THE FROG 95 



point where the perception of the retina is most keen. The blind 

 spot is a little below and behind the yellow spot. 



Examined under fairly high magnification a transverse section 

 of the wall of the eyeball shows the sclerotic to be mainly composed 

 of ordinary hyaline cartilage. The choroid is deeply pigmented 

 tissue with a plentiful supply of blood-vessels. The retina exhibits 

 quite a complex structure and its full details are difficult to make out, 

 save in specially prepared and stained sections. When these are 

 employed the full structure can be seen, but even in ordinary sections 

 viewed under comparatively low magnification the relative position 

 of the parts can be easily made out. Under the low power, the retina 

 appears to consist of a series of superimposed layers. Next to the 

 choroid coat is a pigment layer, and then follow in succession layers 

 termed the rod and cone, the outer nuclear, the outer molecular, 

 the inner molecular, the cellular and the fibrous layers. The explana- 

 tion of this appearance becomes clear when we examine the detailed 

 structure. 



The pigment layer is composed of a number of cells, deeply im- 

 pregnated with pigment, that send processes down between the rods. 



The visual sensory cells are the rods and cones. The rod-cells, 

 which are more numerous than the others, consist of long thread- 

 like cells. Each cell swells out near the inner end to form an enlarge- 

 ment in which the nucleus is situated. These nuclei fall in the outer 

 nuclear layer, the external boundary of which is marked by a thin 

 transparent homogeneous membrane, the outer limiting membrane. 

 Outside this membrane each rod-cell exhibits a spindle-shaped 

 enlargement and then becomes a narrow cylindrical rod, from which 

 the cell is named, both of these parts coming within the designation 

 of rods and cones. The thread-like portion of the cell, passing in- 

 wards from the nucleus, terminates in a small knob-like swelling. 

 The whole structure represents a much modified sensory epithelial 

 cell, the rod-like portion corresponding to the dendron, and the knob- 

 like enlargement the dendrite at the end of its axon. The cone-cell 

 has a very similar structure, the principal difference being that the 

 spindle-shaped swelling of the dendron is larger and more marked, and 

 passes on into a very short pointed rod, hence appearing cone-like. 



Next come a series of connecting elements in the form of more 

 special nerve cells, whose bodies with their nuclei constitute the 

 inner nuclear layer. The dendron of each cell passes out and arbo- 

 rises round the knobs of the rod and cone cells, thus giving rise to 

 the outer * molecular layer. The axon with a large dendrite comes 

 in the inner * molecular layer, where it comes into functional 



* Aborisations of other cells, that have been omitted for the sake of 

 simplicity, are concerned in the formation of these two molecular layers. 



