VISUAL ORGAN. 353 



From the above description of the retina, it is seen that the 

 light stimuli reach the brain in the following way : The rod 

 and cone visual cells, which one may call the first neurones, 

 receive the stimulus. From here it is transmitted to the bipolar 

 cells of the outer ganglionic layer (second neurones), and thence 

 to the cells of the ganglion-cell layer (third neurones), which 

 send fibres through the optic nerve to the brain. The con- 

 nection between these cells is by contact in the two reticular 

 layers. 



The retina has a somewhat different structure in the macula 

 lutea, the papilla n. optici (see Optic nerve), and the ora 

 serrata. 



In the region of the macula lutea the middle or cerebral 

 layer contains a yellow pigment, which is distributed diffusely, 

 so that this part has a yellowish color on the surface. In this 

 neighborhood the inner ganglion-cell layer is distinctly 

 thicker, consisting of as many as nine layers of ganglion 

 cells. The outer ganglionic layer is also wider here. The 

 layer of rods and cones becomes poorer in rods as the macula 

 lutea is approached, so that in this region itself only cone cells 

 are present. In the macula lutea Henle's fibre sheath is 

 especially well developed. 



In the centre of the macula lutea on its inner surface there 

 is a depression, the fovea centralis, in which the retinal layers 

 are distinctly thinner than elsewhere. The nerve-fibre sheath 

 ends here, and both ganglion-cell layers disappear, so that in 

 the fundus fovese itself only a neuro-epithelial layer is found. 

 Owing to the entire absence of the pigmented cerebral layer 

 of the retina, the fundus fovea9 appears colorless. 



In the region of the ora serrata a marked decrease in thick- 

 ness of the retina takes place in consequence of the disappear- 

 ance of the retinal layers. The nerve-fibre and ganglion-cell 

 layers are the first to disappear. The structure of the visual 

 cell layer is altered and the two reticular layers are lost. The 

 outer granular layer fuses with the outer ganglionic layer. 

 At a certain distance from the ora serrata the rod cells dis- 

 appear, and the cone cells change their typical character and 



