256 DR WALTEK II. GASKELL. 



pigment has been pressed into the service of the eye to form 

 the pigmented epithelial layer. 



We can, however, go further than this, for we know definitely 

 in the case of the retina what has been the fate of the epithelial 

 cells lining this tube. They have become the system of support- 

 ing structures known as Milllerian fibres. 



The epithelial layer of the primary optic vesicle can be traced 

 in direct continuity with the lining epithelium of the brain 

 cavity, as a single layer of epithelium cells in the core of the 

 optic nerve, forming the optic stalk, which, in consequence of 

 close contact, becomes the well-known axial layer of supporting 

 cells. This epithelial layer of the optic stalk then expands to 

 form the optic bulb, the outer or dorsal wall of which still 

 remains as a single layer of epithelium, which becomes the layer 

 of pigment epithelium. This layer of epithelium becomes 

 doubled on itself by the approximation of the inner or ventral 

 wall of the optic cup to the outer or dorsal wall in consequence 

 of the presence of the lens, and still remaining a single layer, 

 forms the imrs ciliaris retinm ; then suddenly at the ora sei-rata 

 the single epithelial layer appears to vanish, and the layers of 

 the retina take its place. It has long been known, however, 

 that even throughout the retina this single epithelial layer still 

 continues, being known as the fibres of Mliller. This is how 

 the fact is described in the last edition of Fosters Text-hook of 

 Physiology, p. 1308 : " Stretching radially from the inner to the 

 outer limiting membrane in all regions of the retina are certain 

 peculiar-shaped bodies known as the radial fibres of Miiller, 

 Each fibre is the outcome of the changes undergone by what 

 was at first a simple columnar epithelial cell. The changes 

 are, in the main, that the columnar form is elongated into 

 that of a more or less prismatic fibre, the edges of which become 

 variously branched, and that while the nucleus is retained the cell 

 substance becomes converted into neuro-keratin. And, indeed, 

 at the ora serrata the fibres of Miiller may be seen suddenly to 

 lose their peculiar features and to pass into the ordinary 

 columnar cells which form the ^^ars ciliaris retince." 



It is then absolutely clear that the essential parts of the eye 

 may be considered as composed of two parts — 



1. A tube or diverticulum from the tube of the central nervous 



