866 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



most internally, exists a fine limiting membrane ; second] externally, a 

 layer of nerve-fibres ; third, a la}'er of nerve-cells analogous to the gan- 

 glionic cells of the brain ; fourth, a granular layer consisting of an indis- 

 tinct mass of fine gray granules ; fifth, the inner granular layer, composed 

 of little round granules ; sixth, the intermediate granular layer, in which 

 the granular mass is intermingled with small fibres; seventh, the outer 

 granular layer, analogous to the inner granular layer ; eightl\, a second 

 delicate membranous structure ; and ninth, the layer of rods and cones 

 (Figs. 386 and 387). 



It is this most external layer of the retina which is concerned in the 

 reception of the rays of light and the formation of the image. It con- 

 sists of small, transparent rods packed closely together at right angles to 

 the surface of the retina, while at different intervals between them is seen 

 a small rod expanded at the end so as to form a conical shape. These 

 cones are especially abundant in the jellow spot, where there is a slight 

 depression in the retina and where no rods are present. To reach the 

 layer of rods and cones, the rays of light must evidently pass through 

 all the superimposed layers, and are finally stopped at the choroid, which, 

 with its pigment layer, may be regarded as forming a background for the 

 retina. 



That the nerve-fibres themselves are insensitive to light may be 

 readily determined by proving that the point of entrance of the optic 

 nerve is entirely insensitive to light. If one ej'e is closed and the other 

 fixed on a black spot on a white sheet of paper and some other small 

 body be gradually moved laterally toward the outside of the field of 

 vision, at a certain distance the moving body will entirely disappear from 

 sight, while, if the motion be continued, it will again come into the field 

 of vision ; or if we place two wafers upon a board at a distance of four 

 or five inches apart and stand at about five times this distance, and 

 closing the right eye, with the left look at the right-hand wafer, the left- 

 hand wafer will disappear.; The explanation of this is that when so placed 

 the rays of light from the left-hand wafer will be received directly on the 

 optic nerve, and so indicates tliat the optic fibres themselves are insen- 

 sible to light, and that it is only through the retinal expansion of these 

 fibres that sensations of vision are possible. On the other hand, the 

 macula lutea is the locality of most distinct vision. When we fixedly 

 regard a point with the eye, then the rays of light from that point pass 

 through the middle of the pupil and the centre' of the lens and fall 

 almost on the centre of the retina, directly on the yellow spot. The for- 

 mation of the macula lutea indicates the reason of its especial sensitive- 

 ness to light, while at the same time pointing,out.tne:constituents of the 

 retina which are concerned in the formation of the image. It has been 

 mentioned that the optic fibres surround the yellow spot without passing 



