the retina, 

 with the central part of the picture fall- 

 ing upon the yellow spot. As the things 

 looked at are at different distances from 

 the eye, it is necessary that the lens 

 should be capable of forming clear 

 images of both near and distant objects. 

 The distance between the lens and the 

 plate (i.e. the retina) cannot be altered 



as in a camera, but in order to produce the same result the 

 actual shape of the lens itself is altered ; it is more flattened 



NF 



It 



FIG. 32. Vertical' section 

 through the posterior wall 

 of the eye of a frog ; the 

 section passes through the 

 sclerotic, the choroid, and 

 the entire thickness of the 

 retina. X 300. From 

 Marshall and Gamble. 



B.C., red blood corpuscle ; C., 

 cone ; G., ganglion cell ; I.L., inner 

 limiting membrane ; I.M., inner 

 molecular layer ; I.N., inner nuclear 

 layer ; N.F., layer of nerve fibres ; 

 O.L., outer limiting membrane ; 

 O.M., outer molecular layer ; O.N., 

 outer nuclear layer ; P., pigment 

 cell of retina ; P.C., pigment of 

 choroid ; R., rod ; R.F., radial or 

 Muller's fibre ; S., the cartilaginous 

 sclerotic. 



