RETINAL CHANGES INVOLVED IN VISION 



559 



wall. That part of the cup originally derived from the external surface 

 of the body is turned towards the posterior layer or retinal pigment. From 

 it is developed the special end-organ of vision, viz. the rod and cone layer 

 of the retina. Besides this special sensory epithelium, the retina presents 

 two other sets of neurons through which 

 impulses generated in the sensory epi- 

 thelium must pass before they arrive at 

 the optic nerve. The three relays of 

 nervous elements in the retina have the 

 following arrangement : 



(1) The first relay the sense epithe- 

 lium consists of rods and cones with 

 their nuclei (Fig. 285), the latter being 

 situated in the outer nuclear layer. Each 

 rod presents an external (a) and an in- 

 ternal limb (6). The former, in the eye 

 which has been kept in the dark, has a 

 purplish colour from the presence of 

 rhodopsin or visual purple. From the 

 inner end of the inner limb a fine fibre c 

 passes to its nucleus in the outer nuclear 

 layer, and from the nucleus a central 

 process g passes into the outer molecular 

 layer where it ends freely in a little 

 knob e. The cones, which are thicker 

 than the rods, also possess outer and 

 inner limbs. From the inner limb a 

 thick process containing a nucleus, 

 passes through the external nuclear 

 layer and ends with a broad base e in 

 the outer molecular layer, from which 

 short fibres are given off to come in 

 contact with the bipolar cells of the 

 inner nuclear layer. 



(2) The second relay is formed by 

 the bipolar cells of the inner nuclear 

 layer. Each of these sends off one 

 fibre peripherally to make contact with 

 endings of the rod and cone fibres in 



the outer molecular layer, and another process which passes centrally into 

 the inner molecular layer. Here the process of the rod bipolar forms an 

 arborisation around the body of a cell in the ganglion-cell layer, while 

 processes of the cone bipolars end at various levels in the inner molecula: 

 layer, forming synapses with the dendrites of the ganglion-cells. 



(3) The ganglion-cells, which represent the third relay, receive 

 impulses from the more peripheral parts of the retina and send them towards 



FIG. 285. 



i, a rod ; n, a cone of mammalian 

 retina ; h, external limiting membrane. 

 (R. GREEFF.) 



