624 



THE SENSES. 



cording to Kolliker, which, in radiating from the entrance of the optic 

 nerve, pass directly to the edges of the macula, lose themselves among 

 the nerve cells of its ganglionic layer. The others curve round the 

 borders of the macula on each side, to resume their peripheral direction 

 beyond its limit ; so that the yellow spot itself is not covered, like the 

 rest of the retina, by a continuous superficial layer of nerve fibres. 



Secondly, the nerve cells of the ganglionic layer are more abundant 

 in the macula lutea than elsewhere. Over the greater portion of the 

 retina, according to Schultze, these cells exist, in the ganglionic layer, 

 only in a single plane ; that is, they are arranged side by side, and 

 neither above nor below each other. But in the yellow spot they form 

 several ranges of superimposed cells. On the other hand, toward the 

 centre of the yellow spot the cells diminish in number, and are entirely 

 absent at the fovea centralis. Various other layers, which exist more 

 or less distinctly in surrounding regions of the retina, also diminish in 

 thickness, and disappear toward the centre of the macula lutea. 



Fig. 197. 



DIAGRAMMATIC SECTION OF HITMAN RKTIWA, through the macula lutea and foveu 

 centralis. 1. Internal surface of the retina, in contact with the vitreous body. 2. Gan- 

 glionic layer of nerve cells. 3. Intermediate layers of the retina, disappearing at the centre 

 of the macula lutea. 4. Layer of nuclei, showing the oblique course of the fibres in this 

 region. 5. Layer of rods and cones ; consisting at its central portion exclusively of attenu- 

 ated and elongated cones. C. External surface of the retina, in contact with the choroid. 

 In the middle of the diagram is the depression of the fovea centralis. (Schultze.) 



Thirdly, owing to the modifications described above, the retina, at 

 the situation of the fovea centralis, consists only of its two external 

 layers, namely the layer of nuclei and the layer of rods* and cones. 

 Even these two layers exhibit, at this point, certain important peculiari- 

 ties in the form and arrangement of their elements. 



In the layer of nuclei, the nuclei themselves are present in nearly 

 their usual numbers and position ; but the fibres with which they are 

 connected, instead of passing through the layer in a direction perpen- 

 dicular to the surface of the retina, bend obliquely outward, to reach 

 the more superficial layers of the retina in the external portions, or 

 even beyond the borders, of the yellow spot. Thus this layer is very 

 much diminished in thickness, although it still contains its cell nuclei, 



