190 COMPARATIVE ANATOMY. 



interna and externa), is quite transparent in the fresh con- 

 dition, and consists of two structures which are histologically 

 and physiologically quite different: they are, a supporting part 

 and a nervous part. The former, or so-called fulcrum, which 

 is stretched as on a frame between the limitans interna and externa, 

 is shown in Fig. 156 as a light filigree-like tissue, the nervous 

 portion being indicated by a darker and more granular shade. The 

 latter consists of seven concentric layers, viz. : 



1. Layer of nerve-fibres. 



2. cells. 



3. ,. inner granular or molecular layer. 



4. ,, inner nuclei. 



5. intermediate nuclei or outer molecular layer. 



6. ., outer nuclei. 



7. ,. ., rods and cones with the pigment epithelium. 



Only the two last-mentioned layers (6 and 7) correspond to 

 the proper neuro-epithelium. 



These layers are so arranged that the nerve-fibres lie next 

 to the vitreous humour, that is, internally, while the rods and 

 cones are situated towards the choroid, or are the most external. 

 Thus the terminal members of the neuro-epithelium are turned 

 away from the rays of light falling upon the retina, and the rays 

 must therefore pass through all the other layers before they reach 

 the rods and cones. 



Fishes possess the longest, Amphibians the thickest rods, so that in the 

 latter there are only about 30,000 to a square millimetre, while in Man there 

 are from 250,000 to 1,000,000. 



In Fishes the rods far exceed the cones in number, while in Reptiles and 

 Birds the reverse is the case. The cones of many Reptiles and all Birds are 

 distinguished by the presence of brightly coloured oil-globules, which are 

 also present in those of Marsupials. 



In the retina of all Vertebrates there is a specially modified region of most 

 acute vision. This is called the yellow-spot (fovea central is or macula 

 lutea), and lies in the inner portion of the eye. It is due to the thinning-out 

 of all the layers except that of the rods and cones, and even the rods disappear, 

 only the cones persisting (Fig. 152, Fo). The cones of the macula lutea are 

 distinguished in Fishes and Reptiles by a very elongated and narrow form, 

 while in Amphibia and Mammals they are quite similar to those of the rest of 

 the retina. The physiological function of the pigment-epithelium is to give 

 rise to a colouring material, the so-called visual purple or visual red. 

 This becomes dissipated by the light falling upon the retina, and we may thus 

 compare the latter to a photographic plate, or rather to a whole photographic 

 apparatus, in which the photographer, represented by the pigment-epithelium, 

 by continually laying on new sensitive material (" visual substance ") (purple), 

 renews the plate, removing the old picture. Thus the act of seeing may possibly 

 be looked upon as a photo-chemical process. The rods only possess the visual 

 purple, the cones being without it ; thus in animals the retina of which possesses 

 no rods (e.g. many Reptiles), as well as in the yellow-spot of others, it is wanting. 

 The fact that the visual purple is not present in these Reptiles as well as in 

 certain nocturnal animals (e.g. Caprimulgus, Vespertilio serotinue), and in 

 Fowls and Pigeons, proves, however, that the physiology of vision is not yet 

 thoroughly understood. 



