STUDIES IN TKE EEl'INA. 55 



South African sun. Here the rods, and especially the large 

 inner limbs, are mostly fall of pigment, which can be seen 

 streaming inwards into the retina, no longer forming single 

 fibrils with terminal knobs, but great tangles of refractive 

 matter, which, in the eye (31, a) with dark blackish pigment 

 are dull and blackish, but in the eye (31, b) with bright 

 yellowish-brown pigment are bright yellow-brown. I may 

 say that after seeing how ihe pigmented matter streamed 

 through the retina in the tadpoles brought from the slopes of 

 Table Mouutaiu, I was quite prepared to find something of 

 the kind in the retina of the baboons, but was myself 

 surprised to see how very obvious the escape of the absorbed 

 pigmented matter into the retina is in these cases. The pig- 

 ment is so dense that the colouring matter is not bleached in 

 the rods, nor, indeed, does it undergo much loss of colour 

 throughout its passage through the retina, as it usually does, 

 say, in our indigenous Amphibia. 



We have so far, then, traced the matter absorbed by the 

 rods into the retina as far as the region known as the outer 

 reticular layer. This is in many respects one of the most 

 difficult parts of the retina to understand. The matted and 

 deeply pigmented strands just below this layer in the 

 baboon's eye, as well as the conical expansions of the 

 ordinary " rod fibres," indicate that the absorbed pigmented 

 matter is temporarily stopped by it. But the exact cause of 

 the stoppage at this point I have not succeeded in unravelling. 

 Krause thought that there was a tangential membrane at 

 this place, his " membrana fenestrata," ^ and certainly the 

 first time one sees the outermost layer of nuclei of the middle 

 layer arranged tangentially in a compact row, as shown in 

 figs. 20, a, and 22, c, it is difficult not to think that Krause was 

 right; but a study of older eyes (figs. 20, h, 21, and 24, a), or 

 even of the more used-up parts of younger eyes (fig. 22, a), 

 will show that these nuclei do not belong to any fixed mor- 

 phological structure in the retina such as a membrane, but 



1 I have uufortuucitely never seeu a copy of the book written by Krause 

 under tliib title. 



